AMOS
AustralianMeteorological & OceanographicSociety
Bulletin of the Australian Meteorological & Oceanographic Society Vol 26, No.5, October 2013 ISSN 1035-6576
Contents Editorial ..........................................................................................................................................................................80 President’s Column ........................................................................................................................................................80 News ..............................................................................................................................................................................81 News from the Centres ..................................................................................................................................................83 Obituary .........................................................................................................................................................................85 Science Articles ..............................................................................................................................................................87 S.S. Chand and K.J.E. Walsh—Tropical cyclones in the Fiji region: Impact of ENSO and seasonal prediction ....................87 J.C. Taylor, K.C. Xuereb and W.J. Taylor—Extreme rainfall at Wilsons Promontory 22–23 March 2011 .............................91
Snapshot ..................................................................................................................................................................... 107 Charts from the Past with Blair Trewin: 26 January 1971.......................................................................................... 108 The Research Corner with Damien Irving................................................................................................................... 109
ISSN 1035-6576 Cover picture: A “winter sunset” taken on 31 July 2013 from Melrose (Wellington, New Zealand) looking west. This picture was taken as part of a “100 day project”, which challenged people to do something different for 100 days. The photographer decided to take photographs of meteorological phenomena (each day) for 100 days, which resulted in many stunning images. For more images from Matt’s 100 day project visit: mattevansphotography.co.nz/100-day-project Image: © Matt Evans Photography Unless specifically stated to the contrary, views expressed in the Bulletin are the personal views of the authors, and do not represent the views of the Society or any other organisation or institution to which the author(s) may be affiliated.
Editorial
Current and forthcoming articles This issue of BAMOS has turned out to be fairly large and contains two scientific articles—I will therefore keep this editorial short! The first article comes from Savin Chand, the 2011 Uwe Radok award winner and showcases his work on tropical cyclones in the Fijian region. The second article comes from (James) Taylor et al. and describes the Wilsons Promontory storm of March 2011. I first learned of James’ work at the AMOS National Conference 2013 and discussed the suitability of submitting the report he had already produced to BAMOS. It is great to see this article published as a “Science Article” in BAMOS following our
President’s Column
discussions at the National Conference—one of the many positive outcomes from the conference, I am sure. I hope this will encourage more operational meteorologists to submit their work. It is likely that forthcoming issues of BAMOS will be equally packed into the next year. The December issue will feature a very interesting article on “Fifty years of the World Weather Watch”, which documents the fascinating development of this international system for weather forecasting and warning.
Duncan Ackerley
A new government and the IPCC report Now that the election result is known, we are dealing with a somewhat different landscape to that which we were dealing with previously. In particular, I’m sure that many of you were concerned over the abolition of the Climate Commission (and were probably pleased to see its rapid rebirth as a privately-funded organisation), and the signals that were sent out by the absence of a specific Minister for Science.
It’s natural to interpret these moves as the first step in a series of government attacks on (in particular) climate science, and we will certainly be keeping a close eye on any government moves in this area—either to shut down areas of research, or to place obstacles in the way of communicating that research (something which has become a major issue in Canada). At the same time, it’s clear that there is a wide range of views in the Coalition on the acceptance of mainstream climate science, and it may be that the decisions taken so far are largely symbolic ones to keep those on the more extreme side of that range happy and are not indicative of more substantial intentions. Greg Hunt, the Minister for the Environment (who will be responsible for the Bureau of Meteorology), has made numerous positive comments about the value of climate science and climate information, both before the election—in response to the report of the Senate inquiry on extreme weather events—and after it, and until proven otherwise it seems reasonable to take these at face value. As I write, the first report of the latest round of the Intergovernmental Panel on Climate Change (IPCC) is about to hit the streets, which will focus further attention on the issue. It’s also brought out some fairly predictable misinformation in the media. One particularly egregious example, which many of you will have heard about directly or indirectly, was a front-page story in The Australian (largely imported from the UK Daily Mail), which claimed that the IPCC had halved its estimate of warming. This Bulletin of the Australian Meteorological and Oceanographic Society Vol.26 page 80
claim was based this on false statements about what the previous IPCC report had said. It was our intention to make a complaint to the Press Council but we abandoned this after The Australian retracted the story, albeit inadequately (the retraction certainly wasn’t on the front page). Nevertheless, taking action when our science is blatantly misrepresented in the media is something which we see as being very much within our ambit. We will be playing a substantial role in seeking to communicate the IPCC findings to the community, with a number of events planned by various Regional Centres, the first of them in Melbourne on 3 October. The response to this event has been exceptionally good with over 650 registrations at the time of writing, illustrating the public demand for highquality scientific information on the issue. In other spheres of activity, we are planning to extend our recognition of excellence by extending the range of awards bestowed by AMOS. Council has agreed in principle to the creation of two new awards, one specifically directed at early-career researchers and one at senior scientists beyond the age range targeted by the Priestley Medal. The details of both awards are still being resolved and it is expected that both will be made for the first time in 2015. While we’re on the subject of awards—nominations will have closed for the Priestley Medal and the Christopher Taylor Award by the time you read this; however, there is still time to make new nominations as Fellows of AMOS. Nominations are also welcome for positions on AMOS Council—in addition to the Executive Positions and the Chairs of the Regional Centres, there are six “at large” positions to be elected. These are well suited to people who have a specific area of interest in which they would like to contribute to AMOS.
Blair Trewin
News
Advancing Ozone Research in Australia and New Zealand— the Ozone Science Group Andrew Klekociuk1 and Annie Gabriel2
Australian Antarctic Division, Kingston, Tasmania. Department of Sustainability, Environment, Water, Population and Communities, Canberra, ACT. Addresses for correspondence: andrew.klekociuk@aad.gov.au and annie.gabriel@environment.gov.au 1 2
Man-made ozone depletion is a key climate issue that is gaining stronger recognition as not only having had a significant influence on Southern Hemisphere climate from at least the late 1970s, but which may potentially extend over much of the 21st century. Australia’s heritage in researching ozone and its climate effects largely dates back to the International Geophysical Year (IGY; 1957– 1959) when early ozone measurement programs were established in Australia and at Macquarie Island. Since that time, Australian ozone measurement programs have expanded and currently play important roles in various international monitoring and assessment programs. Since the 1980s, Australia has been at the forefront of modelling efforts to understand the ozone depletion process and its climatic effects. With the recent advent of the sophisticated climate modelling capabilities provided through development of the Australian Community Climate and Earth System Simulator (ACCESS), Australia is now bringing its ozone measurement and modelling activities to a common focus to help address gaps that remain in ozone science. Since 2007, Australian and New Zealand scientists involved in ozone-related research have been able to interact through an informal special interest group known as the Ozone Science Group (OSG). Members of the OSG meet quarterly to share information about developments in ozone science and research, to coordinate their activities and to cooperate on specific projects. The group was established as a means to encourage cooperation among Australia’s stratospheric ozone interests. Stratospheric ozone research and data collection in Australia are carried out by a number of different institutions, including the Bureau of Meteorology (BoM), the Australian Antarctic Division (AAD) (both of which are Department of Sustainability, Environment, Water, Population and Communities portfolio agencies), the Commonwealth Scientific and Industrial Research Organisation (CSIRO), the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) and various universities. Therefore significant efforts to coordinate and make use of existing data collections and expertise, and to publish data and findings are required. Southern Hemisphere observation points are sparse and Australian observations are essential for global emission and trend analysis, and as inputs to global assessments. The overall level of funding for ozone science in Australia Bulletin of the Australian Meteorological and Oceanographic Society Vol.26 page 81
is low, which requires a strategic approach to ensure research undertaken is as relevant and effective as possible. The OSG currently comprises key ozone scientists from the AAD, BoM, CSIRO Marine and Atmospheric Research, the ARPNSA, and a number of research universities including the universities of Melbourne, Wollongong, NSW, Tasmania, Adelaide, the Australian National University, Macquarie University and LaTrobe University. Some ozone scientists from New Zealand are also part of the group and participate in meetings by teleconference. The OSG has undertaken a number of cooperative activities such as; combining information to produce comprehensive reviews of the Antarctic Ozone Holes since 2007, that have been published in the Australian Meteorological and Oceanographic Journal (and its predecessor); producing a combined submission to the draft National Framework for Climate Change Science in late 2008; and preparing Australia’s national report to the triennial Ozone Research Managers meeting in May 2011. In 2012, OSG members took part in celebrations associated with the 25th anniversary of the signing of the Montreal Protocol, which included the preparation of a special series of articles for The Conversation1. Current activities of the OSG include; t
Following up ideas reaised at a special workshop on ozone depletion in the Southern Hemisphere held earlier this year in Buenos Aires under the auspices of the World Climate Research Programme. This meeting highlighted the current state of understanding of the climatic effects of ozone and current gaps in knowledge. Details on this meeting and outcomes can be found in the July 2013 Stratosphere-troposphere Processes And their Role in Climate (SPARC) Newsletter2.
t
Leading, coordinating, authoring and reviewing material for the 2014 Scientific Assessment of Ozone Depletion conducted under the auspices of the Wold Meteorological Organisation and the United Nations Environment Programme. Linkages between climate change and ozone protection, and stratospheric and tropospheric processes are currently areas of focus for the 2014 report. As a result of the coordination and
1
theconversation.com/topics/ozone-layer
2 www.sparc-climate.org/fileadmin/customer/6_Publications/ Newsletter_PDF/41_SPARCnewsletter_Jul2013_web.pdf
cooperation that the OSG has brought to Australian ozone science, Australia is well represented on the Scientific Assessment Panel body of experts putting this report together. t
3
Collaboration to produce chemistry-climate model simulations that are contributing to the First Chemistry-Climate Model Initiative3.
www.met.reading.ac.uk/ccmi/
The OSG is an ideal way for Australian scientists to share information on new and ongoing research, to share data and information, to coordinate their research activities where possible, and to advocate for new or redirected research. Further details on the OSG and how to become involved can be obtained by contacting the authors, or through the contact information provided on the OSG website4. 4 www.environment.gov.au/atmosphere/ozone/ozone-hole/ ozone-science.html
International Association of Meteorology and Atmospheric Sciences (IAMAS) News Tom Beer
National IAMAS Correspondant
A brief report on the Davos Atmosphere and Cryosphere 2013 (DACA-13)—”Air, ice and process interactions” The International Association of Cryospheric Sciences (IACS) and IAMAS held their joint scientific assembly DACA-13 on 8–12 July 2013 in Davos, Switzerland. A total of 989 participants from 52 countries on five continents attended. Scientifically, the assembly covered numerous fields of atmospheric and cryospheric sciences, enriched by snow hydrology, oceanography, natural hazards, economy and risks, and the history of science. These made up an attractive programme consisting of 21 mostly joint Symposia featuring several sessions each. More than 350 posters were on display for the whole week while dedicated poster sessions facilitated deeper discussions with the presenters in attendance. Four distinguished scientists delivered well-attended end-of-day keynote lectures: Thomas Stocker started with the topic “Climate change: Making the best use of scientific information”, Valérie Masson-Delmotte introduced us to “Water stable isotopes and climate in Greenland, from present-day atmospheric monitoring to glacial-interglacial ice core records”, Ronald B. Smith presented “Global aspects of orographic precipitation”, and Georg Kaser closed the scientific part of the assembly talking about “The cryosphere after AR5: more knowledge but also more uncertainty”. The assembly was highly ranked by the attendees. The quality of both talks and posters was generally regarded as excellent and the environment of Davos and its Congress Centre allowed for easy exchanges between colleagues. Indeed, many sessions brought together scientists that would not have otherwise met, and the question “When will the next DACA conference be held?” was often heard. It was a great pleasure to see many early career scientists attending the conference During the Opening Ceremony James Screen (UK) received the first IAMAS Early Career Scientist Medal. Competition for the best student poster was fierce, and at the end of DACA-13, six early career poster awards were presented to Fabiano Monti, Franziska Koch, Heather Archambault, Rianne H. Giesen, Saehee Lim and Narendra Ojha. The cash prizes were sponsored by the Bulletin of the Australian Meteorological and Oceanographic Society Vol.26 page 82
Swiss Meteorological Society and the Swiss Snow, Ice and Permafrost Society. Since 2009, the Swiss National Organizing Committee (SNOC) chaired by Heini Wernli have meticulously planned this event, strongly supported by the two local Davos institutes, the Physikalisch-Meteorologisches Observatorium Davos/World Radiation Center with the Local Organizing Committee chair Werner Schmutz, and the Swiss Federal Institute for Snow and Avalanche Research with Scientific Programme Committee chair Michael Lehning. Several generous sponsors helped in staging this event: the Swiss Academy of Sciences, MeteoSwiss, the Federal Office for the Environment, the Canton of Grisons as well as the World Meteorological Organization and the International Union of Geophysics and Geodesy by awarding travel grants in addition to those provided by IAMAS and IACS. Several commissions and committees of both Associations held business meetings during DACA-13. The IAMAS Executive Committee discussed a resolution about aspects of “geo-engineering” and adopted the bid for a tripleassociation assembly in Cape Town, South Africa, in 2017.
Deadline for the application to IUGG support of scientific meetings (10 October 2013) The program of IUGG support for scientific meetings (e.g. workshops, advanced schools, symposia) is one of the most important means by which the Union and its Associations pursue a goal of promoting geophysics and geodesy through international collaboration. A fraction of the IUGG’s budget is devoted to the support of these scientific meetings. The IUGG Executive Committee places great emphasis on maintaining high scientific standards, coverage of a balanced spectrum of topics, and an appropriately broad and international flavor for the scientific program of the meetings. In that respect, the ICSU rules on non-discrimination in the access of qualified scientists from all parts of the world to any IUGG-sponsored meeting apply. The number of co-sponsored meetings ranges from ten to fifteen (up to US$3,000 each). Accordingly, not all
meeting proposals worthy of support can be awarded
IUGG sponsorship. More information can be found at the IUGG website1. 1
www.iugg.org/meetings/guidelines.php.
News from the Centres
Brisbane Centre News Michael Hewson
Secretary, Brisbane Centre The Brisbane Regional Centre of AMOS will be shortly inviting AMOS members, policy makers and interested stakeholders to a half day forum entitled “Adaptation to Changing Weather in the Home and Working Environments”. It will be held at the Queensland Museum on 26 November 2013. This event will begin with
registration at 12:00 p.m. and will finish with an optional networking / social event after 4:30 pm. Please put this in your diary and plan to attend. A flyer is forthcoming and will contain details of the program, speakers and a nominal fee.
NSW Centre News Fiona Johnson Chair, NSW Centre
The University of Newcastle hosted the AMOS NSW Hunter Valley Seminar on 26 August 2013. It was an excellent showcase of the AMOS members, and what they do in the Hunter Valley. John James, a senior meteorologist from the Bureau of Meteorology at Williamtown, discussed their role in providing meteorological information for the Royal Australian Air Force. Felicity Gamble, a meteorologist at the National Climate Centre, provided an insight into seasonal outlooks and highlighted the recent shift from using statistical to dynamical forecasts. Lastly, Anthony Kiem, a senior lecturer and hydroclimatologist, highlighted the key research areas and activities of the University of Newcastle’s Environmental and Climate Change Research Group (ECCRG). The seminar was well attended, and was an excellent showcase of work being done in the Hunter Valley. Coming up in NSW will be a briefing day that we are organising with the Centre of Excellence for Climate
System Science for CEOs and executives on the IPCC Fifth Assessment Report findings. It is planned for Friday 25 October at the Univerity of New South Wales’ CBD campus. For more information go to the Centre of Excellence website1. Our next technical seminar will be at Macquarie University on Tuesday 29 October from 6:00 p.m. to 8:00 p.m. and will feature a presentation from Dean Howard— the inaugural winner of the AMOS regional award for academic achievement in NSW. Grant Edwards will also present an overview of climate science research at Macquarie University. For more information check out our calendar of events on the NSW regional centre section of the AMOS website2. 1
www.climatescience.org.au/IPCC-Briefing.
2 www.amos.org.au/regionalcentres/list/asset_id/45/cid/19/ parent/0/t/regionalcentres/title/2008%20Calendar
A presentation by Felicity Gamble at the AMOS NSW Hunter Valley Seminar. Image: Andrew Magee. Bulletin of the Australian Meteorological and Oceanographic Society Vol.26 page 83
Melbourne Centre News Nicholas Tyrrell1 and Hamish Ramsay2 1 2
Regional sub-editor, Melbourne Centre Monash University, Clayton, Victoria
The AMOS Melbourne Centre hosted the first of a continuing series of themed, half-day workshops on 6 August 2013. The focus of this meeting was tornadoes. The workshop succeeded in its aim to bring together members from a number of AMOS related communities, with forecasters, weather enthusiasts, students and researchers in attendance. While the interests of these groups are often related, this hasn’t always led to collaboration between them. AMOS wished to address this with an event that included a social gathering and formal presentations—all wrapped up in half a day in order to fit in with our busy lives! The event was held at the Bureau of Meteorology (BoM), and started, as all events held at lunchtime should start, with free pizza. To set the mood of the day the documentary “Deadliest Tornadoes” was screened during lunch. This NOVA documentary detailed the devastating 2011 tornado outbreak in the USA. It gave an overview from scientists working in the field and added important context about the potentially dangerous social impact of tornadoes, and hence the importance of furthering our understanding. The early-afternoon presentations focused on conceptual models of tornadogenesis (i.e. how they form), the historical record of tornadoes in Australia, and the latest tornado season in the U.S.A. Veteran “storm chasers” Clyve Herbert and Jane O’Neill (from Australian Sky & Weather) gave an overview of their recent experience in the U.S.A., where they spent several months roaming the Great Plains in search of extreme weather. Harald Richter
(Centre for Australian Weather and Climate Research) explained the latest ideas and theories on tornadogenesis. John Allen, who returned briefly to home soil from his postdoc in the U.S.A., filled us in on his recent work looking at the historical record of tornadoes in Australia, while Joshua Soderholm briefed us on the upcoming “Coastal Convective Interactions Experiment” based in Queensland. The late-afternoon session focused on recent tornado events in Australia and the U.S.A., as well as forecasting perspectives on severe storms. Tracey Malmborg from the Victorian Regional Office (VRO) gave us insight into what goes on during a typical severe weather day in the forecasting office. Stuart Coombs (VRO) and Hamish Ramsay (Monash University) presented on two tornado outbreaks that occurred earlier this year; one in northern Victoria on 21 March, and the other in Southeast Queensland over the Australia Day weekend. Finally, Harald Richter gave us a detailed analysis on the record breaking Oklahoma tornadoes of May 2013. The day finished with wine and cheese, and a screening of a film that has been an inspiration for many weather buffs of a certain age—”Twister”. The event was well attended, with around sixty people, making it the largest Melbourne Centre AMOS event of recent times outside of the AMOS National Conference. All the presentations and slides have been made available online1. 1 www.amos.org.au/regionalcentres/list/asset_id/110/cid/22/ parent/0/t/regionalcentres/title/tornado2013
Wanted: New AMOS Bulletin editor Do you like writing and communicating? Do you want to meet more AMOS members, and work with leaders in our scientific community? Well this is the opportunity for you! AMOS is currently searching for someone to fill the role of Bulletin editor. The editor is responsible for all aspects of the Bulletin, including writing the editorial, organising the reviews of submitted scientific articles, and collecting and compiling the contents for each issue. There will be support from the current editor, the editor-in-chief and AMOS administrative officer. Some knowledge of Adobe InDesign is helpful, but not necessary. Postgraduate students are encouraged to apply and, if successful, will be financially supported by an honourarium. If you are interested, have any questions or would like to apply, please contact Duncan Ackerley (current editor, duncan.ackerley@monash.edu), Stewart Allen (Editor in Chief, stewart.allen@bom.gov.au) or Jeanette Dargaville (administrative officer, admin_officer@amos.org.au).
Bulletin of the Australian Meteorological and Oceanographic Society Vol.26 page 84
Obituary
Professor Peter Schwerdtfeger 23 December 1935–20 August 2013 John Zillman Bureau of Meteorology, Melbourne, Victoria Emeritus Professor Peter Schwerdtfeger, who was elected to the Fellowship of the Academy of Technological Sciences and Engineering in 1988, died in Adelaide on 20 August 2013, aged 77. His funeral service at the Heysen Chapel, Centennial Park, Adelaide on 10 September 2013 was attended by several hundred friends, family and former academic, professional and community organisation colleagues (including Academy Fellows, many of Peter’s former students and present and retired staff of the Bureau of Meteorology). The many family and institutional speakers at the service included the Vice Chancellor of Flinders University, Professor Michael Barber, and Peter’s long-time Airborne Research Australia (ARA) colleague, Dr Jorg Hacker. Peter was born on 23 December 1935 in Gottingen, Germany and migrated to Australia with his parents, Hans and Hanna Schwerdtfeger, in August 1939. After gaining his BSc and MSc from Melbourne University and a Ph.D. from McGill in Canada (and periods at the University of Cologne and the University of Alaska) he was appointed to the Meteorology Department in Melbourne. At Melbourne he played a key role in developing observational field work and starting a sophisticated micrometeorological program at the Mt Derrimut Research Station. In 1971, Peter was appointed as Professor of Meteorology at Flinders University. He took pride in being the first Professor of Meteorology at an Australian university and, at the time of his appointment, one of the youngest, if not the youngest, Professor at any university in Australia. As Director of the Flinders Institute of Atmospheric and Marine Sciences (FIAMS), he built up a strong team of meteorologists and oceanographers and a large group of enthusiastic “hands-on” graduate students, many of whom went on to key roles in the Bureau of Meteorology and elsewhere in meteorology both nationally and internationally. As a Flinders Professor and gracious host of visitors “from the East”, he enjoyed nothing more than having a nice bottle (or bottles) of wine with a picnic lunch on the hillside not far from Flinders with visiting CSIRO or Bureau lecturers and Honours students. Peter was an observational and experimental meteorologist to his bootstraps and he developed many original approaches to radiation and micrometeorological measurement and data interpretation. He was also passionately committed to the environment in its many dimensions and was a keen gardener and amateur architect. He was inaugurator and long-term President (1983–2009) of the Alexander von Humboldt Fellows Bulletin of the Australian Meteorological and Oceanographic Society Vol.26 page 85
Association in Australia and built strong links with the Humboldt Foundation in Germany. He was a driving force in developing the use of small aircraft for meteorological and other remote sensing programs and, according to Jorg Hacker, Peter was in his element “out in the field messing around with instruments”. He learned to fly relatively late in his career and he and Jorg, in two aircraft, made a formidable team. He was also involved in many community and government advisory roles, including chairing of the South Australian Country Fire Service in the early 1980s and membership of the Antarctic Science Advisory Committee (ASAC) in the 1990s. He served on the Bureau of Meteorology Advisory Board during 2001–2004 and, at the request of the Minister, he represented the Board on the Australian Delegation to the 2003 World Meteorological Congress. Peter was a real polymath and a powerful example, himself, of the breadth of perspective of Humboldtian science. He loved music and, as noted at his funeral service, he had “an uncanny ability to use the English language”. He also had a great “big picture” applications-oriented approach to big social and technological challenges and was, surely, the ultimate authority on towing Antarctic icebergs as a potential source of fresh water for lower latitudes. Peter Schwerdtfeger had a wide circle of friends and professional colleagues across the international meteorological community and he and his wife Arija built strong links into their local community. An added source of pride which Peter shared with his polar meteorological colleagues was “Uncle Werner” who migrated from Germany to the US (Wisconsin) after World War II and, after short periods at universities in Argentina and Australia (Melbourne), went on to become the preeminent international authority on Antarctic climatology. He (Prof. Werner Schwerdtfeger), with Peter on hand here on the local scene, helped initiate a program of collaboration between the University of Wisconsin and the Australian Bureau of Meteorology, which continues to the present day. Peter is survived by his wife Arija, his brother Roland, his son Karl, his daughter India and their families. He was a unique Australian “character” with occasional maverick tendencies and a strong European heritage imbued with the best traditions of German science. He was a greatly liked and admired mentor of young scientists. His passing leaves them with many fine memories but something special will remain missing from the Australian meteorological scene.
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Bulletin of the Australian Meteorological and Oceanographic Society Vol.26 page 86
Science Articles
Tropical cyclones in the Fiji region: Impact of ENSO and seasonal prediction Savin S. Chand1,2 and Kevin J. E. Walsh2
School of Science, Information Technology and Engineering, University of Ballarat, Ballarat, Victoria, Australia. School of Earth Sciences, University of Melbourne, Parkville, Victoria, Australia. Address for correspondence: s.chand@ballarat.edu.au
1 2
Savin Chand was invited to submit this summary article on his work after receiving the AMOS Uwe Radok award for the best Ph.D. thesis in 2011. This article provides that summary—Ed.
1. Introduction Tropical cyclones (TCs) are extreme weather systems in which strong winds, coupled with heavy rainfall, often have devastating consequences for life and property. Understanding the factors affecting TC formation and variability and also predicting TC characteristics at the regional scale a few months in advance both have the potential to provide great social and economic benefits. Tropical cyclone formation in the South Pacific is modulated by large-scale climate drivers such as the El Niño-Southern Oscillation (ENSO) and the MaddenJulian Oscillation (MJO), and also by a climate feature called the South Pacific Convergence Zone (SPCZ). Effects of these drivers and features on TC characteristics can be complex, particularly in the central southwestern tropical Pacific basin that spans the approximate location of the main centre of action of ENSO. Th is makes it possible for TCs to occur there during both El Niño and La Niña conditions, thus giving rise to a somewhat nonlinear ENSO–TC relationship. A better understanding of such a relationship is therefore necessary for developing and implementing improved and cost-effective disaster management and decision-making strategies. A recent Ph.D. thesis of the fi rst author (Chand, 2011a) described the impact of ENSO and MJO on TC characteristics in the Fiji, Samoa and Tonga (FST) regions (170°E–170°W, 5–25°S). Chand (2011a) also explained the development of TC seasonal forecasting systems for the FST region based on state-of-the-art statistical methods. Here we briefly present one of the main results of the work: understanding the impact of ENSO on tropical cyclones in the FST region, and developing appropriate statistical models in the Bayesian framework to predict TCs using ENSO as the main predictor.
2. Impact of ENSO One of the most dominant climatic processes affecting TC activity in the southwest Pacific is the ENSO phenomenon (e.g., Basher and Zheng, 1995; Ramsay et al., 2008). The three phases of ENSO (i.e., El Niño, La Niña and neutral) strongly influence the geographical distribution of TC Bulletin of the Australian Meteorological and Oceanographic Society Vol.26 page 87
activity in the southwest Pacific basin. For example, during El Niño events, TC activity systematically shifts northeastward to the Cook Islands and French Polynesia, extending east-southeast of the Fiji islands (e.g., Basher and Zheng, 1995). Simultaneously, low activity dominates the Coral Sea and Australian regions. In contrast, the reverse occurs during La Niña events when the TC activity is displaced southwestward into the New Caledonia, Coral Sea and Australian regions with relatively low activity east of the dateline. As the centre of oscillation of ENSO spans the FST region, TCs here can occur in both El Niño and La Niña years, yielding a linearly weaker ENSO–TC relationship than for the Australian region or east of the dateline (Basher and Zheng, 1995). Chand and Walsh (2009) reexamined this relationship using a TC track clustering1 method initially proposed by Gaffney (2004), and found distinct TC genesis and track features in the FST region for different ENSO phases. TC tracks from the 1970/71–2005/06 period over the FST region (based on data obtained from the Fiji Meteorological Services) are conveniently described using three separate clusters. These clusters are shown in Figure 1, along with their respective mean regression curves. Two distinct types of TC motions, namely “straight” and “recurver” types are evident for TCs over the FST region. The typical tracks in clusters 1 and 2 (Figures 1a and 1b respectively) are characteristic of straight moving cyclones as shown by their mean regression curves. Although clusters 1 and 2 are of similar types, they differ markedly in other characteristics. For example, tracks in cluster 1 are longer and initially located west of the dateline before moving east, with a mean regression track passing over the Fiji islands. Tracks in cluster 2, on the other hand, are short and highly localised east of the dateline, with a mean regression track passing south of Samoa. The tracks in cluster 3 (Figure 1c) correspond to recurving cyclones. This recurvature occurs west of the Fiji islands, around 170–175°E, 15°S. Climatologically, these results indicate that the Fiji islands lie along the path of both straight moving TCs associated with cluster 1, as well as those of recurving cyclones associated with cluster 3. The Tonga region has a high risk of impact with cyclones associated with cluster 1, many of which also pass over the Fiji islands. 1 TC track clustering refers to grouping of TC tracks based on their similar characteristics, and in our context clustering is based on similar track patterns and their geographical locations (refer to Chand and Walsh, 2009, for details of the clustering method).
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25 S 170 W 160 W 160 W 180 W 160 E 170 E 180 W 170 W 160 W Longitude Longitude Longitude Figure 1: TC tracks (blue) and associated mean regression CLUSTER 3 curves (red) over the FST region between 1970–1971 and 2005–2006 in each of the three clusters. 0 Figure redrawn using information from Chand and Walsh (2009). 5 S The cyclones associated with clusters 1 and 3 pose little or appropriate statistical models in a Bayesian framework to no threat to Samoa but those associated with cluster 2 can forecast TCs in the FST region at least a few months ahead S have10serious impacts. The recurving cyclones largely affect of the November–April TC season. areas west-southwest of the Fiji islands. 15 S 3. Seasonal Prediction Results indicate that around 75% of the straight moving Chand et al. (2010) presented the fi rst comprehensive S TCs 20associated with cluster 1 form poleward of 10°S and study on the development of a seasonal prediction scheme west of the dateline. Similarly, the straight moving TCs for TC counts in the FST region. Here we give only an 25 S 160 Ewith cluster 170 E 2, which 180mainly W 170 Wthe Samoa 160 W associated affect overview of the method and the results. The reader is Longitude of 8°S and east of the dateline. Most region, form poleward referred to Chand et al. (2010) and Chand and Walsh of the TCs that develop equatorward of 10°S are likely to (2012) for more details. take a recurving path associated with cluster 3. Examining each cluster independently shows that the individual Two separate regression models were developed to clusters are highly influenced by ENSO, particularly by predict TC counts in the FST region: (i) for TCs that form conditions associated with El Niño and La Niña events within the FST region (type “FORM”) and, (ii) for TCs (Table 1). For example, clusters 2 and 3 are characteristic that enter the FST region (type “ENTER”). TCs generally of El Niño conditions where TC genesis positions are enter the FST region from the Coral Sea where formation displaced further north-east, consistent with earlier is more common during La Niña years compared to studies over the region (e.g., Basher and Zheng 1995). TCs that in El Niño years. The Poisson regression model2 in cluster 1 are characteristic of both El Niño and La Niña using the Bayesian approach3 is considered optimal for conditions, but those that form during La Niña usually development of a TC prediction scheme in the FST region enter the region from west of 170°E (Chand and Walsh for two reasons (refer to Chand et al., 2010 for details of 2009; Chand et al. 2010 ). the method). First, the observed TCs in the FST region are relatively few from a large number of tropical disturbances Tropical cyclones that form west of 170°E are more often that form each year. Such rare events are often modelled steered into the FST region during La Niña than during El using the Poisson process (e.g., McDonnell and Holbrook, Niño years. In contrast, more TCs are formed in the FST 2004). Second, the Bayesian approach (as opposed to region during El Niño years than during La Niña years. 25oS 160oE
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This adds to a weaker statistical relationship between the total annual number of TCs affecting the FST region and ENSO indices. Accordingly, it is essential to develop separate statistical models for TCs that form within the FST region and for those that enter the domain from the west (of 170°E), in order to obtain more rigorous and stable statistical relations with appropriate ENSO predictors. In the next section, we discuss formulation of
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2 Poisson regression is a form of regression analysis used to model count data, particularly rare events such as the few tropical cyclones that form from a relatively large number of tropical disturbances each year. 3 Bayesian method involves the use of random variables to model all sources of uncertainty in statistical models. This includes not just sources of true randomness, but also uncertainty resulting from lack of information. In Bayesian method, the posterior probability of a random event, TCs in our case, is the conditional probability that is assigned after the relevant evidence is taken into account.
the classical approach) permits availability of model parameters in terms of their posterior distribution. Th is readily facilitates predictive inferences on future cyclone occurrences within a probabilistic framework. A number of predictor combinations affecting TC activity in the FST region are evaluated for statistical FORM and ENTER models through a cross-validation technique. Strong correlations can be identified with the May-July preseason ENSO indices and near-equatorial region largescale environmental parameters (such as relative vorticity and environmental wind shear), enabling prediction of the annual number of TCs associated with FORM and ENTER to be made at least three months before the November-April cyclone season. Table 1: Mean number of TCs (Mean NTC) in each cluster. Asterisks in clusters 2 and 3 indicate TCs forming in the La Niña phase are less than those in the corresponding El Niño and neutral phases (at the 95% significance level obtained using bootstrapping sampling method). Cluster
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Results show that while the ENTER model appears to have good skill over all years, the FORM model has two limitations. First, the model may underestimate (overestimate) the formation for the years where onset of El Niño (La Niña) events is after the May-July pre-season, or where the previous La Niña (El Niño) event continued through the May–July season during its decaying phase. Accordingly, issuing updates of such cases using the October–December early season predictors shows a substantial improvement in the hindcast skill. The other limitation is that the model performance in neutral conditions is quite variable. Overall, no significant skill can be achieved for neutral conditions even after an October–December update. Th is is contrary to the skill obtained during El Niño or La Niña events where model performance is improved substantially after an OctoberDecember early cyclone season update. A supplementary statistical model has been developed using a different approach in order to further improve the seasonal forecasts in the FST region, particularly for ENSO-neutral conditions. Instead of modelling TC counts, we modelled the overall TC activity using Accumulated Cyclone Energy (ACE, Bell et al., 2000) as a binary classification problem. Years of “high TC activity” were defined as those years when ACE values exceeded the sample climatology, and years of “low TC activity” when ACE values were equal to or less than the sample climatology. Annual values of ACE were obtained by Bulletin of the Australian Meteorological and Oceanographic Society Vol.26 page 89
calculating sum of the squares of the estimated 6-hourly maximum sustained wind speed over the lifetime of a TC and then accumulating them for all TCs over the season under consideration. Here we consider a probit regression scheme using Bayesian fitting optimal to model ACE in the FST region (see Chand and Walsh, 2012 for details of this model). A probit regression is considered a proper probability approach for describing binary response data (e.g., Albert and Chib, 1993). Binary response data generally takes a value of one for the occurrence of an event (high TC activity in our case) and zero for the non-occurrence (low TC activity). Various ENSO indices and large-scale environmental conditions that are known to affect ACE are examined as potential predictors of ACE several months in advance. Substantial correlations between ACE and environmental conditions are found up to May-July, thus enabling potential prediction of the state of an upcoming November–April TC season by May–July (approximately five months ahead). Results based on hindcasts clearly suggest that the probit regression model developed here predicts TC activity over the FST region with substantial skill for all years considered in the analysis, particularly for the ENSO-neutral years. The reader is referred to Chand and Walsh (2012) for details of the procedure.
4. Summary Two of the main aspects of Chand (2011a) are presented here: advancing understanding of the impact of ENSO on TC characteristics in the Fiji, Samoa and Tonga (FST) regions, and developing robust statistical models to provide forewarning of TC strikes for improved decisionmaking procedures. Overall, Chand (2011a) provides an improved understanding of TC variability (both at intraseasonal and interannual timescales) in the FST region. It adds to the broad body of scientific literature, which is otherwise limited for the FST region on various aspects of TC characteristics. It also has a significant national benefit for Fiji, Samoa and Tonga by providing a better understanding of TC behavior through its contribution to the improvement of TC forecasting schemes. This may result in significant social and economic benefits through more effective and coordinated preparation for TC events. Results of the work described above are published in a series of papers by Chand and Walsh (2009, 2010, 2011b, 2011c, 2012), and Chand et al. (2010).
Acknowledgments Savin’s PhD was sponsored by Australian Government’s Endeavour Postgraduate Award Scheme. The authors also acknowledge the two referees for their comments.
References Albert, J. and Chib, S., 1993, Bayesian analysis of binary and polychotomous response data, Journal of the American Statistical Association, 88, 669–679. Basher, R.E., and Zheng, X., 1995, Tropical cyclones in the southwest Pacific: Spatial patterns and relationships to
Southern Oscillation and sea surface temperature. Journal of Climate, 8, 1249–1260. Bell, G.D., and Coauthors, 2000, Climate assessment for 1999, Bulletin of American Meteorological Society, 81, S1– S50. Chand, S.S., and Walsh, K.J.E., 2009, Tropical cyclone activity in the Fiji region: spatial patterns and relationship to large-scale circulation, Journal of Climate, 22, 3877– 3893. Chand, S.S., and Walsh, K.J.E., 2010, The influence of the Madden-Julian Oscillation on tropical cyclone activity in the Fiji region, Journal of Climate, 23, 868–886. Chand, S.S., Walsh, K.J.E. and Chan, J.C.L., 2010, A Bayesian regression approach to seasonal prediction of tropical cyclones affecting the Fiji region, Journal of Climate, 23, 3425–3445. Chand, S.S., 2011a, Tropical cyclone characteristics in the Fiji region. Ph.D. Thesis, School of Earth Sciences, University of Melbourne, Australia.
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Chand, S.S., and Walsh, K.J.E., 2011b, Influence of ENSO on tropical cyclone intensity in the Fiji region, Journal of Climate, 24, 4096–4108. Chand, S.S., and Walsh K.J.E., 2011c, Forecasting tropical cyclone formation in the Fiji region: a probit regression approach using the Bayesian fitting, Weather and Forecasting, 26, 150–165. Chand, S.S., and Walsh, K.J.E., 2012, Modeling Seasonal tropical cyclone activity in the Fiji region as a binary classification problem, Journal of Climate, 25, 5057–5071. Gaffney, S.J., 2004, Probabilistic curve-aligned clustering and prediction with regression mixture models, Ph.D. Thesis, University of California, Irvine, CA, USA. McDonnell, K.A., and Holbrook, N.J., 2004, A Poisson regression model of tropical cyclogenesis for the Australian–southwest Pacific Ocean region, Weather and Forecasting, 19, 440–455. Ramsay, H.A., Leslie, L.M., Lamb, P.J., Richman, M.B. and Leplastrier, M., 2008, Interannual variability of tropical cyclones in the Australian region: role of large-scale environment, Journal of Climate, 21, 1083–1103.
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The Australian Meteorological and Oceanographic Society (AMOS) is holding its 20th Annual National Conference in Hobart, Tasmania from 12-14 February 2014 at the Hotel Grand Chancellor. This conference is the major annual event for AMOS, attracting more than 350 national and international scientists from diverse fields. AMOS 2014 will provide a unique opportunity for Australian scientists to present cutting edge research in the weather, climate and ocean sciences, with a strong focus on southern hemisphere research. For further information please visit www.amos2014.org.au or contact admin_officer@amos.org.au. Kindly sponsored by:
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Bulletin of the Australian Meteorological and Oceanographic Society Vol.26 page 90
Australian Government Department of Sustainability, Environment, Water, Population and Communities Australian Antarctic Division
Extreme rainfall at Wilsons Promontory 22–23 March 2011 James C. Taylor1, Karin C. Xuereb2 and William J. Taylor3
a/Senior Meteorologist, Victorian Regional Forecast Centre, Bureau of Meteorology, 1010 Latrobe Street, Docklands, Victoria 3008, Australia Senior Meteorologist, Climate and Water Division, Bureau of Meteorology, 700 Collins St, Docklands, Victoria 3008, Australia. 3 Transport Analyst (retired), Beaumaris, Victoria 3193, Australia. Address for correspondence: j.taylor@bom.gov.au 1 2
1. Introduction
2. Event Details
On 22 March 2011 an extreme rainfall event occurred at Wilsons Promontory (southern Victoria) causing flooding and multiple landslides, which resulted in major refurbishment works. A Davis Weather Station (DWS) rain gauge mounted on the roof of the Tidal River National Parks Office recorded 377.4 mm of rain from midnight on 21 March to 5 a.m. on 23 March. The Average Recurrence Interval (ARI) for this event is estimated to be in excess of 3000 years.
Extreme rainfall resulted in widespread devastation over a central swath of Wilsons Promontory extending from the east to west coasts. Multiple significant landslips destroyed roads and walking tracks, some extending from the Promontory’s highest peaks to sea level. Figure 1 illustrates the landslips on the southern slope of Mount Oberon located approximately 2 km south-east of Tidal River. Flooding in Tidal River resulted in the rebuilding and refurbishment of 25 accommodation, administration and utility buildings. The destruction of a bridge over Darby River on the northern side of the Promontory resulted in the stranding and eventual airlifting of around 500 campers and staff out of the flood affected region.
Daily rainfall (24 hours to 9:00 a.m.1) recorded by the DWS on 23 March was 360.2 mm. This is close to Victoria’s official highest daily rainfall of 375.0 mm recorded at Tanybryn in the Otway Ranges associated with drought breaking rains on 22 March 1983. The next highest official daily rainfall total for Victoria is 318.6 mm, recorded at Mount Wellington in the Great Dividing Range in central Gippsland on 28 June 2007, associated with an east coast low pressure system. To document this exceptional event, this paper discusses the effect of the rainfall at Wilsons Promontory and meteorological aspects of the event. An estimate of the ARI is given and other recent events in the region surrounding Bass Strait are summarised to give an indication of the prevalence of such events. This leads into a discussion of the effect of potential increased sea surface temperatures (SSTs) due to global warming.
1 All times are given as Eastern Daylight Savings Time (AEDT), which is UTC+11 hours, unless stated.
Figure 1: Looking north from Oberon Bay at the southern slope of Mount Oberon. Multiple landslips shown (highlighted by the white circles).
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The official Bureau of Meteorology (BoM) rain gauge at Tidal River is a 203 mm gauge, which is the standard rain gauge universally used by World Meteorological Organization contributing meteorological agencies and has a maximum capacity of 250 mm. In the 24 hours to 9:00 a.m. on 22 March this rain gauge recorded 11 mm; however, over the next 18 hours the gauge overflowed and therefore had in excess of 250 mm. National Parks own a DWS that has a rain gauge located on the roof of the Tidal River National Parks Offices as shown in Figure 2. This rain gauge has a tipping bucket that records rainfall data to a computer in the National Parks offices. The total rainfall recorded from 12:00 a.m. on 22 March to 5:00 a.m. on 23 March was 377.4 mm. In the 24 hours to 2:30 a.m. the DWS recorded 375.4 mm. However, official daily rainfall amounts are defined by the BoM as, “the rain that has fallen in the 24 hours to 9:00 a.m”. The
Figure 2: Rain gauge of the DWS (black cylindrical shape) located above air conditioning ducts on the roof of Tidal River National Parks Offices.
Figure 3: Google Earth image showing referenced locations. DWS rainfall data indicated that 360.2 mm was recorded in the 24 hours to 9:00 a.m. on 23 March. Heavy rainfall was also recorded at Moe with thunderstorm activity; this extended southwards through the regions of Fish Creek, Foster and Waratah Bay where widespread totals in the order of 100–150 mm were recorded (refer to Figure 3 for location information). Figure 4 shows the Australian Water Availability Project (AWAP) analysis of the 24-hour rainfall to 9:00 a.m. on 23 March for the South Gippsland region. This was a special run of the analysis to include both official BoM rainfall data and unofficial rainfall observations collected in the region. The plotted observations are a selection from these data. The analysis makes best use of the available data but may
not be entirely accurate over the topographically complex Wilsons Promontory where only a few observations were available (all shown). The rainfall recorded by the DWS rain gauge is potentially Victoria’s second highest daily rainfall total; however, the rain gauge site does not comply with official BoM guidelines and the type of rain gauge is not a BoM standard. As such the extreme rainfall collected via the DWS will be recorded as a notable event but will not be an official record. However, DWSs do have a good reputation for being reliable. Although it is not certain when the rain gauge was last calibrated or how the rooftop site may have influenced the rainfall record, the fact that the official BoM rain gauge
Figure 4: The Australian Water Availability Project (AWAP) analysis of the 24-hour rainfall to 9:00 a.m. on 23 March for the South Gippsland region. Includes official BoM rainfall observations and unofficial observations collected in the region. The plotted observations are a selection from these data.. Bulletin of the Australian Meteorological and Oceanographic Society Vol.26 page 92
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Figure 6: Mean Sea Level Pressure (MSLP) analysis for the Australian region for (a) 11:00 p.m. 21 March, (b) 11:00 a.m. 22 March, (c) 11:00 p.m. 22 March, (d) 11:00 a.m. 23 March, (e) 11:00 p.m. 23 March, (f) 11:00 a.m. 24 March.
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is sited within 50 m of the National Parks offices and recorded in excess of 250 mm, combined with the extent to which landslips and flooding occurred, indicates that the DWS recorded rainfall is likely to be reliable. Figure 5 shows the 30-minute rainfall recorded during the event as well as the accumulated total. The three shaded areas indicate three-hourly periods when close to 100 mm was recorded. While there were common meteorological factors through each of these periods, the independent characteristics are also of interest and will be described in more detail in the section discussing the meteorological specifics of the event (Section 5).
3. General Situation A low-pressure system was cut off in easterly flow over southern New South Wales (NSW) on 21 March, with an associated trough that extended southwards to Bass Strait and northwards to north-east NSW. The low had moved across southeastern South Australia (SA) where ten locations had record March daily rainfall totals on 21 Bulletin of the Australian Meteorological and Oceanographic Society Vol.26 page 94
March. Figure 6 shows the evolution of this low from 11:00 p.m. on 21 March to 11:00 a.m. on 24 March at 12-hourly intervals. Figure 7 shows the corresponding infra-red (IR) satellite imagery for each Mean Sea Level Pressure (MSLP) chart in Figure 6. By the morning of 22 March, a low-pressure centre had developed in the trough in western Bass Strait as the initial low moved to the southern NSW coast bringing very heavy rainfall to the Illawarra region and South Coasts of NSW (Figure 6b). Cathcart, Mount Darragh (inland from Merimbula, elevation 880 m) recorded daily rainfall of 397.8 mm on 22 March. The low over western Bass Strait and the low off the south coast of NSW remain linked by a trough overnight on 22 March and into 23 March (Figures 6c and 6d, respectively). During this period a mesoscale low developed over eastern Bass Strait approximately 70 km south of Lakes Entrance. Vigorous and deep moist easterly flow was incident on Wilsons Promontory’s steep terrain until the mesoscale
Figure 8: SST deciles for the Australian region for the 12 months leading up to July 2011.
low tracked to the south of Wilsons Promontory early on the morning of 23 March (Figure 6e). Another heavy rain event occurred on 23 March over the northeast of Tasmania as the southern NSW low and associated trough moved southwards towards the region (Figure 6f). Gray, near the northeast coast of Tasmania, recorded daily rainfall of 327.2 mm on 24 March, this will be discussed further in Section 7.
4. Antecedent Conditions During 2010, one of the strongest La Niña events (as measured by the Southern Oscillation Index) on record developed. This, coupled with very warm sea surface
temperatures (SSTs) to the north of Australia and the eastern Indian Ocean (see Figure 8), contributed to make 2010–2011 Australia’s wettest two year period on record (Special Climate Statement 38). Research in seasonal prediction (see, for example, Drosdowsky and Chambers, 2001; Power et al., 2006; Ummenhofer et al., 2009) has shown that such conditions increase the chance of above average rainfall in the southeast of the continent. Historically, there is not a strong link between rainfall and the El Niño Southern Oscillation (ENSO) over coastal Victoria and western Tasmania (including South Gippsland) relative to other parts of the continent (Risbey et al., 2009). However, La Niña events with very strongly
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Figure 9: Comparison of strong La Niña events; Top shows rainfall deciles for (a) the 24 months to March 1976 and (b) the 24 months to March 2012; Bottom shows the SOI for the years (c) 1969–1976 and (d) 2008–2013, which lead up to and include the periods in (a) and (b), respectively.
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Figure 10: ACCESS-A 850 hPa (approximately 1500 m above sea level) wind vectors for 3:00 p.m. on 22 March 2011. Arrows indicate wind direction and coloured contours indicate wind velocity in knots (kts). positive Southern Oscillation Index (SOI) values have typically been associated with widespread, above-average rainfall across most of eastern Australia. For example, Figure 9 shows that above average to very much above average rainfall occurred south of the Great Dividing Range during the sustained La NiĂąa period in the 24 months to March 1976. A similar pattern is observed for the 24 months to March 2012. The strongly positive SOI is also shown for these periods.
During the period from 1 March to 22 March 2011, rainfall of 48.2 mm was recorded at the official BoM rain gauge at Tidal River, with two notable rain days of 23.8 mm on 10 March and 15.0 mm on 14 March. No rain was recorded in the four days prior to 22 March; however, the recent above-average rainfall would have primed the ground for runoff, likely contributing to the flooding and severe landslides that occurred on 22 March during the extremely heavy rainfall.
During the five months leading up to March 2011, the official BoM rain gauge at Tidal River recorded monthly rainfall from close to average (October 2010) to more than double the monthly average (February 2011). Only three months had below average rainfall in the twelve months leading up to March 2011.
5. Meteorological Specifics of the event
In this report, the 11:00 a.m., 22 March model run of the 0.11° (~12 km) resolution Australian Community Climate and Earth-System Simulator A (ACCESS-A) has been used to examine the meteorological specifics of the event.
Figure 11: ACCESS-A 250 hPa geopotential height contours (dashed red lines - x101 m) and wind field (kts) for 5:00 p.m. on 22 March 2011. Bulletin of the Australian Meteorological and Oceanographic Society Vol.26 page 96
Figure 12: The Cooperative Institute for Meteorological Satellite Studies (CIMSS) Water Vapour image with upper atmospheric motion vectors (blue in the 100–250 hPa layer, yellow in the 251–350 hPa layer, green in the 351-600 hPa layer) overlayed for 5:00 p.m. 22 March 2011. Blue / red dashed line denotes approximate position of trough / ridge respectively. Provided courtesy of UW-Madison SSEC/CIMSS Gale-force easterly winds were established through eastern Bass Strait leading up to midday on 22 March. The 10:00 a.m. radiosonde ascent from Melbourne airport indicated deep levels of moisture up to an altitude of approximately 5000 m (not shown). The trough associated with the low in western Bass Strait extended close to Cape Otway, through Port Phillip and along the southern part of Victoria’s Great Dividing Range.
tropospheric jetstreak through SA and NSW. Figure 11 shows jet core wind speeds at 250 hPa in the order of 100–120 kts (51–62 m s-1). Upper divergence occurs in this region of a jetstreak in the Southern Hemisphere due to ageostrophic flow caused by the deceleration at the exit of the jet streak. Southern NSW and Victoria, including Wilsons Promontory, lie in this favourable region of upper divergence.
A very strong low level east-to-northeasterly jet of approximately 55–60 kts (28–31 m s-1) was simulated by ACCESS-A at approximately 1500 m above sea level, south of the trough over Bass Strait and Wilsons Promontory (see Figure 10). The simulated wind profile indicated easterly winds near the surface backing to northeasterly at 3000 m and north to northeasterly at 5500 m. Such wind profiles are associated with warm air advection (WAA) that acts to lift an air mass. A lifting air mass will cool until it reaches saturation (cloud) and will then rain as water droplets grow to a point where the uplift cannot sustain their weight.
The second process is the upper divergence associated with a trough tilted to the north-east over the midlatitudes in the Southern Hemisphere, commonly known as a negatively tilted trough. Figure 11 shows a strongly negatively tilted trough in the vicinity of Wilsons Promontory. Atmospheric motion vectors shown in Cooperative Institute of Meteorological Satellite Studies imagery (Figure 12) indicates that Wilsons Promontory lies in the highly favourable upper divergent region downstream of the strongly negatively tilted trough (blue dashed lines) and upstream of a ridge (red dashed lines).
Divergence in the upper tropospheric airflow will enhance up-motion through the atmosphere, particularly when the atmosphere is characterised by low stability. As horizontal momentum near the surface would be transferred to vertical momentum rapidly by the topography of Wilsons Promontory, any enhancement of up-motion due to favourable upper flow would act to increase the vertical moisture profile. Model guidance indicated that the tropopause for Wilsons Promontory was near to or higher than 250 hPa during this event. There are two processes relevant to this event that were associated with the complex structure of the atmosphere near to or just below the tropopause. The first process is the right exit region of the slow moving upper Bulletin of the Australian Meteorological and Oceanographic Society Vol.26 page 97
It is likely that the jetstreak and the strongly negatively tilted trough combined to produce significant upper tropospheric divergence in the vicinity of Wilsons Promontory during the event. More evidence of the favourable nature of the upper-level flow is provided by the strong cyclogenesis that occurred over eastern Bass Strait during the event (discussed in more detail at the end of this section). Wilsons Promontory is the southernmost tip of the Australian mainland and juts significantly southwards into Bass Strait. The topography is steep with multiple peaks above 500 m, the highest of which is Mount Latrobe (754 m), 5.8 km to the east-north-east of Tidal River. There are a number of valleys which, from east to west, rise from sea
h
Figure 13: Google Earth image looking west over Wilsons Promontory. East / West orientated valleys are marked at the saddles with the main north-south running ridgeline. Boulder Saddle in the south (to the left of image), Windy Saddle and Sealers Creek / Tidal River Valley. More similarly orientated valleys are observable to the north of the Sealers Creek / Tidal River Valley. level quite steeply to become saddles through the centre of the Promontory then fall to sea level again (see Figure 13). The easterly wind flow into such topography will result in sharp lifting of the air. The orientation of the valleys also has potential to create convergence of the wind flow through them to enhance the topographical lifting. The enhanced up-motion created by the topographical lifting of the gale force easterly flow, the WAA and the favourable upper level flow in the region primed the environment for heavy rainfall. It is common for the upstream side of a ridge and the ridgetop to experience the greatest rainfall in topographicallyenhanced flow, with rainfall easing rapidly on the lee side due to subsidence. Given that Tidal River (on the lee side of the main north-south running ridge line of Wilsons Promontory, see Figure 13) recorded 377.4 mm during the event, it is highly likely that higher rainfall totals would have occurred close to ridge-top level. Landslips seem to have been more prominent on the southern faces of the east-west orientated valleys. Flooding occurred in catchments on both sides of the main north-south ridgeline but had particularly severe effects on the lee
side. The relatively modest rainfall recorded at Sealers Cove (east coast of Wilsons Promontory, see Figure 13) of 70 mm in a National Parks rain gauge, indicates the strong gradient in rainfall from east to west across the Promontory. There is also a strong north to south gradient in rainfall given that the Automatic Weather Station (AWS) at the lighthouse near the southern tip of Wilsons Promontory only recorded 39.4 mm for the event (see Figure 4). However, without ridge-top level rainfall observations it is difficult to be confident about where the region of maximum rainfall would have occurred. The complex topography of Wilsons Promontory would have been a strong factor in determining rainfall distribution. However, the question of why such extreme rainfall was observed at Tidal River on the lee side of the main north-south orientated ridge line is hard to answer. Funnelling and convergence of the easterly flow through the east-west orientated valleys could well be a contributor. Another possibility may be the formation and location of a standing wave. Standing waves can often form over topography when the flow is perpendicular to the main ridgeline with cloud and precipitation occurring within the standing wave. It is usual for such a wave pattern to
ridge
Figure 14: Illustration of the potential location of a standing wave in relation to ridge-top level to produce maximum rainfall on the lee side of the ridge. Bulletin of the Australian Meteorological and Oceanographic Society Vol.26 page 98
(a)
(c)
Barinsdale Radar
Line along RHI Wilsons Promontory Barinsdale Radar
(b) Wilsons Promontory
Figure 15: (a) Plan Position Indicator (PPI) (b) Range Height Indicator (RHI) and (c) Constant Altitude Plan Position Indicator (CAPPI) at 9000 m from Bairnsdale radar at 12:00 p.m. on 22 March. The CAPPI indicates reflectivities higher than 9000 m on the southern side of the trough in the easterly flow.
be orientated on the upstream side or at ridge level. It may
be the case that for Wilsons Promontory, a standing wave was shifted just to the lee side of the main north-south orientated ridgeline due to the very strong easterly flow, potentially allowing the region of heaviest rainfall to be just to the lee side of the ridge (Figure 14).
Smith (1979) lists a number of variables including the width (in the direction of flow) and height of a mountain range as being influential on the airflow over and around the mountain barrier. If the width and height are very large (for example the Great Dividing Range) then it is more likely that rainfall will be maximised on the upwind slope near the ridge top. If the width and height are very small then the airflow is less likely to be affected significantly by the obstacle. The width (~24 km) and height (~500 m) of Wilsons Promontory may well be favourable for rainfall to be advected to the lee side. Goldreich et al. (1996) studied a lee side rainfall maxima observed over Mount Carmel in Israel, which is smaller in width (~5 km) and similar in height (~500 m) to Wilsons Promontory. They conclude that the lee side rainfall maximum is likely due to wind flowing around the mountain and creating convergence on the lee side. They discuss advection of the rain by strong flow over the mountain as potentially having some influence. It is worth noting that a similar recent gale force easterly event (discussed in more detail in Section 7) had a twoday total to 9:00 a.m. on 4 June 2012 of 154 mm at Tidal River. Again the highest recorded rainfall totals were on Bulletin of the Australian Meteorological and Oceanographic Society Vol.26 page 99
the lee side of the main north-south orientated ridge with Sealer’s Cove only recording around 50–70 mm. It is likely that a combination of factors allowed the extreme rainfall to be observed at Tidal River on the lee side of Wilsons Promontory’s main north-south orientated ridge line. It would be useful to model flow over terrain with similar width and height to Wilsons Promontory using wind profiles similar to those which occurred during these events to better understand the rainfall distribution across the Promontory. The following sections refer to the three-hourly rainfall sub events indicated in Figure 5.
a. Sub Event 1: 22 March 12:00 p.m. to 3:00 p.m.— 92.6mm Radar imagery indicated that the moisture with the easterlies on the southern side of the trough was a lot deeper than that indicated by the Melbourne radiosonde flight. Radar reflectivities from the Bairnsdale radar were reaching 9500–10000 m in altitude to the east of Wilsons Promontory at midday (Figure 15). The coldest tops (brightest pixels) on the IR imagery for 11:00 a.m. on 22 March in Figure 7 (top right) also indicate that deep convective cells were present in the cloud band on the southern side of the trough. Figure 15 shows that the Bairnsdale radar is located more than 150 km, from Wilsons Promontory. At this distance the radar’s ability to accurately depict the reflectivities in
Figure 16: Design Rainfall Intensity Chart for Tidal River with black points indicating the DWS data. The 12- and 24-hour (1-day) durations are the most significant values above the 1 in 100 year (green upper) curve.
the region of Wilsons Promontory is greatly reduced due to factors such as beam spreading and attenuation (loss of energy due to scattering and absorption). Although the reflectivities observed in the region of Wilsons Promontory were significantly less than might be expected given the rainfall observed, they still provide useful information for the post analysis of the event. In the three hours to 3:00 p.m. on 22 March the Bairnsdale radar showed a continuous stream of radar reflectivities moving across Wilsons Promontory with embedded higher reflectivities indicating that persistent rain was likely mixed with periods of heavier rain as more enhanced convective cells moved through. This phenomenon is commonly described as a train effect (by BoM operational meteorologists) as the enhanced convective cells within the rain move across the same region one after the other. During this period, thunderstorm activity was developing to the west and north of the Latrobe Valley, likely due to convergence in the region of strong horizontal shear at the trough, as modelled by ACCESS-A (see Figure 9). There was also some shower activity that moved onto the coast just north of Seaspray at about 2:30 p.m. and was tracking towards Yarram.
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b. Sub Event 2: 22 March 5:00 p.m. to 8:00 p.m.— 106.4 mm Bairnsdale radar reflectivities indicate that shower and rain activity was moving generally from the north to north-east, to south to south-west. The shower tracking toward Yarram was moving anomalous to this general flow as it had a more westward trajectory. At approximately 3:00–3:30 p.m. this shower began to move towards the south to south-west as it crossed Yarram. At this time another shower about 15 km off the coast, south to southeast of Yarram, also began to move in a westward direction anomalous to the general flow. At 5:00 p.m. the preceding lighter rainfall at the DWS began to increase as some more enhanced showers moved in from the north to north-east. At 6:00 p.m. both of the showers described above reached Wilsons Promontory simultaneously. Then, for the next two hours, approximately 20–25 mm of rain was recorded every 30 minutes due to another train effect. During this time thunderstorm activity had moved across the Latrobe Valley. The Victorian State Emergency Service (SES) Situation Report issued on the morning of 23 March indicated that approximately 140 mm of rainfall was recorded in the region of Moe and Thorpdale. A significant number of houses were affected by flash flooding. Road closures and landslips also occurred due to the heavy rain. This activity moved southwards across the
Design Rainf all Frequency curv e f or a point located near Tidal Riv er (Victoria)
CRC-FORGE estimates Interpolation Curve EA/Bureau 2013 IFDs PMP 23 March 2011 event Upper and Low er Confidence Limits
Design Rainfall Depth (mm)
1000
800
600
400
200
0 100
101
102
103 104 105 106 107 Average Recurrence Interval (years)
108
109
Figure 17: Design Rainfall Frequency curves for a point near Tidal River. Upper and lower confidence limits using two orders of magnitude from the PMP AEP. The 377.4 mm line shown corresponds with an ARI of 25000 years. regions surrounding Mirboo North, Foster and Waratah Bay during the afternoon and evening with widespread rainfall in the order of 100–150 mm (see Figure 4).
c. Sub Event 3: 22 March 10:00 p.m. to 23 March 1:00 a.m.100.7mm As the storms that originated near the Latrobe Valley moved southwards across Waratah Bay, the Bairnsdale radar indicated rainfall re-intensifying across Wilsons Promontory at 10:30 p.m. The shear line that triggered the initial thunderstorm activity near the Latrobe Valley was moving southwards and weakening. By this time shower and thunderstorm activity had extended eastwards along the trough line and the remnants of this were steered towards the south-west in the direction of Wilsons Promontory. Up to 10:00 p.m. the DWS data indicated east to northeast winds in the range of 15 to 25 km hr-1. The 10:30 p.m. DWS data showed the wind direction changing to westerly below 5 km hr-1. The weakening shear line was approaching the vicinity at this time and thunderstorms were over Waratah Bay. It is likely that the DWS wind change was associated with outflow from thunderstorm activity across Waratah Bay helping to strengthen the shear line in the area. At the same time the automatic weather station (AWS) at the lighthouse indicated east to northeast winds of 58 km hr-1. Convergence between the light westerly flow at the DWS and the east to northeasterly winds would have likely contributed to the development of enhanced shower and rain activity over the western side of Wilsons Promontory. Enhanced reflectivities in this region observed by the Bairnsdale radar imagery supported this hypothesis. Rainfall of 13–28 mm was observed every half an hour by the DWS from 10:30 p.m. until midnight. Rainfall recorded by the DWS eased after this period as radar imagery indicated the focus shifted to southern parts of Bulletin of the Australian Meteorological and Oceanographic Society Vol.26 page 101
the Promontory, however steady falls of 5–10 mm were recorded up to 2:00 a.m. by the DWS. The easing of the rain was likely due to the southward movement of the low level east to northeasterly jet as the trough moved southwards. Automatic weather station observations indicate that a low had developed just east of Orbost by 5:00 p.m. Strong cyclogenesis of the low is identifiable via the Bairnsdale radar imagery as the low moved southwestwards to be approximately 30 km off the coast south of Lakes Entrance at 9:30 p.m., 50 km off the coast from Seaspray at midnight then passing over Deal Island at 3:30 a.m. The Bairnsdale radar indicated that there was enhanced rainfall spiralling around the western and southern quadrants of the low as it moved to the south-west. The deep convection associated with this rainfall is identifiable from the IR imagery in Figure 7c and 7d with very cold tops in a cloud band spiralling around the low as it moved to the southwest. This more intense rain stayed offshore from Wilsons Promontory. Between midnight and 4:00 a.m., 8 mm of rainfall was recorded by the lighthouse AWS.
6. Average Recurrence Interval of the event For planning purposes it is useful for National Parks to have an idea of how likely the recurrence of an event of this nature might be. The method for estimating design rainfalls for Australia (Institution of Engineers Australia, 1987) has recently (June 2013) been updated by Engineers Australia and the BoM2. The design rainfall is the rainfall depth for a particular duration and probability of occurrence that is used in the design of hydraulic structures. The highest rainfall depths for durations from 0.5 to 24 hours were extracted from the DWS data and plotted on the design rainfall Intensity-Frequency-Duration (IFD) chart (Figure 16). The plotted points for the 12- and 24hour durations lie well above the 1% annual exceedance 2
www.bom.gov.au/water/designRainfalls/revised-ifd
probability (AEP), which corresponds to an average recurrence interval (ARI) of 100 years. As can be seen in Table 1, both the 12- and 24-hour duration rainfall amounts are around 2.5 times the 1 in 100 year rainfall amount. It was decided to focus on the 24-hour duration ARI for this event. The maximum 24-hour rainfall recorded by the DWS was 375.4 mm and occurred in the 24 hours up to 2:30 a.m. on 23 March. Note that the daily rainfall in the 24-hours to 9:00 a.m. on 23 March was 360.2 mm, somewhat less than the maximum 24-hour rainfall for the event required for ARI calculations. To estimate design rainfall to lower probabilities, to 1 in 2000 years, the Co-operative Research Centre for Catchment Hydrology Focussed Rainfall Growth Estimation (CRC FORGE) technique (see Nandakumar et al., 1997) has been developed. The method is usually used in engineering design of hydraulic structures. The method substitutes space for time to estimate design rainfall to such low probabilities from much shorter rainfall records (at the most approximately 100 years). The 24-hour CRC FORGE point rainfall estimates for Tidal River, give 233 and 238 mm for the 1 in 1000 and 1 in 2000 year AEPs respectively, which are more than 100 mm less than the 375.4 mm recorded. Therefore the ARI for this event exceeds 2000 years. Application of generalised methods of Probable Maximum Precipitation (PMP, World Meteorological Organization, 2009) provides design rainfall at the extreme low end of AEP of around 10-6. It represents a physical upper limit of the rainfall depth that could fall at a particular location over an area. The PMP for a 1 km2 area at Tidal River is 950 mm for a duration of 24 hours using the Generalised Southeast Australia Method (GSAM) (see Minty et al., 1996). An estimate of rainfall of such low probability is itself subject to error. Estimating the AEP of the PMP is even more problematic. Guidance provided by Laurenson and Kuczera (1999) suggest an AEP of 10-7 for areas less than 100 km2 in area with notional upper and lower limits of two orders of magnitude. This gives an AEP for the PMP of somewhere between 10-5 and 10-9 for the Tidal River event. The point design rainfall values from the 2013 IFDs and the CRC FORGE estimates are plotted in Figure 17. Also plotted is the estimate of PMP for a 1 km2 area, which is the closest available to a point estimate. This has been assigned an AEP of 10-7 following guidance from Laurenson and Kuczera (1999). The difference between point rainfall and an areal 1 km2 rainfall is small enough to be negligible when considering the errors involved in calculating the PMP and the AEP. Siriwardena and Weinmann (1996) have published areal reduction factors for a range of AEPs that convert a point rainfall to an area. They suggest an implied areal reduction factor of 1.0 for a 1 km2 area over a duration of 24 hours, therefore the 1 km2 PMP is also valid for a point. An interpolation procedure described in Siriwardena and Weinmann (1998) is a method adopted in hydrological Bulletin of the Australian Meteorological and Oceanographic Society Vol.26 page 102
engineering to interpolate between CRC FORGE estimates and the PMP. The method has been applied using the CRC FORGE and PMP estimates for a location near Tidal River over 24 hours. The interpolation curve gives an estimated ARI of 25 000 years for this rainfall event (Figure 17). Table 1: Rainfall amounts from the DWS data for different durations compared with the 1 in 100 year rainfall amounts as calculated by the updated design rainfall estimate technique. Rainfall amounts for differing durations at Tidal River Duration (hrs)
0.5
1.0
3.0
6.0
12.0
24.0
Rain Depth (mm)
30.2
50.8
107.0
173.6
307.8
375.4
IDF 100 years (mm)
32.7
45.7
73.4
94.9
119.8
149.5
Event % of 100 year ARI
92.4
111.2
145.8
182.9
256.9
251.1
Upper and lower confidence intervals have been drawn assuming limits of two orders of magnitude for the AEP of the PMP and 20% for the 100 year ARI estimate. The upper and lower limits for the AEP of the PMP are notional limits from Laurenson and Kuczera (1999). Confidence limits for the Engineers Australia / BoM 2013 IFDs are not available yet. The techniques for estimating ARIs of events with return periods greater than 100 years become more uncertain with increasing return periods. This is reflected in the very large confidence limits of the PMP. The methods used for estimating the return periods are current best practice used in hydrology. Although the estimate of the ARI is quantitative, there are large errors associated with these estimates and these are reflected in the confidence limits, which become much wider for larger ARI. Using these techniques it can be seen from Figure 17 that the ARI of the March 2011 event could range from 3000 to 200,000 years. As the AEP is the probability of on event of equal magnitude or greater occurring in a year, it carries an urgency that the ARI lacks. What may or may not happen in 25,000 years from now is of little concern to most people. The probability of a repeat event next year is more likely to get attention. Although the AEP for this event seems very low, it is worth noting that it is three-times more likely than the chance of a First Division win in Tattslotto (for two games a week for a year, the First Division win probability is 1 in 81,450.63). During the design of infrastructure in locations such as Wilsons Promontory it is well worth considering that the AEP may be as high as 1 in 3000 for this event.
7. Recent events with some similar characteristics and comparison with previous records The extreme rainfall of the 22–23 March 2011 occurred as the deep moist easterly flow on the southern flank of a broad trough with embedded meso-scale low pressure system was incident on Wilsons Promontory. The strength 3
en.Wikipedia.org/wiki/Lotteries_in_Australia
Figure 18: MSLP Analysis for 5:00 p.m. on 12 January 2011.
Figure 19: MSLP Analysis for 11:00 p.m. on 23 March 2012.
of this easterly flow on Wilsons Promontory’s steep terrain, combined with favourable upper level divergence, due to a combination of the negatively tilted upper trough and the right exit region of a 200 hPa jet streak, created a highly favourable environment for heavy rainfall. The general slow movement of this pattern allowed three significant rainfall sub-events to occur in a 12–13 hour period between 12:00 p.m. 22 March to 1:30 a.m. 23 March. Two of these events were associated with train effect motion of deep convective cells over Wilsons Promontory, the third event was likely associated with enhanced convergence associated with thunderstorm outflow.
Falmouth recorded 282 mm and Scamander 278 mm on the northeast coast of Tasmania in the 24 hours to 9:00 a.m. on 13 January. Both of these stations broke Tasmania’s previous January daily rainfall record of 247 mm set at the Springs on Mount Wellington in 1917 (see Special Climate Statement 25, 2011). Unofficial but credible reports of up to 400 mm were recorded in two days in the region (Special Climate Statement 25, 2011).
The following section explores some recent synoptic configurations with similar characteristics to the 22–23 March 2011 event. Intense rainfall occurred with all of these events; three of the four events occurred during the strong La Niña event that started in 2010 and finished early in 2012.
b. 24 hours to 9:00 a.m., 24 March 2012
Figure 18 indicates that this rainfall was associated with a cut-off low in easterly flow advecting moisture laden air to the northeast of Tasmania. The moist easterly air mass that affected Wilsons Promontory on 22–23 March also impacted northern and eastern Tasmania, particularly as the main trough line moved southwards with strong easterly flow impacting the Tasmanian north-east coast in the 24 hours to 9:00 a.m. on 24 March.
For comparison, the MSLP patterns for Victoria’s two highest daily rainfall events are also included.
Gray, near the northeast coast, recorded 327.2 mm with widespread totals over the north-east in the order of 150– 200 mm. Two-day totals up to 9:00 a.m. on 24 March of 452.4 mm at Gray, just short of the Tasmanian all time two-day record of 467.8 mm at Mathinna in April 1929 (Special Climate Statement 30, 2011).
a. 12–13 January 2011 The Melbourne Airport atmospheric sounding at 9:00 p.m. on 13 January indicated a record precipitable water value of 65.2 mm, approximately 300% higher than the monthly average precipitable water (the depth of water in a 1 m2 column of the atmosphere that would be achieved if all the water were to precipitate as rain).
Figure 20: MSLP Analysis for 5:00 p.m. on 10 February 2012.
Bulletin of the Australian Meteorological and Oceanographic Society Vol.26 page 103
Figure 19 shows that this rainfall was again associated with a broadscale cut-off low in easterly flow advecting moisture-laden air over the northeast of Tasmania.
Figure 21: MSLP Analysis for 10:00 p.m. AEST (UTC+9) on 3 June 2012.
(a)
(b)
Figure 22: MSLP analysis for 11:00 a.m. on (a) 21 March 1983 and (b) 22 March 1983. to the March 2011 event, significantly less rainfall (in the c. 10–11 February 2012 order of 50–70 mm) was recorded on the east coast of Pennyroyal Creek on the western side of the Otway Wilsons Promontory. This provides another example of Ranges received 223.0 mm of rain, mostly between 8:30 very high rainfall occurring in gale force easterly flow on p.m. to 11:30 p.m. on 9 February, associated with severe the lee side of the main north-south orientated ridge at thunderstorm activity. The station record is 130 years Wilsons Promontory. long but in one night the monthly, long-term record for Overnight on 4 June the focus of this rainfall shifted to February was broken. central and eastern Gippsland as a deep low developed off The following night a mesoscale low, that followed a the south-east coast of NSW. Every major river system in similar path to the mesoscale low described in Section central and eastern Gippsland went to a major flood alert 5c, moved from eastern Bass Strait, just south of Wilsons as widespread daily rainfall totals of 100-150 mm were Promontory then west towards Port Phillip by late on 11 recorded in the region. February. Bairnsdale radar reflectivities indicated that the Figure 21 shows that the rainfall and wind were associated heaviest rainfall fell mainly over Bass Strait, but as the rain with mesoscale low pressure systems embedded in a broad band associated with the low briefly clipped the southern dip in the easterly flow. tip of Wilsons Promontory, the AWS at the lighthouse recorded approximately 36.4 mm of rain, most falling in e. Previous Victorian rainfall records the hour leading up to 5:00 a.m.. Victoria’s official highest daily rainfall of 375.0 mm was Figure 20 indicates that the general situation was a dip in recorded at Tanybryn in the Otway Ranges on 22 March the easterly flow bringing moisture-laden air across Bass 1983. Figure 22 shows that this rain was associated with a Strait. cut-off low in easterly flow that moved southwards from NSW to western Victoria during 21–22 March. The low d. 3–5 June 2012 bought widespread rainfall to eastern Australia, relieving Another gale force easterly event on 3–4 June 2012 resulted severe drought conditions. in a two-day total (to 9:00 a.m. on 4 June) of 154 mm at -1 The next highest official daily rainfall total for Victoria Tidal River. Wind gusts to 96 km hr were recorded at the is 318.6 mm recorded at Mount Wellington in the Great lighthouse AWS. Significant asset damage associated with Dividing Range in central Gippsland on 28 June 2007. the wind was reported at Wilsons Promontory. Similar
Figure 23: MSLP analysis for 10:00 p.m. on 27 June 2007.
Bulletin of the Australian Meteorological and Oceanographic Society Vol.26 page 104
Figure 23 shows that this rain was associated with a cut-off low in easterly flow located off the NSW coast. Widespread major flooding occurred through Gippsland associated with this low. The similarity of all these events occurring within broadscale troughs in easterly flow, most of which have an associated cut-off low pressure system, indicates the significance of such a synoptic configuration for very heavy rainfall over southeastern Australia. Further analysis of these events is required to determine what amount of upper divergence may have been present for each event; however, topographical enhancement of rainfall is likely to be common amongst all the events.
8. Discussion Rainfall variability across eastern Australia is heavily controlled by sea-surface temperature variability in the Australian region. Typically, rainfall is influenced by ENSO conditions in the tropical Pacific and the Indian Ocean Dipole (IOD, Risbey et al., 2009). For the south-east, the phase relationship between ENSO and the IOD is an important factor, with La Niña combined with a negative IOD being the most favourable for rainfall over the region (Ummenhofer et al., 2011; Risbey et al., 2009). Two aspects of tropical SST forcing are relevant to the magnitude of rainfall changes over Australia. The first is the influence of elevated local SSTs on available precipitable water vapour. The other is the warmer than average local tropical waters to the north and east of the continent with which La Niña events tend to be associated. Similarly, the negative phase of the IOD is characterised by warmer than average water off the north-west of Western Australia. These conditions are typically associated with SST gradients across both ocean basins providing favourable circulation for increased rainfall over eastern Australia. While future circulation changes due to global warming are highly uncertain in the Australian region, a consistent result from climate modelling studies over the last two decades has been a global intensification of the hydrological cycle of the order of 2–4% (IPCC, 2007), and possibly higher in recent observational studies (Durack et al., 2012) due to elevated atmospheric and ocean temperatures. An intensification of the hydrological cycle is associated with increased rainfall intensities globally. Australian region SSTs have risen by around 0.9°C since 1900, broadly consistent with global SST trends (Fawcett 2012). As such, the typically higher than average SSTs in coastal waters experienced during La Niña events historically have been warmer in recent decades and tended to record-breaking during 2010 and 2011. Figure 8 indicates that significantly warmer than average SSTs surrounded the Australian continent during the 12 months to June 2011, satisfying both aspects (local increased SSTs and local tropical increased SSTs) relevant for increased rainfall for the Bass Strait region. The number of very heavy rainfall events in the region of Bass Strait during this period is notable. It is also notable that the synoptic type for nearly all the heavy rainfall events discussed in this pattern has been cut-off lows in easterly flow. Bulletin of the Australian Meteorological and Oceanographic Society Vol.26 page 105
The rainfall recorded by the DWS at Tidal River was extraordinary in that there were three periods of three hours where close to 100 mm of rain was observed. Any one of these three hourly events is noteworthy, but to have three short period heavy rainfall events, each with somewhat different characteristics and at the same location is remarkable. The ARI of such an extreme event is of high importance for public safety and engineering of infrastructure for regions such as Wilsons Promontory. It is unlikely that rainfall of this magnitude will be recorded at the same location in the near future; however, how low this chance may be is worth questioning. The ARI of this event may range from 3000 to 200,000 years. If the recent studies indicating that the hydrological cycle will intensify as atmospheric and ocean temperatures continue to rise (leading to increased rainfall intensities) prove to be valid, then in the future, lower estimations of ARIs may be necessary. Taking into account the lower estimate of ARI for the Tidal River event and the trend towards higher SSTs in the future, it may be prudent for those engaged in engineering of infrastructure in the region to be more conservative in design.
Acknowledgements Many people have given their time to contribute to this paper. Thanks to Brett Mitchell and Jim Whelan of Parks Victoria for helping with post event damage surveys and providing any further information requested. Thanks to Geoff Feren for his insight into the upper level meteorology and to Kevin Parkyn for his feedback on the general meteorology surrounding the event. Thanks to Dr Robert Fawcett for providing the AWAP analysis of the 24-hour rainfall over South Gippsland. Thanks to Dr Karl Braganza for his feedback, input and advice on the potential impacts of climate change for events such as this. Thanks to Ivor Blockley, Oliver Lemmel and Tony Bannister for helping with data retrieval. Thanks also to the University of West Madison Space Science and Engineering Centre (SSEC) and the Cooperative Institute for Meteorological Satellite Studies for providing highly useful imagery for the upper analysis of this event. Thanks also to Google Earth (image 25/8/2011 from 2012 GeoEye) for Figures 3 and 13.
References Drosdowsky, W., and Chambers, L.E., 2001, Near-global sea surface temperature anomalies as predictors of Australian seasonal rainfall, Journal of Climate, 14, 1677– 1687. Durack, P.J., Wijffels, S.E., and Matear, R.J., 2012, Ocean salinities reveal strong global water cycle intensification during 1950 to 2000. Science, 336 (6080), 455–458. DOI: 10.1126/science.1212222 Goldreich,Y., Freundlich, A., and Pinhas A., 1996, Rainfall anomaly over the lee side of Mount Carmel (Israel) and the associated wind field, Journal of Applied Meteorology, 36, 748–762
Institution of Engineers Australia, 1987, Australian rainfall and runoff. A guide to flood estimation, Volume 1, Book II – Design Rainfall Considerations, Reprinted Edition, 1998.
Siriwardena, L., Weinmann, P.E., 1998, A technique to interpolate frequency curves between frequent events and probable maximum events. Cooperative Research Centre for Catchment Hydrology, Report 98/9.
IPCC, 2007, Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
Siriwardena, L., and Weinmann, P.E., 1996, Derivation of areal reduction factors for rainfalls in Victoria for rainfall durations 18–120 hours. Cooperative Research Centre for Catchment Hydrology, Report 96/4.
Laurensen, E.M. and Kuczera, G., 1999, Annual exceedence probability of probable maximum precipitation, Australian Journal of Water Resources, 3(2), 189–198. Nandakumar, N, Weinmann, P.E., Mein R.G., Nathan R.J., 1997, Estimation of extreme rainfalls for Victoria using the CRC-FORGE method (for rainfall durations 24 to 72 Hours), Co-operative Research Centre for Catchment Hydrology, Report 97/4. Minty, L.J., Meighen, J., and Kennedy, M.R., 1996, Development of the generalised Southeast Australia method for estimating probable maximum precipitation, Hydrometeorological Advisory Service, Bureau of Meteorology, HRS Report No. 4. (www.bom.gov.au/water/ designRainfalls/pmp/gsam.shtml). Power, S., Haylock, M.,Colman R., and Wang, X., 2006, The predictability of interdecadal changes in ENSO and ENSO teleconnections, Journal of Climate, 8, 2161–2180. Risbey, J., Pook, M., McIntosh, P.,Wheeler, M. and Hendon, H., 2009, On the remote drivers of rainfall variability in Australia. Monthly Weather Review, 137 (10), 3233–3253. Fawcett R.J.B., Trewin B.C., Braganza, K., Smalley, R.J., Jovanovic B. and Jones, D.A., 2012, The sensitivity of Australian temperature trends and variability to analysis methods and observation networks. CACWR technical Report, 50.
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Smith, R. B., 1979, The influence of a mountain on the atmosphere, Advances in Geophysics, 21, 87–230. Special Climate Statement 25, 2011, Exceptional January rainfall in northern Tasmania, Australian Bureau of Meteorology publication. www.bom.gov.au/climate/ current/statements/scs25.pdf Special Climate Statement 30, 2011, Heavy rainfall and flooding in northeast Tasmania, Australian Bureau of Meteorology publication. http://www.bom.gov.au/ climate/current/statements/scs30.pdf Special Climate Statement 38, 2012, Australia’s wettest two year period on record; 2010-2011, Australian Bureau of Meteorology publication. www.bom.gov.au/climate/ current/statements/scs38.pdf Ummenhofer, C.C., England, M.H., McIntosh, P.C., Meyers, G.A., Pook. M.J., Risbey, J.S. Sen Gupta, A. and Taschetto, A.S., 2009, What causes Southeast Australia’s worst droughts? Geophysical Research Letters, 36, L04706, DOI:10.1029/2008GL036801. Ummenhofer C.C., Sen Gupta, A., Briggs, P.R., England, M.H., McIntosh, P.C., Meyers, G.A., Pook, M.J., Raupach, M.R., and Risbey, J.S., 2011, Indian and Pacific Ocean influences on Southeast Australian drought and soil moisture, Journal of Climate, 24, 1313–1336. World Meteorological Organization, 2009, Manual on ecipitation (PMP), WMO–No. 1045, Geneva.
Snapshot
Summer storm in Gothenburg, Sweden 13 August 2013 Etienne Rebuffet
This photogenic thunderstorm was captured at approximately 10:00 p.m. (local time), looking in a northwest direction from Gothenburg. The storm persisted for some time, which allowed this stunning picture of sheetlightning (inside the cloud) and also cloud-to-ground (CG) strikes. As can be seen, the CG in the right side of the image emanated from the anvil and connected with the surface outside of the storm—also known as a “bolt from the blue”.
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The storm also appears to be tilted from the low-to-upper levels (right to left in the image) and therefore the wind shear may have acted to separate the up and downdrafts. This effect may have been the reason for its longevity. If you have an image of the weather near you to share, send it to duncan.ackerley@monash.edu, or post it on the AMOS Facebook page. — Ed
Charts from the Past with Blair Trewin
26 January 1971 The La Niña summer of 1970–1971, the first of several in the early and middle 1970s, was a very wet one through many parts of eastern Australia. The November-February period was the wettest on record in a number of regions, including much of south-east Queensland and north-east New South Wales, the far south-east of New South Wales, and much of Gippsland. There were many floods during the course of the summer, both on inland and coastal rivers. (One casualty of the wet weather was the scheduled New Year Test in Melbourne, abandoned without a ball bowled). A blocking high persisted in the Tasman for two weeks in late January and early February. In the first few days of this pattern, there was a slow-moving trough over eastern Australia. Whilst this pattern did not result in widespread general rain, there were outbreaks of scattered thunderstorms on several days. The first major outbreak occurred on the afternoon of the 24th in the Melbourne area, with flash flooding in many suburbs, especially in the north-east where 55 mm was recorded in an hour at Watsonia. Later, 125 mm in an hour was reported on the 26th near Barraba in northern New South Wales, while the following day, 3–4 cm hail was reported in the Hobart area with a tornado at Risdon Vale. By far the most significant event of this period, though, occurred in Canberra on the evening of the 26th. After a hot, humid day (with maximum temperatures reaching 32.2°C in Canberra, and the low 40s at a number of sites in western NSW), thunderstorms developed southeast of Canberra and moved slowly northwest. Initially, the most severe impacts were southeast of the city (at Burra, 110 mm of rain fell in 90 minutes with 3–4 cm hail, and
a funnel cloud was reported at Mount Rob Roy), but by 7.30 p.m. the major storm cell had become near-stationary over the southern part of the, then newly-developed, Woden Valley. Over the next hour, estimates from private gauges indicated falls of 90–100 mm in Farrer and Torrens (the highest official reading was 79 mm, in Mawson; the airport, which largely missed the storm, had only 14 mm). This rain resulted in extreme flash flooding in the Yarralumla Creek, which drains the valley. A kilometrelong section of Yarra Glen, the main road north to the central city, was inundated, catching many vehicles in the rising waters. Seven lives were lost in total, five of them in one car which was swept downstream. In terms of casualties it was one of the worst floods in Australia in the last 50 years. It is likely that the flood was exacerbated by the storm’s direction of movement (meaning that the heaviest local rain in the lower catchment coincided with the arrival of peaks from upstream), and possibly also by the lack of vegetation in suburbs still under construction. Flooding of a different sort was occurring on the other side of the country, where Cyclone Rita touched the coast before moving offshore again. Glen Florrie, inland from Onslow, had 407 mm for 24 hours ending on the 26th, while Learmonth’s 227 mm was its second-highest daily total on record. The blocking pattern went on to produce two more major flooding episodes before it finally broke down—one on 30–31 January in central Gippsland, one from 5 February in the Snowy catchment and on the NSW South Coast (covered in the April 2007 BAMOS).
Synoptic chart for 0000 UTC (1000 AEST), 26 January 1971
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The Research Corner with Damien Irving
Picking the right programming language One of the really nice things about data analysis in the weather and climate sciences is that our profession has pretty much universally settled on Network Common Data Form (netCDF) as the file format for storing our data. As such, a number of command line utilities known as the NetCDF Operators (NCO) and Climate Data Operators (CDO) have been developed for performing common tasks on these files. The former focusses on simple data curation (e.g. viewing the contents of a file, selecting a subset of the data or editing the metadata within a file), while the latter provides for simple statistical analysis (e.g. calculating a climatology, percentile, correlation or heat wave index). Of course, all these tasks could be achieved by writing your own code, but it’s much quicker to use these command line utilities. With freely available utilities like NCO and CDO out there, it’s tempting to think you might be able to avoid ever having to write your own code. While this would be great, the unfortunate reality is that most of us do some pretty serious data processing from time to time, which goes far beyond the limits of what NCO and CDO can do. In a typical weather and climate science institution, people will use one or more of Fortran, C, Matlab, Python, NCAR Command Language (NCL), IDL, Ferret or R to do this. This is an intimidating list for the uninitiated, so people often find it difficult to decide which language to use. In fact, even when people have settled on an option, they often spend a lot of time wondering whether they should switch to a different one. Hopefully, the following summary will assist with these tough decisions.
A brief summary of the options… Fortran and C are known as “type-safe”, “compiled”, “lowlevel” or “system programming” languages. Code written in these languages isn’t particularly concise, intuitive or programmer friendly. The pay-off is that it’s easier for the computer to interpret, meaning it runs really quickly. Matlab and IDL are proprietary software, meaning that you or your employer must purchase an annual license in order to use them. They are complete scientific computing environments that consist of not only their own “dynamically typed”, “interpreted”, “high-level” or “scripting” programming language, but also built in functions for data analysis and visualisation, as well as fancy graphical user interfaces for viewing your data while you analyse it. High-level languages are popular because the resulting code is much shorter and more intuitive, which reduces the time required to write it. However, there’s a price to pay for writing concise, easy to understand code—it runs slower than low-level code. R, Ferret and NCL are somewhat similar to Matlab and IDL, in that they are complete scientific computing environments with their own high-level language and lots of built-in functions. The associated graphical user Bulletin of the Australian Meteorological and Oceanographic Society Vol.26 page 109
interface isn’t as fancy (or is simply absent) and the documentation isn’t as good, but to some extent that’s because you get what you pay for (i.e. they’re free). The greatest strength of NCL is that it has lots of weather and climate specific functions and creates attractive images, while R has the most extensive library of statistical functions. Ferret has many functions that are useful for analysing oceanographic data, and is particularly popular with that community. The major problem with the environment-specific coding languages that come with Matlab, IDL, NCL, Ferret and R is that they are (to varying degrees) fairly simple and primitive. This means they lack some of the general functionality associated with non-environment-specific, “fully-fledged” high-level languages like Python. It’s also difficult (or impossible) to link them with code written in other languages, whereas Python has lots of tools for this. This flexibility that Python offers would theoretically allow you to build your own Matlab-like scientific computing environment, however in practice such a task would be an absolute nightmare. It’s therefore very fortunate that the Department of Energy has already done this for the weather and climate science community. In producing Ultrascale Visualization-Climate Data Analysis Tools (UV-CDAT), they’ve basically bundled together all the useful data analysis packages written in Python—including many that they’ve written themselves—into a complete weather and climate science computing environment.
How to pick one… Computers are so fast these days that unless you’re writing code that is many thousands of lines long (e.g. like a global climate model, which are typically written in Fortran), the biggest bottleneck in your data processing will be the speed at which you can write your code, not the speed at which it can be executed. This is why scientific computing languages / environments like Matlab, IDL, R, NCL, Ferret and UV-CDAT have become so popular—high-level code is much faster to write, especially when you have tools available to view your data as you go. You should therefore weigh up the pros and cons of these six environments and simply pick one (i.e. consider the license fees, requirements of your work, what your colleagues use, etc.). I personally think that UV-CDAT is the best choice, but the truth of the matter is that you can find highly effective weather and climate scientists using any one of these six options. The key is that once you’ve picked one, you need to commit to learning it really well. In my experience it’s useful to become highly proficient in one language, so that you can make efficient use of that language for the majority of your work. You can then pick up the basics of other languages on an “as-needs” basis, for those occasional tasks that can’t be achieved with your language of choice.
Calendar
2013
25–30 1st International Summit on Tornadoes and Climate Change, Chania, Crete, Greece.
December
June
9–13 AGU Fall Meeting, San Francisco, USA.
9–13 21st Symposium on Boundary Layers and Turbulence, Leeds, UK.
2014 February 2–6 94 AMS Annual Meeting, Atlanta, USA. th
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March 24–28 9th International Conference on Air Quality— Science and Application, Garmisch-Partenkirchen, Germany.
14–19 15th International Conference on Atmospheric Electricity (ICAE 2014), Norman, OK, USA. 16–20 21st Conference on Applied Climatology, Boulder, CO, USA. 16–20 17th Symposium on Meteorological Observation and Instrumentation, Boulder, CO, USA. 17–20 42nd Conference on Broadcast Meteorology, Olympic Valley, CA, USA.
July 7–11 14th Conference on Cloud Physics, Westin Copley Place, Boston, MA, USA.
31–4 April 31st AMS Conference on Hurricanes and Tropical Meteorology, San Diego, California, USA.
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May
2015 January
12–15 2 Conference on Atmospheric Biogeosciences, Portland, OR, USA. nd
12–15 31st Conference on Agricultural and Forest Meteorology, Portland, OR, USA.
4–8 95th AMS Annual Meeting, Phoenix, USA.
June 22–2 July 26th General Assembly of the International Union of Geodesy and Geophysics, Prague, Czech Republic.
Australian Meteorological and Oceanographic Journal
Articles — Vol. 63 No. 2, June 2013 Puri et al. Implementation of the initial ACCESS numerical weather prediction system. Jones et al. An updated analysis of homogeneous temperature data at Pacific Island stations. Davis Towards the development of long-term winter records for the Snowy Mountains. Johnson et al. An evaluation of the precipitation forecasts of the Poor Man’s Ensemble for wintertime rainfall across the southern portion of Australia.
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Li et al. Rainfall distribution of five landfalling tropical cyclones in the northwestern Australian region. Regular features: Pepler. Seasonal climate summary Southern Hemisphere (winter 2012): dry conditions return to Australia. Wu. Quarterly numerical weather prediction model performance summary—January to March 2013.
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Trewin, B., 2001, Extreme temperature events in Australia. Ph.D. Thesis, School of Earth Sciences, University of Melbourne, Australia.
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2013 Bulletin of the Australian Meteorological and Oceanographic Society ISSN 1035-6576
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