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Wind: Global Systems

Chapter 10. Wind: Global Systems. General Circulation of the Atmosphere. General refers to the average air flow, actual winds will vary considerably. Average conditions help identify driving forces. The basic cause of the general circulation is unequal heating of the Earth’s surface

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Wind: Global Systems

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  1. Chapter 10 Wind: Global Systems

  2. General Circulation of the Atmosphere • General refers to the average air flow, actual winds will vary considerably. • Average conditions help identify driving forces. • The basic cause of the general circulation is unequal heating of the Earth’s surface • Warm air is transferred from the Tropics to the Poles • Cool air is transferred from the Poles to the Tropics

  3. General Circulation of the Atmosphere • Single Cell Model • Assume • uniform water surface • Sun always directly overhead the Equator • Earth does not rotate Result: huge thermally direct convection cell rising at equator and sinking at poles(Hadley) • Three Cell Model • Allow earth to spin = three cells (Hadley, Ferrell, Polar) • Alternating belts of pressure starting with L at Equator (L H L H) (Eq, Sub Trop, Sub Polar, Polar) • Alternating belts of wind with NE just North of Equator

  4. General Circulation of the Atmosphere • Average Surface Wind and Pressure: The Real World • Semi-permanent high and lows • Northern vs. Southern Hemisphere • Major features shift seasonally with the high sun • North in July • South in December

  5. General Circulation of the Atmosphere • General Circulation and Precipitation Patterns • Rain where air rises (low pressure) • Less rain where air sinks (high pressure) • Average Wind Flow and Pressure Patterns Aloft • North-South temperature and pressure gradient at high altitudes creates West-East winds, particularly at mid to high latitudes.

  6. Jet Streams • 100-200 kt winds at 10-15km, thousands of km long, several 100 km wide and a few km thick (polar and subtropical) • Observations: Dishpan Experiment • Illustrates waves, with trough and ridge, develops in a rotating pan with heat on the exterior and cold at the center.

  7. Jet Streams • Polar and Subtropical Jet • Established by steep temperature and pressure gradients between circulation cells. • Between tropical-mid-latitude cell (subtropical) and mid-latitude-polar cell (polar) • Gradients greatest at polar jet • Topic: Momentum • Low-latitudes: atmosphere gains momentum • High-latitudes: atmosphere losses momentum • Conservation of Momentum

  8. Jet Streams • Other Jet Streams • Tropical easterly jet stream • Low-level jet (nocturnal) • Polar night jet streams

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