Essentials of Meteorology An Invitation to the Atmosphere
7th Edition Ahrens
Full download at link: https://testbankpack.com/p/solution-manualfor-essentials-of-meteorology-an-invitation-to-the-atmosphere-7thedition-by-ahrens-isbn-128546236x-9781285462363/
Chapter 7
Atmospheric Circulations
Chapter Outline
Scales of Atmospheric Motion
Eddies Big and Small
Local Wind Systems
Thermal Circulations
Focus on an Observation
Eddies and “Air Pockets”
Sea and Land Breezes
Mountain and Valley Breezes
Katabatic Winds
Chinook (Foehn) Winds
Focus on a Special Topic
Snow Eaters and Rapid Temperature Changes
Santa Ana Winds
Desert Winds
Seasonally Changing Winds The Monsoon
Global Winds
General Circulation of the Atmosphere
Single-Cell Model
Three-Cell Model
Average Surface Winds and Pressure: The Real World
The General Circulation and Precipitation Patterns
Ahrens Essentials of Meteorology 7e
Instructor’s Manual
Westerly Winds and the Jet Stream Atmosphere-Ocean Interactions
Global Wind Patterns and Surface Ocean Currents
Winds and Upwelling
El Niño and the Southern Oscillation
Other Atmosphere-Ocean Interactions
Summary
Key Terms
Questions for Review
Questions for Thought and Exploration
Summary
Air motions of a wide variety of types and sizes are examined in this chapter. Atmospheric circulations ranging from short-lived microscale phenomena to the semi-permanent circulation patterns found in the earth's global circulation are compared and contrasted.
The first portion of the chapter begins with a classification of the scales of atmospheric motion. Starting with the smallest phenomena (eddies), the role of wind shear and the resulting turbulent eddies that canform in clear air are described. These small-scale features are of considerable practical importance because they present a hazard to aviation. The formation of thermal circulations is then covered in some detail. Sea, lake, and land breezes are common examples of thermal circulation and students may have lived in or visited a region where these occur. The role of seasonal changes in the development of the Asian monsoon, which resembles a large thermal circulation, is presented. Several additional local scale winds including mountain and valley breezes, katabatic winds, chinook, and the Santa Ana winds are described.
The second half of the chapter addresses the earth's global scale wind and surface pressure patterns. Single-cell and three-cell models describe the underlying cause of the atmosphere's general circulation. Despite some unrealistic assumptions, the three-cell model contains many of the surface features found in the real world. These features and their seasonal movement can have an important effect on regional climate. Approximately 70% of the earth's surface is covered with oceans; thus weather and climate are strongly affected by interactions between the atmosphere and oceans. The chapter features discussions of the El Niño/Southern Oscillation phenomenon, as well as other atmosphere-ocean interactions including the North Atlantic, Arctic, and Pacific Decadal Oscillations.
Teaching Suggestions, Demonstrations and Visual Aids
1. Using the concept of horizontal pressure gradient, discusswhy a fan is effective in cooling a house on a hot summerdayonly whentwo ormore windows or doors are open.
Ahrens Essentials of Meteorology 7e
Instructor’s Manual
2 Clouds produced by some of the mesoscale and global scale wind circulation patterns discussed in this chapter can be seen on satellite images. The ITCZ is often very clearly defined on a full or half disk photograph from a geostationary satellite.
3. On a satellite image, sea breeze convergence zones may be visible along the eastern or western coastline of Florida. Ask students why a strong convergence zone will form along the west coast one day and then along the east coast another day.
4 During the winter, bring a surface weather map to class and compare temperatures to the north and thesouth of the polar front. There is often a very sharp temperature gradient. Locate the polar jet stream on an upper-level chart and determine the region where the maximum winds are found.
5 Construct a box approximately 12" wide x 12" high x 4" deep. The front side of the box should be constructed out of clear plastic. Place two empty cans at opposite ends of the bottom of the box. Fill one can with dark soil, the other with water. Place a 150 or 200 Watt bulb so that light shines equally onto the two cans in the box. After a few minutes, carefully introduce some smoke into the chamber from a hole near the middle of the bottom edge of the plastic. With some care, a circular circulation will be visible.
6. If there is a particularly windy area on campus, such as a ‘wind tunnel’ between adjacent buildings, discuss the possible reasons for its existence.
7. Using realtime weather imagery, have students locate the prevailing westerlies and the polar easterlies.
8. Using Internet sources such as the Weather Underground (www.wunderground.com), examine current surface and upper-level winds at locations in the tropics, the middle latitudes, and the Arctic or Antarctic. Discuss the relationships between the observed winds and those expected based on the threecell model of the general circulations.
9. Examine current winds along the east and west coasts of the United States, and discuss the relationships between wind direction and upwelling.
10. Discuss the interesting role of katabatic winds in creating a suitable habitat for penguins in Antarctica. A good source for this is http://earthobservatory.nasa.gov/IOTD/view.php?id=41161.
Student Projects
1. Have students research and plot the path of some of the early voyages of discovery made aboard sailing ships (Columbus or Magellan, for example). Do the routes appear reasonable in light of what the students know about the earth's general circulation pattern?
2. Have students collect climatological data for their location (or another region) during one or two strong El Niño events. Select two or three periods when there was not a strong El Niño to act as a control. What effectsmight the El Niño have onlocal weather or climate? Dothestudents see anyevidence of this in their climate data?
3. Have students summarizethe weather conditions prevailing during a period when strong Santa Ana
Ahrens Essentials of Meteorology 7e
Instructor’s Manual
winds are being observed in southern California.
4. Have students research and summarize the weather conditions that produce any local scale winds that are unique to their area. Do these local winds have any important effects on the local climate? The 'weather history' feature of the Weather Underground website (www.wunderground.com) may be useful for this activity.
5. Have students construct a wind rose for their city or town. Can students find any evidence that data of this type has been used by city planners?
Answers to Questions for Review
1. Microscale: diameter of a few meters or less, time scale of seconds to a few minutes. Example: turbulent eddies. Mesoscale: diameter of a few kilometers to hundreds of kilometers, time scale of hours to a day. Example: thunderstorms. Synoptic scale: diameter of several hundred to a thousand kilometers, time scale of days to weeks. Example: high and low pressure systems. Global scale: spatial scale of entire earth, time scale of weeks to months. Example: longwaves in the westerlies.
2. Wind shear is the change in wind speed or direction with increasing altitude. When the shear exceeds a critical value, waves break into large swirls with significant vertical movement. When this happens in clear air it's called clear air turbulence.
3.Seefigure7.4inthetextforagoodexampleofadiagramthatcanshowhowthermal circulationdevelops. Thermal circulationdevelops in situations where two adjacent areas experience differential heating. A good example is when the land near a body of water heats up more rapidly during the day than the water body near it. A good diagram will show the heating and expansion of air over the land during the day, causing a lower pressure area over the land. The relatively cooler, denser, higher pressure air over the water will tend to move toward the land. The opposite effect will take place at night, when the land cools more rapidly than the water, resulting in the air above the land becoming more dense and higher in pressure. Now the cool air over the land will blow out onto the water.
4 During the day, the land heats more quickly than the adjacent water, and the intensive heating of the air above produces a shallow thermal low. The air over the water remains cooler than the air over the land; hence, a shallow thermal high exists above the water. The overall effect of this pressure distribution is a sea breeze that blows from the sea toward the land. At night, the land cools more quickly than the water. The air above the land becomes cooler than the air over the water, producing a distribution of pressure, such as the one shown in Fig. 7.5b.With higher surface pressure now over the land, the wind reverses itselfand becomesa landbreeze a breeze that flowsfrom thelandtowardthe water. Temperature contrasts between land and water are generally much smaller at night; hence, land breezes are usually weaker than their daytime counterpart, the sea breeze.
5. Valley breeze, because rising air will expand and cool adiabatically.
6. The ideal setting for a katabatic wind is an elevated plateau surrounded by mountains, with an opening that slopes rapidly downhill. When winter snows accumulate on the plateau, the overlying air grows extremely cold and a shallow dome of high pressure forms near the surface. Along the edge of the plateau, the horizontal pressure gradient force is usually strong enough to cause the cold air to flow across the isobars through gaps and saddles in the hills. Along the slopes of the plateau, the wind continues
downhill as a gentle or moderate cold breeze. If the horizontal pressure gradient increases substantially, such as when a storm approaches, or if the wind is confined to a narrow canyon or channel, the flow of air can increase, often destructively, as cold air rushes down-slope like water flowing over a waterfall.
7. Chinook winds blow down mountain slopes. As air descends it warms adiabatically. Chinook winds are dry because the air's moisture has been lost as the air ascended on the upwind side of the mountain.
8. a. Santa Ana winds are warm because of compressional heating of the already warm, dry desert air. b. As air descends from the elevated desert plateau, it funnels through mountain canyons in the San Gabriel and San Bernardino Mountains, finally spreading over the Los Angeles basin and San Fernando Valley. The wind often blows with exceptional speed in the Santa Ana Canyon (the canyon from which it derives its name). These warm, dry winds develop as a region of high pressure builds over the Great Basin. The clockwise circulation around the anticyclone forces air downslope from the high plateau.
9. Dust devils form on clear, hot days over a dry surface where most of the sunlight goes into heating the surface, rather than evaporating water from vegetation. The atmosphere directly above the hot surface becomes unstable, convection sets in, and the heated air rises. Wind, often deflected by small topographic barriers, flows into this region, rotating the rising air. Depending on the nature of the topographic feature, the spin of a dust devil around its central eye may be cyclonic or anticyclonic, and both directions occur with about equal frequency
10. a. During the winter, the air over the continent becomes much colder than the air over the ocean. A large, shallow high-pressure area develops over continental Siberia, producing a clockwise circulation of air that flows out over the Indian Ocean and South China Sea. Subsiding air of the anticyclone and the downslope movement of northeasterly winds from the inland plateau provide eastern and southern Asia with generally fair weather. Hence, the winter monsoon, which lasts from about December through February, means clear skies (dry season), with winds that blow from land to sea. b. In summer, the wind flow pattern reverses itself as air over the continents becomes much warmer than air above the water. A shallow thermal low develops over the continental interior. The heated air within the low rises, and the surrounding air responds by flowing counterclockwise into the low center. This condition results in moisture-bearing winds sweeping into the continent from the ocean. The humid air converges with a drier westerly flow, causing it to rise; further lifting is provided by hills and mountains. Lifting cools the air to its saturation point, resulting in heavy showers and thunderstorms. Thus, the summer monsoon of southeastern Asia, which lasts from about June through September, means wet, rainy weather (wet season) with winds that blow from sea to land. This differs from the winter monsoon, where continental air is blowing out to the ocean so the air is not moist like it is in the summer monsoon.
11. For placement of semi-permanent pressure systems on the earth refer to Figure 7.29 in the text. The diagram drawn should include Polar highs at each pole, followed by subpolar lows, subtropical highs and the ITCZ Equatorial Zone. For placement of pressure systems and prevailing winds see Figure 7.25 in the text. The diagram shown should include Polar Easterlies, Westerlies (in mid-latitudes), NE Trade Winds (in Northern Hemisphere low latitudes), SE Trade winds (in Southern Hemisphere low latitudes).
12. The westerlies.
Essentials of Meteorology 7e Instructor’s Manual13. When we compare the January and July maps, we can see several changes in the semipermanent pressure systems. The strong subpolar lows so well developed in January over the Northern Hemisphere are hardly discernible on the July map. The subtropical highs, however, remain dominant in both seasons. Because the sun is overhead in the Northern Hemisphere in July and overhead in the Southern Hemisphere in January, the zone of maximum surface heating shifts seasonally. In response to this shift, the major pressure systems, wind belts, and ITCZ shift toward the north in July and toward the south in January.
14. During the summer, the Pacific high drifts northward to a position off the California coast. Sinking air on its eastern side produces a strong upper-level subsidence inversion, which tends to keep summer weather along the West Coast relatively dry. Along the East Coast, the clockwise circulation of winds around the Bermuda high brings warm, tropical air northward into the United States from the Gulf of Mexico and the Atlantic Ocean, causing frequent precipitation.
15. Since the polar front is a boundary separating the cold polar air to the north from the warm subtropical air to the south, the greatest contrast in air temperature occurs along the frontal zone.
16. The equator-to-pole temperature gradient is stronger in winter than in summer.
17. As the wind blows over the oceans, it causes the surface water to drift along with it. The moving water gradually piles up, creating pressure differences within the water itself. This leads to further motion several hundreds of meters down into the water. In this manner, the general wind flow around the globe startsthemajorsurfaceoceancurrentsmoving.Majoroceancurrentsdonotfollowthewindpatternexactly; rather, they spiral in semiclosed circular whirls called gyres In the North Atlantic, the prevailing winds blow clockwise and outward from the subtropical highs, while the ocean currents move in a more or less circular, but still clockwise, pattern. As the water moves beneath the wind, the Coriolis force directs the water to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This deflection causes the surface water to move at an angle between 20° and 45° to the direction of the wind. Hence, surface water tends to move in a circular pattern as winds blow outward, away from the center of the subtropical high.
18. Summer winds tend to parallel the coastline of California. As the wind blows over the ocean, the surface water beneath it is set in motion. As the surface water moves, it bends slightly to its right due to the Coriolis effect. The water beneath the surface also moves, and it too bends slightly to its right. The net effect of this phenomenon is that a rather shallow layer of surface water moves at right angles to the surface wind and heads seaward. As the surface water drifts away from the coast, cold, nutrient-rich water from below rises (upwells) to replace it. Upwelling is strongest and surface water is coolest where the wind parallels the coast, such as it does in summer along the coast of northern California.
19. a. A situation in which El Niño conditions last for many months, and a more extensive ocean warmingoccurs.This extremely warm episode occursat irregular intervalsof twoto sevenyearsandcovers a large area of the tropical Pacific Ocean. b. They reverse in a seesaw pattern: high at one end and low at the other.c During exceptionally warm El Niños, drought is normally felt in Indonesia, southern Africa, andAustralia,whileheavyrainsandfloodingoftenoccurinEcuadorandPeru.IntheNorthernHemisphere, a strong subtropical westerly jet stream normally directs mid-latitude cyclonic storms into California and heavy rain into the Gulf Coast states. The total damage worldwide due to flooding, winds, and drought may exceed many billions of dollars. A major El Niño event can also pump so much heat into the atmosphere that global temperatures rise by several tenths of a degree Fahrenheit for a few months.
20. Unusually cold surface temperatures.
21. Over the Atlantic there is areversal of pressure (called the North Atlantic Oscillation, or NAO) that has an effect on the weather in Europe and along the east coast of North America. For example, in winter, if the atmospheric pressure in the vicinity of the Icelandic low drops, and the pressure in the region of the Bermuda-Azoreshighrises,thereisacorrespondinglargedifferenceinatmosphericpressurebetweenthese two regions that strengthen the westerlies. The strong westerlies in turn direct strong storms on a more northerly track into northern Europe, where winters tend to be wet and mild. During this positive phase of the NAO, winters in the eastern United States tend to be wet and relatively mild, while northern Canada and Greenland are usually cold and dry. The negative phase of the NAO occurs when the atmospheric pressure in the vicinity of the Icelandic low rises, while the pressure drops in the region of the Bermuda high. This pressure change results in a reduced pressure gradient and weaker westerlies that steer fewer and weaker winter storms across the Atlantic in a more westerly path. These storms bring wet weather to southern Europe and to the region around the Mediterranean Sea. Meanwhile, winters in Northern Europe are usually cold and dry, as are the winters along the east coast of North America.
22. Cold.
23. During the warm (or positive) phase, unusually warm surface water exists along the west coast of North America, while over the central North Pacific, cooler than normal surface water prevails. At the same time, the Aleutian low in the Gulf of Alaska strengthens, which causes more Pacific storms to move into Alaska and California. This situation causes winters, as a whole, to be warmer and drier over northwestern North America. Elsewhere, winters tend to be drier over the Great Lakes, and cooler and wetter in the southern United States. The cool (or negative) phase finds cooler-than-average surface water along the west coast of North America and an area of warmer-than-normal surface water extending from Japan into the central North Pacific. Winters in the cool phase tend to be cooler and wetter than average over northwesternNorthAmerica,wetter overtheGreat Lakes,andwarmer anddrierinthesouthernUnited States.
Answers to Questions for Thought and Exploration
1. In early morning you would likely experience the mountain breeze, with winds blowing down the mountain slope.
2. The very cold Antarctic air is quite dense, and it tends to 'drain' down the valleys, creating strong katabatic winds.
3. The equator-to-pole temperature gradient must reverse directions.
4. Cheyenne, Wy is located very near to the foot of a large mountain range. These mountains form an obstruction, which results in eddy motions, commonly referred to as mechanical turbulence. Mechanical turbulence produces a frictional affect (drag) on the air flow that is much larger than caused by molecular viscosity, thus resulting on decreased wind velocity and increased snow deposition in Cheyenne, relative to the countryside nearby.
5. Strongest sea breezes: west coast. Strongest land breezes: east coast.
6. Because of the Ekman Spiral, the average movements of surface water down to a depth of about
Ahrens Essentials of Meteorology 7e
Instructor’s Manual
100 m is at right angles to the surface wind direction. Icebergs, which may extend downward to depths greater than 100 m, move with this surface water at nearly right angles to the surface wind direction.
7. The fastest winds are found in the jet stream core. Clear air turbulence is found above and below the jet stream core.
8. This is due mainly to the reversal of winds associated with the summer and winter monsoon.
9. Upwelling is strongest in summer and when the winds blow parallel to the coast. Strong upwelling conditions bring cold water to the surface. In winter when upwelling is not as strong, the surface water is not as cold.
10. Large subtropical highs tend to be centered over the oceans off the western margin of continents in both hemispheres. Winds around the highs blow clockwise (from the north along the western margin of the continents) in the Northern Hemisphere, and counterclockwise (from the south along the western margin of continents) in the Southern Hemisphere.