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Aerosol Particle Redistribution in a Deep Convective Cloud Environment Gerard Devine School of Earth and Environment The University of Leeds September 6, 2005


0.1

Introduction

Aerosol particles in the atmosphere impact on a multitude of earth systems. Amongst these, they contribute to the radiative balance of the planet by directly absorbing and scattering incoming solar radiation, and indirectly through their ability to modify cloud albedo and lifetime. Aerosols also impact on tropospheric chemistry by providing sites for a range of heterogeneous reactions to occur, and they affect the dynamics of the troposphere by modifying surface temperatures and associated atmospheric stability. Uncertainties in the magnitude of the above impacts exist due to the complexity and large variability of the temporal and spatial patterns of the aerosol particles. One factor that leads to such complexity is the behaviour of aerosol particles in convective cloud environments. Using the Met Office Model Large Eddy Model (LEM), the mechanisms by which such cloud environments control and dictate aerosol populations in their vicinity is being investigated.

0.2

Convective Cloud Environment Simulation

The convective environment used in this study is that from the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA-COARE) campaign. The LEM, used in the context of a Cloud-resolving Model (CRM), is used to simulate a 6-day period (20-26 December) from this campaign. The same period has been used by the GEWEX Cloud System Study (GCSS) Working Group 4 as part of their CRM intercomparison projects. Observations averaged over the entire Intensive Flux Array(500 by 500km approx.) are used to initialise and force the model. The model is run in 2D with a horizontal domain of 256km at 1km resolution, and a variable vertical resolution stretching to 20km. Several episodes of deep convection exist within this period and include the periods 30 to 40 hours, 60 to 75 hours, and 100 to 125 hours, where both single cloud events and larger systems are evident. Figures 1 and 2 show Hovmoller plots of horizontally averaged cloud field (y-t) and surface rain rate (x-t) for the entire 6-day period, respectively. Horizontally-Averaged Cloud Mixing Ratio (LQ,Rain,Ice,Snow,Graupel) (g/kg) 20

altitude(km)

0.62

10

0.47

(g/kg)

0.94 0.78

15

0.31 0.16 0.00

5 0 0

20

40

60

80

100

120

140

Time Hours

Figure 1: Hovmoller diagram(y-t) of horizontally averaged total cloud (g/kg) during the 6-day TOGACOARE simuation.

0.3

Tracer Studies

The goal of the overall study is to examine the behaviour of an aerosol population in a deep convective cloud environment. To aid with this, a series of tracer studies was carried out in order to gain a better understanding of some of the individual processes that contribute to the overall convection-aerosol system.

0.3.1

Passive Tracers

A series of tracers was implemented into the TOGA COARE run in order to examine how the altitude at which the tracers are implemented affects the degree to which they are transported vertically, and i


Surface Rain Rate 140

120

73.13 100 60.94

Time

36.56

mm/hr

48.75

80

24.38

60

12.19 0.00

40

20

0 -100

-50 0 50 Horizontal Distance (km)

100

Figure 2: Hovmoller diagram(x-t) for the CRM-modelled surface rain rate (mm/hr) during the 6-day TOGA-COARE simulation. subsequently how efficiently they are mixed throughout the domain. Five horizontally and vertically homogenous tracers, referred to as tracers A, B, C, D, and E, were initialised at heights 0 to 2km, 4 to 6km, 8 to 10 km, 12 to 14km, and 16 to 18kms, respectively. Although simple in nature, these studies can give insight into the fate of a passive material with sources at different altitudes within the atmosphere, and can mimic, for example, material emitted from the earth’s surface, aerosol layers advected into the convective domain at mid-tropospheric levels, or material previously tranported by convection to the upper troposphere (UT). The evolution of the five tracers is shown in Hovmoller plots (y-t) and vertical profiles given in Figures 3 to 7. Horizontal mean of Tracer A 20 Time=0 hrs Time=20 hrs Time=40 hrs Time=60 hrs Time=80 hrs Time=100 hrs Time=120 hrs Time=140 hrs 15

1.0

15

altitude(km)

Horizontally-Averaged Tracer A 20

10

altitude(km)

0.8 0.7

10

0.5 0.3

5

0.2

5

0 0

0.0

20

40

60

80 Time Hours

100

120

0 0.0

140

0.2

0.4

0.6 Tracer A

0.8

1.0

1.2

Figure 3: (a) Hovmoller diagram(y-t) of horizontally averaged Tracer A (normalised to initial maximum concentration) (b) Profiles of horizontally averaged Tracer A at 0, 20, 40, 60, 80, 100, 120, 140 hrs(normalised to initial maximum concentration)

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Horizontal average of Tracer B 20 Time=0 hrs Time=20 hrs Time=40 hrs Time=60 hrs Time=80 hrs Time=100 hrs Time=120 hrs Time=140 hrs 15

1.0

15

altitude(km)

Horizontally-Averaged Tracer B 20

10

altitude(km)

0.8 0.7

10

0.5 5

0.3 0.2

5

0 0

0.0

20

40

60

80 Time Hours

100

120

0 0.0

140

0.2

0.4

0.6 Tracer B

0.8

1.0

1.2

Figure 4: (a) Hovmoller diagram(y-t) of horizontally averaged Tracer B (normalised to initial maximum concentration) (b) Profiles of horizontally averaged Tracer B at 0, 20, 40, 60, 80, 100, 120, 140 hrs(normalised to initial maximum concentration) Horizontal mean of Tracer C 20 Time=0 hrs Time=20 hrs Time=40 hrs Time=60 hrs Time=80 hrs Time=100 hrs Time=120 hrs Time=140 hrs 15

1.0

15

altitude(km)

Horizontally-Averaged Tracer C 20

10

altitude(km)

0.8 0.7

10

0.5 5

0.3 0.2

5

0 0

0.0

20

40

60

80 Time Hours

100

120

0 0.0

140

0.2

0.4

0.6 Tracer C

0.8

1.0

1.2

Figure 5: (a) Hovmoller diagram(y-t) of horizontally averaged Tracer C (normalised to initial maximum concentration (b) Profiles of horizontally averaged Tracer C at 0, 20, 40, 60, 80, 100, 120, 140 hrs(normalised to initial maximum concentration) Horizontal mean of Tracer D 20 Time=0 hrs Time=20 hrs Time=40 hrs Time=60 hrs Time=80 hrs Time=100 hrs Time=120 hrs Time=140 hrs 15

1.0

15

altitude(km)

Horizontally-Averaged Tracer D 20

10

altitude(km)

0.8 0.7

10

0.5 0.3

5

0.2

5

0 0

0.0

20

40

60

80 Time Hours

100

120

140

0 0.0

0.2

0.4

0.6 Tracer D

0.8

1.0

1.2

Figure 6: (a) Hovmoller diagram(y-t) of horizontally averaged Tracer D (normalised to initial maximum concentration (b) Profiles of horizontally averaged Tracer D at 0, 20, 40, 60, 80, 100, 120, 140 hrs(normalised to initial maximum concentration)

iii


Horizontal mean of Tracer E 20 Time=0 hrs Time=20 hrs Time=40 hrs Time=60 hrs Time=80 hrs Time=100 hrs Time=120 hrs Time=140 hrs 15

1.0

15

altitude(km)

Horizontally-Averaged Tracer E 20

10

altitude(km)

0.8 0.7

10

0.5 0.3

5

0.2

5

0 0

0.0

20

40

60

80 Time Hours

100

120

140

0 0.0

0.2

0.4

0.6 Tarcer E

0.8

1.0

1.2

Figure 7: (a) Hovmoller diagram(y-t) of horizontally averaged Tracer E (normalised to initial maximum concentration (b) Profiles of horizontally averaged Tracer E at 0, 20, 40, 60, 80, 100, 120, 140 hrs(normalised to initial maximum concentration) Both Tracer A and Tracer B get transported rapidly from their original positions in the lower atmosphere to altitudes of 13km during the first phase of convection at around 13 hours. Figure 3 shows that at around 20 hours into the simulation, the vertical transport of Tracer A has resulted in a maximum at 10 km altitude, with lower concentrations being observed in the mid-troposphere. With time, mixing by further convection acts to smooth out the vertical profile, and by 100 hours into the simulation, the tracer has become well mixed throughout the troposphere. The fact that the initial inputted concentration is the only source to the domain allows for this eventual smooth profile. Because the initial Tracer A layer becomes quickly depleted through convective pumping, the lack of a constant source to the UT leads to the inevitable extinction of the upper tropospheric maximum. Further tests will look at similar tests with a constant source into the lowest layers of the atmosphere. Figure 8 shows the rapid vertical transport of Tracer A in the strong updrafts asssociated with the development of two convective clouds (located at -25km and 80km) at the beginning of the simulation. Both cells have shown updraft speeds of up to 8 m/s throughout a large depth of the troposphere. These strong updrafts have enabled each storm to transport plumes of Tracer A to cloud top at 13km. Figure 9 shows that 2 hours later plumes of tracer A exist in the UT at concentrations of up to 60% of the initial layer concentration. As well as showing the vertical transport of Tracer A, Figures 8(b) and 9 show evidence of boundary layer cleanout. Strong localised reductions in Tracer A concentration is evident directly below both clouds described above. Figure 10 shows the evolution of these ’clean’ patches with time near the surface (79m) and show how concentrations drop to approximately 40% of the initial concentration over the space of one convective cloud event. This may be a result of convective pumping removing this material to the upper regions of the troposhere. However, 8(a) shows evidence of mid-tropospheric air being brought down in cloud downdrafts directly above these patches. It is possible that it is this ’clean’ air being brought down from above in such downdrafts that is causing such localised reductions in concentration. Further studies have been carried out to examine this issue and will be addressed later. What is also interesting from Figure 10 is the timescale of these patches of clean air. Although these patches appear to form very quickly, they become ’filled in’ again relatively quickly by what can only be horizontal mixing with polluted air adjacent to the patch. This result gives a first insight into horizontal spatial variability of aerosol in a convective environment. This will also be developed at a later stage.

iv


Vertical Velocity at 14.0Hours

Tracer A at 14.0Hours

20

20

15 altitude(km) Hours

8.15 6.15 4.15 2.15

10

m/s

altitude(km) Hours

15

0.15 -1.86 -3.86

5

1.0 0.8 0.7 0.5

10

0.3 0.2 0.0

5

0

0 -100

-50

0 Horizontal Distance (km)

50

100

-100

-50

0 Horizontal Distance (km)

50

100

Figure 8: (a) Vertical velocities at time=14hrs overplotted with contours of cloud at 0.1, 1, 2, and 5g/kg. (b) Tracer A concentrations at time =14hrs(normalised to initial maximum concentration) with contours of cloud at 0.1, 1, 2, and 5g/kg Tracer A at 16.0Hours 20

altitude(km) Hours

15

1.0 0.8 0.7 0.5

10

0.3 0.2 0.0

5

0 -100

-50

0 Horizontal Distance (km)

50

100

Figure 9: Tracer A concentrations at time =16hrs(normalised to initial maximum concentration)

Tracer A Concentration at

79 meters

140 120 1.0

Time

100

0.8 0.7

80

0.5 60

0.3 0.2

40

0.0

20 0 -100

-50 0 50 Horizontal Distance (kms)

100

Figure 10: Horizontal Hovmoller diagram (x-t) of Tracer A concentration at 79m (normalised to initial maximum concentration)

Figures 4 and 5 show the evolution of Tracer B and Tracer C, initialised between 4 and 6kms, and 8 and 10 kms, respectively. Both tracers experience similar vertical transport to cloud top level as Tracer A. Figure 11 compares this transport of each of Tracers A, B, C to the UT ( 11km). Although Tracer A is initialized at the lowest altitude, it experiences the most efficient transport to the UT where, as already mentioned concentrations of 60% of the initial concentration are observed, compared to v


less than 30% of the original Tracer B and C concentrations. This is due to the fact that plumes of Tracer A become ingested directly into the updraft air that originates at lower altitudes of the troposphere. With an increase in altitude, the degree to which tracer gets ingested directly into the updraft decreases. At this point, entrainment of tracer into the rising thermal begins to play a part in determining its ability to reach higher altitudes. Figure 12 shows how a plume of lower altitude air is transported vertically through the Tracer C layer but at the same time entraining some of Tracer C into the cloud walls and cloud top, enabling some degree of transport of this tracer. As well as the upward transport of Tracer B and C, Figures 4 and 5 show that the predominant transport is towards the surface. This happens on two timescales. One is the smaller-scale rapid downward transport in cloud-scale downdrafts as shown in Figure 13. The second is the gradual sinking of tracer air caused by domain-wide subsidence associated with the transport of air forced upwards in the convective cells. Both the Tracer D and Tracer E layers are only marginally affected by the evolution of the convective field during the period of the simulation. At around 105 hours into the simulation, Tracer D appears to have mixed sufficiently downwards that it gets trapped in the tropospheric subsidence thus enabling transport down to levels around 7.5 kms by the end of the simulation. Tracer A Concentration at 11003 meters

Tracer B Concentration at 11003 meters

140 120

140

120

120

0.6

0.2 100

0.5

80

0.3 60

Time

0.4

0.2

0.1

80

0.1 60

0.1

0.1 40 20

0.0

20

0

0.2 0.1

60

0.1

100

0.0 40

0.0

20

0 -50 0 50 Horizontal Distance (kms)

0.2

80

0.0 40

0.0

-100

0.2 100

0.2 Time

100

Time

Tracer C Concentration at 11003 meters

140

0 -100

-50 0 50 Horizontal Distance (kms)

100

-100

-50 0 50 Horizontal Distance (kms)

100

Figure 11: (a) Hovmoller diagram(x-t) of Tracer A at 11003m altitude (normalised to initial maximum concentration initial maximum concentration) (b) Hovmoller diagram(x-t) of Tracer B at 11003m altitude (normalised to initial maximum concentration initial maximum concentration) (c) Hovmoller diagram(x-t) of Tracer C at 11003m altitude (normalised to initial maximum concentration initial maximum concentration)

Tracer C at 14.0Hours 20

altitude(km) Hours

15

1.0 0.8 0.7 0.5

10

0.3 0.2 0.0

5

0 -100

-50

0 Horizontal Distance (km)

50

100

Figure 12: 2D Plot of Tracer C at time = 14 hours(normalised to initial maximum concentration) with contours of cloud at 0.1, 1, 2, and 5g/kg

vi


Tracer B at 14.0Hours 20

altitude(km) Hours

15

1.0 0.8 0.7 0.5

10

0.3 0.2 0.0

5

0 -100

-50

0 Horizontal Distance (km)

50

100

Figure 13: 2D Plot of Tracer B at time = 14 hours(normalised to initial maximum concentration) with contours of cloud at 0.1, 1, 2, and 5g/kg

0.3.2

Tracer sinks

The passive tracer studies were developed to include the examination of how a convective field can move an aerosol population from different regions into a ’sink region’. These sink regions have been classified into liquid cloud, rain water, and ice particles. Again, although this study has little physical realism it can give an insight into how efficiently convective cloud can remove cloud condensation nuclei (CCN), ice nuclei (IN), and smaller particles that are ’washed’ out in rainfall depending on the altitude at which they are either formed or advected into the region. The study was carried out using the same tracer altitudes as before. Tracers A to E are assumed to be CCN and are removed from the system immediately in the presence of liquid water. Tracers F to J are assumed to be smaller particles and are removed from the system immediately in the presence of rainfall. Finally, Tracers K to O are assumed to be IN and hence are removed in the presence of ice. These studies were also carried out using the TOGA-COARE simulation and run for the first four days of the simulation. Figure 14 shows plots of horizontally averaged liquid water, rain, and ice during the 72 hour simulation. Horizontally-Averaged Liquid Water Mixing Ratio (g/kg)

Horizontally-Averaged Rain Mixing Ratio (g/kg) 20 0.94 0.78

10

0.47 0.31 0.16 0.00

5 0 0

20

40

60

0.13 0.11

15 altitude(km)

0.62

(g/kg)

altitude(km)

15

0.09

10

0.07 0.04 0.02 0.00

5 0 0

80

Time Hours

20

40

60

80

Time Hours

Horizontally-Averaged Ice Mixing Ratio (g/kg) 20

altitude(km)

0.22

10

0.16

(g/kg)

0.32 0.27

15

0.11 0.05 0.00

5 0 0

20

40

60

80

Time Hours

Figure 14: (a) Vertical Hovmoller diagram(y-t) of liquid water during the TOGA-COARE 4-day simulation (b) Vertical Hovmoller diagram(y-t) of rain during the TOGA-COARE 4-day simulation (c) Vertical Hovmoller diagram(y-t) of ice during the TOGA-COARE 4-day simulation

vii

(g/kg)

20


The fate of Tracers A-E is shown in Figure 15, Tracers F to J in Figure 16, and Tracers K to O in Figure 17. Horizontally-Averaged Tracer A

Horizontally-Averaged Tracer B

20

20

1.0

15

1.0

15

0.8

altitude(km)

altitude(km)

0.8 0.7

10

0.5 0.3

0.7

10

0.5 0.3

0.2

5

0 0

0.2

5

0.0

20

40

60

0 0

80

0.0

20

40

Time Hours

60

80

Time Hours

Horizontally-Averaged Tracer C

Horizontally-Averaged Tracer D

20

20

1.0

15

1.0

15

0.8

altitude(km)

altitude(km)

0.8 0.7

10

0.5 0.3

0.7

10

0.5 0.3

0.2

5

0 0

0.2

5

0.0

20

40

60

0 0

80

0.0

20

40

Time Hours

60

80

Time Hours

Horizontally-Averaged Tracer E 20

1.0

15 altitude(km)

0.8 0.7

10

0.5 0.3 0.2

5

0 0

0.0

20

40

60

80

Time Hours

Figure 15: Vertical Hovmoller diagram(y-t) of (a) Tracer A, (b) Tracer B, (c) Tracer C, (d) Tracer D, (e) Tracer E, all removed in liquid water

Horizontally-Averaged Tracer F

Horizontally-Averaged Tracer G

20

20

1.0

15

1.0

15

0.8

altitude(km)

altitude(km)

0.8 0.7

10

0.5 0.3

0.7

10

0.5 0.3

0.2

5

0 0

0.2

5

0.0

20

40

60

0 0

80

0.0

20

40

Time Hours

60

80

Time Hours

Horizontally-Averaged Tracer H

Horizontally-Averaged Tracer I

20

20

1.0

15

1.0

15

0.8

altitude(km)

altitude(km)

0.8 0.7

10

0.5 0.3

0.7

10

0.5 0.3

0.2

5

0 0

0.2

5

0.0

20

40

60

0 0

80

Time Hours

0.0

20

40

60

80

Time Hours

Horizontally-Averaged Tracer J 20

1.0

15 altitude(km)

0.8 0.7

10

0.5 0.3 0.2

5

0 0

0.0

20

40

60

80

Time Hours

Figure 16: Vertical Hovmoller diagram(y-t) of (a) Tracer F, (b) Tracer G, (c) Tracer H, (d) Tracer I, (e) Tracer J, all removed in liquid water

viii


Horizontally-Averaged Tracer K

Horizontally-Averaged Tracer L

20

20

1.0

15

1.0

15

0.8

altitude(km)

altitude(km)

0.8 0.7

10

0.5 0.3

0.7

10

0.5 0.3

0.2

5

0 0

0.2

5

0.0

20

40

60

0 0

80

0.0

20

40

Time Hours

60

80

Time Hours

Horizontally-Averaged Tracer M

Horizontally-Averaged Tracer N

20

20

1.0

15

1.0

15

0.8

altitude(km)

altitude(km)

0.8 0.7

10

0.5 0.3

0.7

10

0.5 0.3

0.2

5

0 0

0.2

5

0.0

20

40

60

0 0

80

Time Hours

0.0

20

40

60

80

Time Hours

Horizontally-Averaged Tracer O 20

1.0

15 altitude(km)

0.8 0.7

10

0.5 0.3 0.2

5

0 0

0.0

20

40

60

80

Time Hours

Figure 17: Vertical Hovmoller diagram(y-t) of (a) Tracer K, (b) Tracer L, (c) Tracer M, (d) Tracer N, (e) Tracer O, all removed in liquid water

0.4

Surface Fluxes

Further simulations have been carried out with the inclusion of a surface flux. As opposed to using a tracer for this simulation, the flux of Dimethyl Sulphide (DMS) from the ocean surface was modelled. DMS was chosen for a number of reasons. Firstly, parametrization schemes exist for the oceanic surface flux of DMS, and are a function of surface temperature, seawater DMS concentration, and surface wind speeds. In this study the scheme by Liss and Merlivat (1986) was employed. Secondly, DMS is an important pre-cursor to sulphate aerosol (see section 0.5) and therefore its distribution within a convective environment can have consequences on the production of such aerosol. Finally, a primary sink for DMS (through oxidation by the OH radical) can be modelled to create a simple but at the same time semi-realistic ’closed’ system for DMS. This simulation has been developed further to look at the effect of sub-grid variability (in this case the variability of surface wind speeds). The surface fluxes of DMS and the domain DMS content will be compared using the CRM-resolved winds at 10m and using a domain mean wind speed. Results from this may give an insight into inaccuracies in larger scale models that calculate surface fluxes based on wind speeds on a GCM or regional-scale model. Additionally, a third test has been included to compare the use of ECMWF-derived winds for the same time period, evolving on a 6-hourly time period. Figure 18 compares the surface fluxes of DMS from the three different schemes. Further results from these tests will be shown at the internal seminar.

ix


0.5

Sulphate aerosol scheme

At present, the model has been developed with the implementation of a sulphate aerosol scheme. The sulphate scheme itself is based primarily on that resident in version 5.5 of the Met Office Unified Model(UM) and includes variables for the representation of dimethyl sulphide(DMS), sulphur dioxide(SO2 ), and mass mixing ratios of sulphate aerosol in the Aitken and accumulation mode size range. Sources and sinks of sulphur into and out of the system are included as well as some of the various chemical exchanges between the sulphur variables. Fluxes of sea-salt from the ocean surface has also been implemented into the model but as yet has not been used. The sulphate scheme has also been run through the TOGA-COARE simulation and processes such as chemical fluxes from the ocean surface, aerosol scavenging, as well as transport have been studied. Some of the results from this simulation will be shown at the internal seminar.

x


S in DMS flux (CRM resolved winds)

S in DMS flux ( Domain mean surface wind)

100

100

80

80

80.69

24.69

67.25

20.57 16.46

40.35

12.34

Time (hours) ng/m2.s

53.80

26.90 40

8.23 40

13.45

4.11

0.00

0.00

20

20

0

0 -100

-50 0 50 Horizontal Distance(kms)

100

-100

-50 0 50 Horizontal Distance(kms)

100

S in DMS flux (ECMWF wind field) 100

80

30.21 25.18 60

15.11

ng/m2.s

Time (hours)

20.14

10.07 40 5.04 0.00

20

0 -100

-50 0 50 Horizontal Distance(kms)

100

Figure 18: Surface Hovmoller diagram(x-t) of (a) DMS Flux (CRM-resolved winds), (b) DMS Flux (Domain mean winds), (c) DMS Flux (ECMWF winds) xi

ng/m2.s

60

Time (hours)

60


Figure 19: Schematic of implemented aerosol scheme

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TEST TITLE  

THIS IS A TEST

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