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The General Circulation of the Atmosphere

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General Circulation of the Atmosphere

Large scale flow of the atmosphere

Focus on both upper level and lower level winds

Definitions:– Zonal winds: – Meridional winds:– Westerly winds; come from the west– Southwest winds, come from the southwest

East-WestEast-WestNorth-southNorth-south

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Atmospheric Scales of Motion

Scale Time Scale Distance Scale Examples

Macroscale -Planetary Weeks to years Westerlies,

trade winds

-SynopticDays to weeks Cyclones, anticyclones and hurricanes

Mesoscale Minutes to days Land-sea breeze,thunderstorms and tornadoes

Microscale Seconds to minutes Turbulence, dust devils and gusts

500 – 25,000 miles500 – 25,000 miles

50 – 3,000 miles50 – 3,000 miles

1 – 50 miles1 – 50 miles

< 1 mile< 1 mile

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Conservation of Angular Momentum

Describes motion of air/earth on a rotating planet

Says if all winds blew from one direction (east/west), planet’s rotation rate would have to change

If the atmosphere speeds up (stronger westerly winds) then the solid Earth must slow down (length-of-day increases).

So, winds are westerly some places, easterly at others.

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Single Cell Model

Early description of general circulation George Hadley (1685-1768) developed this model

Assumptions:– Earth is primarily heated in the tropics– Thermally direct circulation results from heating

differences– Low pressure at equator, high-pressure at the poles

Surface heat imbalance produces air movement to balance.

Not realistic, because it violates COAM Not realistic, because it violates COAM

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Three Cell Model

Proposed to explain how the Earth’s heat balance is maintained

Good simple model of global circulation

Terms:– Hadley Cell: The tropical circulation

ITCZ - intertropical convergence zone Horse Latitudes:

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Global wind patterns

Must conserve angular momentum, so direction varies Westerlies: what we experience here in U.S.

– Weather in east coast usually starts here. Trade Winds: from NE in NH, SE in SH

– E.g. Hawaii– El Nino: when trade winds reverse directions, which

cools W. Pacific and warms E. Pacific oceans Doldrums

– Area of no wind (ITCZ) where only air movement in up!

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Semi-permanent pressure systems

Part of 3-cell model Exist due to seasonal changes and land-sea

differences (specific heat) Change with the seasons. Land:

– predominantly high pressure in winter– predominantly low pressure in summer

Water:– predominantly low pressure in winter– predominantly high pressure in summer

Think of cold areas as having the high pressure. Warm areas/low pressure

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ITCZ Seasonal variations

• Intertropical Convergence Zone (ITCZ)

• Is a semi-permanent low-pressure system where the trade winds meet and converge, forcing rising motion

• ITCZ changes with season•This is a reflection of the changing location of the Hadley Cell

• The ITCZ follows the sunTo the north in JuneTo the south in December

Positions of intertropical convergence zone in January and July

Positions of intertropical convergence zone in January and July

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Westerly winds in the upper atmosphere

The Jet Stream Caused by differences in temperatures at the surface,

or uneven heating of the surface Higher heights (of pressure levels) exist in the tropics. Pressure gradient exists across middle latitudes Pressure gradient force is stronger in winter than

summer– larger temperature gradient.

Upper atmosphere winds are predominately – westerly in both hemispheres.

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The Jet Stream

Due to large differences in temperature at the surface– Strongest in winter

Flow from west to east in a wavy pattern ~35,000 feet above sea level Illustrates that atmosphere is full of waves that

– Bring heat from equator to poles– Bring cold air from poles to equator– Control our weather here in the mid-latitudes

Fronts, low-pressure troughs, and high-pressure ridges

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