Atmospheric Circulation

Air Pressure Experiments Lessons from Paper Cup experiment: 1. Air pressure is present everywhere Air “tries to” move from an area of higher pressure to an area of low pressure Lesson from Pop Bottle experiment: 3. Warm air occupies more space than an equal number of molecules of cold air

Wind Power Generation in Southern Alberta

“Don’t try this at home”

The speed and direction of wind is determined by three forces: Pressure Gradient Force Inertial Coriolis Force Friction Force

H L Pressure Gradient Force Definition: The difference in atmospheric pressure per unit distance PGF acts at right angles to isobars of equal pressure H L 102.2 99.8 101.4 100.6 600 km Pressure Gradient Force = 2.4 kPa / 600 km = 0.4 kPa / 100 km

Where is the PGF forecast to be strongest today ? Regina or Lethbridge? Solution: Check the spacing of the isobars of equal surface pressure Source: http://weatheroffice.ec.gc.ca/data/model_forecast/592_100.gif

The Inertial Coriolis Force Objects moving in an “absolute” straight line between two points on the Earth’s surface are deflected: To the RIGHT in the N hemisphere To the LEFT in the S hemisphere Why ? The Earth rotates more quickly at the equator.

Visualizing the Coriolis Force Source: NASA

The Friction Force H L Surface roughness decreases wind speed Reduces impact of Inertial Coriolis Force Winds cross isobars, spiralling out of ANTICYCLONES (H), and into CYCLONES (L) H L

Can you infer wind direction and relative speed from this map ? weather.unisys.com

Sea level pressure: Altitude Correction Source: Ahrens (1994)

Weather symbols and wind barbs

Classic Low Pressure System In Temperate Latitudes 0600h GMT APRIL 5 2003 NORTH- EAST WINDS SHARP COLD FRONT WARM, MOIST SOUTHERLY FLOW www.atmos.washington.edu

Cold Front

Arctic high pressure drives cold arctic air behind low

Warm Front Not as steep a division as in a cold front                                                                                                                                                                                                                          Not as steep a division as in a cold front It takes longer to scour out surface air (warm air rises)

The weather pattern right now COOL NW WIND WARM FRONT WARM, SOUTH WIND COLD FRONT HURRICANE ISABEL

Main Low and High Pressure Zones Atmospheric Circulation Main Low and High Pressure Zones Equatorial Low Pressure Trough Subtropical High Pressure Cells Subpolar Low Pressure Cells 4. Weak Polar High Pressure Cells

Atmospheric Circulation Overview POLAR CELL FERREL CELL HADLEY CELL

Equatorial low pressure trough (warm, wet) High solar angle Consistent daylength Convergence Heating ITCZ shifts with season

Hadley Cells 1. Warm, moist air rises in equatorial low Cools, condenses, and causes heavy rain Outward flow to subtropical high at high altitude Air descends in subtropical high Heats, compresses and becomes very dry The subtropical high provides the gradient for trade winds and westerlies eg. Bermuda/Azores and Pacific/Hawaii highs

Strahler and Strahler (2002)

Ferrel Cells Between subtropical highs and subpolar lows Poleward transport of excess heat through eddies and migration of lows toward polar front Strong low pressure develops in a belt around Antarctica, near the Aleutians and near Iceland Lows strongest in winter (shift and diminish periodically, especially in the summer) Why ? Water much warmer than land in winter leading to lower pressure over oceans

H L Air tends to be unstable in low pressure (tendency to rise) Air tends to be stable in high pressure (tendency to fall) (more on stability in next class)

WINTER SUMMER Generalized Overview of Seasonal Surface Pressure

Average Global Surface Pressure in January and July Can you explain the monsoon season of the Indian sub- continent with this chart ?

Polar High Pressure Cells Tendency for higher pressure near poles than at the polar front Anticyclonic flow develops Weak and variable polar easterlies result (stronger in southern hemisphere) In northern hemisphere winter, the polar front usually lies over Canada and Russia, (further south than in the summer)

Geostrophic Winds 500 mbar height map Lower heights where air is cold Airflow parallel to isobars in upper troposphere Why ? Combination of PGF and Coriolis force Source: http://weatheroffice.ec.gc.ca/data/model_forecast/134_100.gif

Effect of Air temperature on 500 mb heights Source: Ahrens (1994)

Upper Atmospheric Circulation Jet Streams A band of wind in the upper troposphere 150 – 500 km wide 0.9-2.2 km thick Speeds may exceed 300 km/h Polar Jet Stream: Between Polar and Ferrel cells Subtropical Jet Stream: Between Hadley and Ferrel Cells

Source: http://apollo.lsc.vsc.edu

Source: http://apollo.lsc.vsc.edu

Jet Stream Cross Section “Rivers” of strong wind where cold and warm meet 18 000 m 12 000 m Tropopause height 6 000 m Discontinuity or step in tropopause height See: www.avsim.com/avwx/avsim_wxus_jetstream.html

Subtropical Jet Stream Polar Jet Stream Meanders from 30-70° N or S Moves more poleward in summer Influences (and is influenced by) storm paths Subtropical Jet Stream Meanders from 20-50° N or S May occur simultaneously with Polar Jet in NA

Rossby Waves The polar jet stream follows the Rossby Waves Rossby Waves are undulations in the upper-air westerlies extending from the middle to upper troposphere Form along the polar front Mechanism of poleward heat transport

(Strahler and Strahler, 2002)

Smaller Scale Winds 1. The Land-Sea Breeze Land heats up in morning sun, causing air molecules to rise Air pressure drops over land as molecules removed http://cimss.ssec.wisc.edu/wxwise

High pressure develops over the ocean In the evening, the land cools faster than the sea and the process is reversed

2. Mountain Valley Breezes Daytime The sun heats the hillslope, causing air to move up the slope - a convection loop forms Night Night radiation cools the slopes Cooler, denser air moves downslope Source: http://apollo.lsc.vsc.edu

3. Katabatic Winds Air cools on a plateau or sloping terrain, becomes more dense and descends Winds get faster and faster downslope Relatively warm water at base can further increase winds, which can be very strong as a result Can occur on large scale (eg. Greenland, Antarctica) Also referred to as gravity drainage winds

Oceanic Circualtion Water piles up around equator due to trade winds Along western edge of oceans, water spills N and S along shorelines of continents (also downwelling) Upwelling occurs near east edge of oceans (west coasts)

Upwelling of cool waters

The Thermohaline Circulation (1) Intensive cooling at the ocean surface in North Atlantic (2) Northward transport of salty surface water from lower latitudes (both increase the density).