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Chapter 7: Atmospheric Circulations

Chapter 7: Atmospheric Circulations. Scales of atmospheric motions Eddies - big and small Local wind systems Global winds Global wind patterns and the oceans. Scales of Atmospheric Motions. Microscale: meters - kilometers Mesoscale: km – a few hundred km

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Chapter 7: Atmospheric Circulations

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  1. Chapter 7: Atmospheric Circulations • Scales of atmospheric motions • Eddies - big and small • Local wind systems • Global winds • Global wind patterns and the oceans

  2. Scales of Atmospheric Motions • Microscale: meters - kilometers • Mesoscale: km – a few hundred km • synoptic scale: a few hundred km – a few thousand km • planetary scale: a few thousand km and larger Q1: what is the scale of atmospheric boundary layer turbulence? a) microscale, b) mesoscale, c) synoptic scale Q2: what is the scale of weather fronts? a) microscale, b) mesoscale, c) synoptic scale Q3: what is the scale of lake breeze over the Great Lakes a) microscale, b) mesoscale, c) synoptic scale

  3. Fig. 7-2, p. 171

  4. Eddies - Big and Small • eddy or turbulent eddy: caused by convection (heating or cooling), wind shear (or near surface wind), or waves • Rotor: caused by mountain waves wind shear: change of wind speed or direction with height

  5. Clear-air turbulence: caused by wind shear; important for aviation Billow clouds: caused by mountain waves in a wind shear zone Q4: what could cause bumpy aircraft flight in the upper troposphere? a) clear-air turbulence, b) rotor, c) billow clouds

  6. Local Wind Systems • Thermal Circulations: warm air rises and cool air sinks • Warm air leads to H in the air (i,e, pushing isobar up); • Air moves from H to L; • increases surface pressure (i.e., pushing near-surface isobar up) over cool place; • Leads to circulation from cool place to warm place near surface • Pay attention to the change of isobars with height

  7. Sea and Land Breezes • sea breeze: from sea to land • land breeze: land to sea • sea breeze front: clouds • Florida sea breezes • Sea and land breezes also occur near the shores of large lakes, such as the Great Lakes • Pay attention to the change of isobars with height Q5: which is stronger in general? a) sea breeze, b) land breeze, c) the same

  8. Q6: When do you expect to see the thunderstorm in summer in Florida? a) 10am, b) noon, c) 3pm, d) 6pm Fig. 7-6, p. 175

  9. height Q7: given the isobars as left, what is the near-surface wind direction? a) from A to B b) from B to A Q8: During the day, if you stand on beach, what would be the wind direction due to sea breeze? a) from sea to beach b) from land to sea A B

  10. Seasonally Changing Winds - the Monsoon • Monsoon wind system: change with season • India and eastern Asian monsoon • Global monsoons Q9: What are the differences and similarities between monsoon and sea/land breeze? A: Monsoon system is much greater in geographic area; changes with season; sea/land breeze changes with diurnal cycle; both due to horizontal temperature difference

  11. North American monsoon Q10: Coriolis force is important for monsoon circulation. Is it as important for sea breeze as for monsoon? a) yes b) no Q11: Still, is Coriolis force important for sea breeze? a) yes b) no

  12. Mountain and Valley Breezes • The nighttime mountain breeze is sometimes calledgravity winds or drainage winds, because gravitycauses the cold air to ‘drain’ downhill. Q12: Which is stronger in general? a) valley breeze, b) mountain breeze, c) the same • valley breeze: daytime; from valley to top • mountain breeze: nighttime; from top to valley

  13. Katabatic Winds • Strong drainage wind from cold elevated plateau down steep slope • Katabatic winds are quite fierce in parts of Antarctica, with hurricane-force wind speeds. • Bora: a cold, gusty northeasterly wind along the Adriatic coast in the former Yugoslavia

  14. Chinook (Foehn) Winds • It is called a Foehn along the leeward slopes of Alps. Q13: Which wind is weakest in general? a) drainage wind, b) katabatic wind, c) Chinook wind • Chinook winds: one type of drainage wind; warm and dry wind down the eastern slope of the Rocky Mountains

  15. Chinook wall cloud indicates that chinook is coming Fig. 7-14, p. 180

  16. Santa Ana Winds • Santa Ana wind: warm, dry wind from the elevated desert plateau down to southern California • compressional heating • Could have very strong wind • wildfires Q14: which wind comes from elevated desert plateau? a) Chinook wind; b) Santa Ana wind; c) Katabatic wind; d) mountain breeze Q15: The drainage wind over the lee side of the Rocky Mountain is a) Chinook wind; b) Santa Ana wind; c) Katabatic wind; d) mountain breeze

  17. Desert Winds • dust and sand storms: occurs over arid and semiarid regions • dust devils – from surface; usually with a diameter of a few meters and a height of <100 m Q16: What is the difference between tornado and dust devil? A: Tornado is larger horizontally and deeper vertically; from cloud base down

  18. General Circulation of the Atmosphere • cause: unequal net heating of the earth’s surface and atmosphere • effect: atmospheric circulation and ocean currents to transport heat from the equator to the poles Fig. 2.17 on p. 43

  19. Single-cell Model • basic assumptions: no rotation • Originally proposed by George Hadley in England in the 18th Century • Hadley cell Q17: Why is the single-cell model wrong? A: Because single cell does not exist due to earth’s rotation • UK’s Hadley Centre for Climate Research is named after George Hadley.

  20. Three-cell Model • model for a rotating earth; realistic over the tropics in winter hemisphere: Hadley cell; doldrums; subtropical highs trade winds; intertropical convergence zone (ITCZ) • Over mid- and high latitudes: Ferrel cell and polar cell do not play major roles; westerlies in the upper troposphere; polar front; polar near-surface easterlies Q18: if near-surface wind is southwesterly over NH midlatitudes, what is the direction of upper troposphere wind? a) westerly, b) easterly, c) southerly, d) northerly

  21. Fig. 7-21, p. 185

  22. Average Surface Winds and Pressure: The Real World • semipermanent highs: Bermuda high & Pacific high Pacific high moves north in summer; Bermuda high moves west in summer • Semipermanent low: Icelandic low it moves north in summer • Siberian high in winter due to very cold air • Aleutian low in winter due to storm track • ITCZ stays in the warm hemisphere (e.g., NH in July) • There are three semipermanent highs in SH

  23. Fig. 7-22a, p. 188

  24. Fig. 7-22b, p. 189

  25. The General Circulation and Precipitation Patterns • Most of the world’s thunderstorms are found along the ITCZ. • Low rainfall over the subtropical regions • Fronts and precipitation over the subpolar lows Q19: which is correct? a) desert causes subtropical high; b) subtropical high causes desert

  26. Westerly Winds and the Jet Stream • jet streams • subtropical jet stream • polar front jet stream Low-level jet stream over the Central plains of the U.S. (within 2 km above surface), bringing moist and warm air to form nighttime thunderstorms

  27. (warm) Gulf Stream; (warm) Kuroshio Current; (cold) California Current; (cold) Canary Current; Equatorial Current and Counter Current in the Pacific Global wind drives ocean current Q20: These ocean circulations are consistent with wind of a) high pressure system; b) low pressure system Fig. 7-29, p. 193

  28. Winds and Upwelling • Upwelling is strongest when wind is parallel to the coastline Q21: Why is ocean coldest in northern California? A: wind is parallel to the coastline; upwelling is strongest; cold deep water is brought to surface

  29. El Niño and the Southern Oscillation Q22: what is the El Niño? A: warming, pressure decrease, and weakened upwelling over the central and eastern Pacific; trade wind weakened as well; (cooling, pressure increase over western Pacific) • La Niña: opposite Q23: What is the Southern Oscillation (SO)? A: oscillation of surface pressure over tropical western and eastern Pacific • ENSO: El Nino and SO are closely related Q24: What is teleconnections? A: local changes affect weather in remote regions. • SST animation: http://www.cdc.noaa.gov/map/clim/sst_olr/sst_anim.shtml

  30. Thermocline is incorrect in bottom panel Fig. 7-32, p. 196

  31. ENSO is an example of a global-scale weather phenomenon. Q25: What is the El Nino effect on winter weather in the U.S.? A: Northwestern U.S. usually has a warmer winter, Southeast usualy has a wetter winter, and often Arizona has a wetter winter El Nino effect

  32. Other Atmosphere-Ocean Interactions • North Atlantic Oscillation: based on pressure difference between Bermuda and Iceland • Arctic Oscillation: pressure difference between about 45oN and Arctic • Pacific Decadal Oscillation: Pacific surface temperature pattern changes every 30-50 years

  33. Over the tropical Pacific, PDO pattern is not very different from ENSO; Over midlatitude Pacific, PDO pattern is different from ENSO Fig. 7-36, p. 199

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