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Atmospheric Circulations

Atmospheric Circulations. Chapter 7. A Fluid Atmosphere. Easier to envision atmospheric motions if we think of our atmosphere as a fluid That’s what it really is Air motions are similar to those in any fluid Oceans, rivers, etc.

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Atmospheric Circulations

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  1. Atmospheric Circulations Chapter 7

  2. A Fluid Atmosphere • Easier to envision atmospheric motions if we think of our atmosphere as a fluid • That’s what it really is • Air motions are similar to those in any fluid • Oceans, rivers, etc. • All kinds of circulations ranging from microscopic whirls to large storms (low pressure systems) to global wind patterns (jet streams)

  3. Water vs Air “Eddies” Gulf Stream

  4. Scales of Motion • In meteorology, scales of motion describe the size and duration of circulations • Smallest scale we’re concerned with is the microscale, largest is the global scale • And everything in between

  5. Scales of Motion Global Synoptic Meso Micro

  6. Scales of Motion

  7. Scales of Motion • Often, it’s hard to tell what kind of motions are going on since you can’t see air • Can’t see turbulence in a plane right?

  8. Lenticular cloud

  9. Scales of Motion • Sometimes you can see what’s going on when motions are visible in clouds

  10. Local Wind Systems • Interesting topic because they exist almost everywhere • Coastal regions - Seabreezes • Mountains - Chinook winds, mountain/valley breezes • Often due to 1) terrain or 2) temperature differences from one place to another • If due to 2) - we call it a thermal circulation

  11. Thermal Circulations • These circulations are due to differences in temperature • Like between air over land vs air over water • The temp differences cause something to happen to the pressure • And what happens when the pressure is different from place to place?? • WIND

  12. Thermal Circulations • Initially, if temps are uniform, so are pressure surfaces • Going upward, pressure surfaces are horizontal • But what if air to the south heats up?

  13. Thermal Circulations • Remember from earlier, if a column of air warms, it stretches vertically • So, the pressure surfaces are now higher in the warmer air causing a pressure gradient force - wind

  14. Thermal Circulations • To the south, a “thermal low” pressure develops at the surface • Due to air moving toward north aloft • To the north, high pressure develops at surface • Due to air piling up aloft

  15. Thermal Circulations • So, air begins to move from high to low at both the surface and aloft • Air at the surface warms near the low and rises • Air aloft cools over the surface high and sinks

  16. Sea Breezes • Sea breezes are a good example of a thermal circulation • Form in tropical/sub-tropical regions in the warm season • People from coastal areas of FL know about these - nice at the beach but reeeaaall dark inland • Can also have “lake breezes” and “river breezes” which form in the same way

  17. Sea Breezes • Same as before except now the water/land is causing the contrast in temp/pressure • Thermal low develops over land and wind at the surface moves from water to land

  18. Sea Breezes • As before, air rises in the vicinity of the thermal low and often produces thunderstorms inland • That’s why the summer weather at the coast is usually better than weather inland (in terms of sunshine)

  19. Sea Breezes-lightning T-storms East Coast Sea Breeze Sea Breeze in panhandle

  20. Sea Breezes • How would you know that a sea breeze has passed? • Temp ? • Decreases (air off of the water is cooler) • Humidity ? • Increases (air off of the water is more moist) • Wind ? • Shifts to onshore and increases in speed • Sky ? • Clear after passage, cloudy before

  21. Sea Breezes • Florida gets hit often by thunderstorms due to sea breezes • Sometimes sea breezes from both coasts converge over the central peninsula (Orlando in particular)

  22. T-storm frequency

  23. Sea Breezes • Which raises an interesting question……

  24. Land Breezes • Remember from several weeks ago we said that land heats up and cools down quicker than water? • Happens at night causing a reversal of the sea breeze • Called a land breeze

  25. Land Breezes • Air over land cools quickly at night • So now air over water is warmer than air over land • Winds switch to offshore • generally weaker than the sea breeze

  26. Monsoon • What kind of image does that word conjure up? • Lots of heavy rain? • Floods? • Actually it means: • A seasonal shift in wind patterns • So it doesn’t directly address rain at all • The monsoon is a thermal circulation just like the sea breeze but it occurs as a seasonal cycle - not a daily cycle

  27. Asian Monsoon • Asian monsoon is the most well developed one • Weaker monsoon over the desert southwest of N. America • Very cold air over the continent during winter • Causes high pressure and flow toward the water • This also keeps moisture offshore • So, very dry over land in the winter

  28. Asian Monsoon • Things (winds) change during the summer as the land heats up and low pressure develops • Now wind moves onshore and brings with it very moist air • This region, particularly central India, gets absolutely hammered w/ rain • Some places over 400 inches a year! (most of it coming in 6-7 months!)

  29. Asian Monsoon • Why so much rain (other than the obvious)? • 1) Convergence of air enhances upward motion • 2) Terrain Lots of mountains

  30. Chinook Winds • “Snow eaters” • Warm, dry winds on the eastern side of the Rocky Mountains • Due to compressional heating as air descends • Enhanced if clouds form on the western side • Can cause warming of >40ºF in 1 hour!

  31. Chinook Winds • “Chinook wall cloud” - precursor of Chinook winds • 20ºF when this pic was taken - 60ºF same time next day

  32. Chinook Winds • How they work • Any air which descends will warm by compression - compressional heating • main source of warmth for Chinooks • But if clouds form on the western side of the mountains, the Chinook wind will be even warmer and drier than w/o clouds • Why? • Have to go back to the moist/dry adiabatic stuff • Remember: moist rate=6ºC/1000m, dry rate=10ºC/1000m

  33. Chinook Winds • Air which starts at 10ºC on the western side, ends up at 18ºC on the eastern side. • Due to cooling at the moist rate starting at 1000 m…..but warming at the dry rate going down • So, clouds enhance the Chinook winds Cooling at moist rate Warming at dry rate

  34. Chinook Winds • Very localized - just near the mountain base • As warm air hits the cold air below, the boundary can oscillate • Cities near the boundary can experience rapid back-and-forth changes in temperature

  35. Chinook Winds • One case in South Dakota: • 5:30 a.m. : -4ºF • 9:40 a.m. : 54ºF • 10:30 a.m.: 11ºF • 10:45 a.m.: 55ºF

  36. General Circulation • From earlier, there are all kinds of local wind systems - many more than I went over • All kinds of smaller circulations too • But what about the global wind patterns? • If we average winds over a long period of time, the small stuff disappears and we are left with the “global scale winds” or “general circulation” of the atmosphere • Keep in mind - just an average • Can be different at any particular instant

  37. General Circulation • A look at the average global winds can give us insight into • 1) Driving mechanisms behind the winds • 2) Model of how heat is transferred from the tropics to the poles • Already established the fact that uneven heating of the earth is what causes weather • Including wind • In a nutshell: tropics get excess energy while polar regions have an energy deficit • Wind tries to balance things

  38. Single Cell Model • Very simplified model but still gives us some insight. 3 Key assumptions: • 1) All water over the earth • 2) Sun always over equator • 3) No rotation • Result: • High pressure at the poles • Low pressure at the equator • 1 huge convective cell in each hemisphere • Called “Hadley Cells” after the guy who postulated the idea.

  39. Single Cell Model • Cold air flows toward tropics from poles (H to L) at the surface • Warm air flows toward poles from tropics aloft • Just like a sea breeze circulation • So, some of the excess energy at the tropics is transferred to the poles • What if we get rid of assumption #3 and let the earth rotate?

  40. Three Cell Model • Now we have 3 cells in each hemisphere rather than one. • More realistic • Still have H pressure at poles and L pressure at equator • Also now, H and L pressure at 30º and 60º respectively H L H L H L H

  41. Three Cell Model • Look at the vertical motion where the cells meet: • Equator: Rising air • Lots of rain here • 30º: Sinking air • Most of the world’s deserts lie near this latitude • 60º: Rising air • This is where most big mid-latitude storms form • So this model does a better job with surface winds

  42. Three Cell Model • Not too bad huh? • Everything is shifted to the south due to seasons • Cloud bands where the model says low pressure should be • Rising air Today

  43. Surface Winds • Intertropical Convergence Zone (ITCZ) • near the equator • tropical systems form here • Trade winds • named such because sailing vessels used these winds to travel west • not much wind poleward of the trades (horse latitudes) • Westerlies • where we live • Polar Easterlies

  44. The Real World • Still two assumptions in the three cell model • all water/no land and sun always over equator • These factors change things a bit and give us the real pressure/wind fields…….

  45. The Real World • Semi-permanent features • Lows near 60º • Highs near 30º January

  46. The Real World • Strength and position of these features changes with the seasons • The Highs on either side of the U.S. have a great impact on weather at each coast July

  47. Semi-permanent Highs • Generally sinking air on the east side of highs and rising air on the west side • Summers are dry on the west coast and wet on the east coast

  48. Semi-permanent Highs • LA (west coast) gets almost no rain during the summer • Atlanta (east) gets a great deal of rain during the summer months

  49. Jet Streams • Fast flowing “rivers” of air at upper levels • 33,000-46,000 ft usually • On average: 100’s - 1000’s miles long, 100 miles wide, 1 mile deep • Existence first confirmed by military pilots during WWII • Suspected beforehand due to fast moving cirrus clouds • Important because they can show us where cold air is and where storms will track • Also if storms will intensify

  50. Jet Streams • Like all winds, jets occur due to horizontal differences in pressure • Sharp north/south temperature/pressure differences aloft • The temperature contrast is at a maximum during the winter • Jet streams are strongest during winter season

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