20 terms

General Circulation

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General circulation
Refers to planetary-scale winds and pressure, features that appear in the time-averaged state (average of the atmosphere)
-Represents "average air flow around the globe".
-Created by unequal heating at earth's surface (the difference in temperature between the equator and the poles)
-General circulation's function is to transport heat pole ward
Climate and general circulation of the atmosphere are related to
Energy balance
Transport processes
The three cell model
What happens in general circulation
Intense heating at equator. Warm air rises from equator and circulates symmetrically to north and south pole; cold air sinking at the poles, circulated back to equator for reheating/circulation

3 cell circulating model, Rising air -> low pressure -> transfer heat/moisture to poles; Middle latitudes air sinks -> high pressure
Single Cell model
-Hypothetical circulation for a non-rotating Earth.
-Developed by Hadley.
-Assumes surface is uniformly covered by water, sun is always directly over the equator, and earth does not rotate.
-Induces the formation of the ITCZ
ITCZ
Intertropical Convergence Zone. Hovers over equator.

Is the birthplace of tropical storms.
What happens in Single Cell model?
-Maximum heating at equator, causes air to become buoyant and rise (air is warm).
-Then it begins to diverge toward the poles
-Once it hits the pole, it gets cool and starts to sink back to the surface and return to the equator
-Moves from an area of high pressure to low pressure
Problems with Single Cell Model
-One big circulation cell; air circulation around the globe would be simple (and the weather boring) if the Earth did not rotate and the rotation was not tilted relative to the Sunh
-Thought to be too simple because it doesn't take into account the Coriolis force in the Northern hemisphere, which deflects south moving surface air to produce easterly winds at all latitudes (and hus neglects Earth's rotation)
-Sinking air near the 30 degree mark
Three Cell Model
Proposed by William Ferrel
-Single cell is broken up; circulations are divided in each hemisphere into three cells:
- Hadley Cell (tropics/subtropics)
- Ferrel Cell (middle latitudes)
- Polar Cell

-Earth still heated at the equator and cooling at the poles.
-Still assumes surface is covered by water and sun over equator
-BUT NOW INCLUDES the CORIOLIS FORCE WHICH ROTATES THE EARTH
What happens in the Three Cell Model?
-HADLEY cell circulates air between topics and subtropics (approximates real world well). Warm air rises at the equator
-As the flow moves poleward, it begins to cool and sinks at 20°-35° latitude.
-Trade winds meet at the equator, in a region with a weak pressure gradient, called the doldrums.
-FERREL cell circulates air in the middle latitudes.
-Circulation (at 60°-90°) within a POLAR cell produces polar easterlies, or surface flows that move toward the equator.
-POLAR cell circulates air at the poles (Ferrel and Polar cells do not approximate the real world as well)
Problems with Three Cell Model
-Doesn't account for land/continents
-For upper air motions, the three-cell model is unrepresentative (ex. the Ferrel cell implies easterlies in the upper atmosphere where westerlies dominate)
are thermally-driven
Polar, Ferrel and Hadley cells
Hadley Cell
• Strong solar heating leads to rising air which
diverges towards the poles.
- Creates Intertropical Convergence Zone (ITCZ)
• Aloft, air moves poleward but acquires a west to east motion.
• Air descends at 20°-30° latitude to form subtropical highs.
- Creates desert conditions
• Air moves towards the equator at the surface and a weak Coriolis force creates the NE trade winds.
-Pressure and winds associated with Hadley cells are close
approximations of real world conditions
Ferrel Cell
The westerly circulation of surface winds (prevailing westerlies) between 30°-60° latitude
• Indirectly caused by the turning of the Hadley and Polar cells.
• Air flows north away from the subtropical high towards the subpolar lows, and acquires a large westerly component
- westerlies
Polar cell
-Circulates air at the poles.
-Circulation (at 60°-90°) within a polar cell produces polar easterlies, or surface flows that move toward the equator.
Jet streams
Area of air above where two air masses of different temperatures converge (e.g. a cold front meeting a warm front). Transport heat pole ward (cold air south and warm air north)

Often have troughs and ridges

More commonly form in the winter, when there is a greater difference between the temperature of the cold continental air masses and warm oceanic air masses.
Polar Front
Narrow, strongly sloping boundaries between warm and cold air (polar front). Pretty narrow bands.
-Strong areas of westerly wind
-Located in upper troposphere, near tropopause
-LEADS TO strong temperature gradients, pressure gradient forces, and the FORMATION of the POLAR JET STREAM
Polar jet
A jet stream found in the upper troposphere above the polar front. 60°.

-Result of the strong temperature contrast across the front (the meeting of warm and cold air).

-Responsible for winter weather

-Spring jet streams = tornadoes

-Low pressure

-Connects mid-latitude to polar regions
Subtropical jet
A jet stream common in the upper troposphere on the poleward side of the Hadley cells. Connects subtropics to mid latitudes

Produced by the conservation of angular momentum.

Weaker than polar jet
Rossby Waves
-Horizontal undulations (isobars are are wavy) in the flow path of the upper air westerlies. Also called long waves.

-Transport energy (warm air poleward and cold air equaotorward. Affects upper level convergence and divergence

-Important for cloud development
easterly trade wind
Driven by a surface pressure pattern of higher pressure in the eastern Pacific and lower pressure in the west. When this pressure gradient weakens, so do the trade winds. The weakened trade winds allow warmer water from the western Pacific to surge eastward, so the sea level flattens out.