9/25/2014. Scales of Atmospheric Motion. Scales of Atmospheric Motion. Chapter 7: Circulation of the Atmosphere

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1 Chapter 7: Circulation of the Atmosphere The Atmosphere: An Introduction to Meteorology, 12 th Lutgens Tarbuck Lectures by: Heather Gallacher, Cleveland State University Scales of Atmospheric Motion Small- and large-scale circulation: Microscale Mesoscale Macroscale Scales of Atmospheric Motion Microscale winds: The circulation is small and chaotic. They can last from seconds to minutes. They can be simple gusts, downdrafts, and small vortices, such as dust devils. 1

2 Scales of Atmospheric Motion Mesoscale winds: They can last from minutes to hours. They are usually less than 100 km across. Some mesoscale winds (thunderstorms and tornadoes) also have a strong vertical component. Scales of Atmospheric Motion Macroscale winds: These winds are the largest wind patterns. These planetary-scale patterns can remain unchanged for weeks at a time. Smaller macroscale circulation is called synoptic scale. These wind systems are about 1000 km in diameter. Smaller macroscale systems are tropical storms and hurricanes. Scales of Atmospheric Motion 2

3 Scales of Atmospheric Motion Structure of wind patterns: Global winds are a composite of motion on all scales. Hurricanes appear as a large cloud moving slowly across the ocean. The large cloud contains many mesoscale thunderstorms. The thunderstorms consist of numerous microscale bursts. Land and sea breezes Local Winds Local Winds Mountain and valley breezes 3

4 Local Winds Chinook (foehn) winds Chinooks are warm dry winds that sometimes move down slopes of mountains. These winds can bring on drastic changes in temperature. The winds will melt snow cover rapidly. The Native American word chinook means snow-eater. Similar winds are called foehns in Europe. Local Winds Katabatic (fall) winds: These winds originate when cold, dense air begins to move. Better known katabatic winds have local names, such as mistral, which blows from the French alps to the Mediterranean. Country breezes: These breezes are mesoscale winds. They are caused by the uneven heating of urban and country areas. This results in the flow from country to urban areas. Single-cell circulation model Hadley model Hadley proposed that the contrast in temperatures between the poles and the equator creates a large convection cell in both the Northern and Southern hemispheres. Global Circulation 4

5 Three-cell model Global Circulation Global Circulation A three-cell circulation model was proposed in the 1920s. Warm air rises at the equator (Hadley cell). As the flow moves poleward, it begins to cool and sinks at latitude. Trade winds meet at the equator, in a region with a weak pressure gradient, called the doldrums. The westerly circulation of surface winds (prevailing westerlies) between latitude is called the Ferrel cell. Circulation (at ) within a polar cell produces polar easterlies; surface flows that move toward the equator. Pressure Zones Drive Winds Idealized zonal pressure belts: The equatorial low is an intertropical convergence zone (ITCZ). Subtropical highs (STH) are high-pressure zones in the belts about latitude on either side of the equator. Polar highs near the Earth s poles are where the polar easterlies originate. 5

6 9/25/2014 Pressure Zones Drive Winds Pressure Zones Drive Winds Semi permanent pressure systems: The real world January pressure and wind patterns Pressure Zones Drive Winds Semi permanent pressure systems: The real world July pressure and wind pattern 6

7 Monsoons Monsoon refers to a seasonal reversal of winds. The Asian monsoon, which affects India and its surrounding areas, China, Korea, and Japan. The monsoon is driven by pressure differences. The North American monsoon occurs in the southwestern U.S. and northwestern Mexico. This monsoon is driven by the extreme temperatures, which generate a low-pressure center over Arizona and results in a circulation pattern that brings moist air from the Gulf of California and from the Gulf of Mexico, to a lesser degree. Monsoons Monsoons 7

8 Why Westerlies? The Westerlies The Westerlies Waves in the westerlies: Westerlies flow in wavy paths that have long wavelengths. The longest wave patterns are known as Rossby waves, which usually consist of 4 6 waves that encircle the globe. Rossby waves can have a large impact on weather. Jet Streams Jet streams: Embedded in westerlies Widths vary from less than 100 km to more than 500 km. Speeds can attain kph. Polar and subtropical 8

9 The polar jet stream is the most prevalent. It occurs along a major frontal zone, the polar front. The jet stream moves faster in winter. During the winter, occasionally it moves north south. Jet Streams Jet Streams The subtropical jet stream is a semipermanent jet stream over the subtropics. It is a west-to-east current, centered at 25 N and S. It is mainly a winter phenomenon. The subtropical jet stream is slower than the polar. Jet Streams Jet streams and Earth s heat budget 9

10 Global Winds and Ocean Currents The Coriolis force deflects surface currents poleward, which form nearly circular patterns of ocean currents called gyres. The Gulf stream is strengthened by westerly winds and continues northeastward. Global Winds and Ocean Currents Importance of ocean currents: Ocean currents have an important on climate, which helps maintain the Earth s heat balance. Cold currents offshore result in a dry climate. Warm offshore current produce a warm moist climate. Global Winds and Ocean Currents Ocean currents and upwelling: Upwelling, a wind-induced vertical movement, is the rising of cold water from deeper layers to replace warmer surface water. It occurs where winds blow parallel to the coast toward the equator. 10

11 El Niño and La Niña and the Southern Ocean El Niño and La Niña and the Southern Ocean El Niño is a gradual warming of eastern Pacific waters in December or January. La Niña is the opposite of El Niño and refers to colder-than-normal ocean temperatures. El Niño and La Niña and the Southern Ocean Impact of El Niño: It is noted for its potentially catastrophic impact on weather and economies of Chile, Peru, Australia, and other countries. Arid areas can receive a lot of precipitation. A change in surface water temperature can kill fish. El Niño has been recognized as part of the global atmospheric circulation pattern. 11

12 El Niño and La Niña and the Southern Ocean Impact of La Niña: La Niña is also an important atmospheric phenomenon. In the western Pacific, wetter than normal conditions occur. There are also more frequent hurricanes in Atlantic. Southern oscillation: This is the seesaw pattern of atmospheric pressure between the eastern and western Pacific. Winds are the link between pressure changes and the ocean warming and cooling associated with El Niño and La Niña. Global Distribution of Precipitation Zonal Distribution of precipitation Global Distribution of Precipitation Distribution of precipitation over the continents 12

13 End Chapter 7 13