Transcript Met 10 - De Anza College

MET 10
The General Circulation of the
Atmosphere
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General Circulation of the Atmosphere
MET 10
 Large scale flow of the atmosphere
 Focus on both upper level and lower level winds
 Definitions:
– Zonal winds: East-West
– Meridional winds: North-south
– Westerly winds; come from the west
– Southwest winds, come from the southwest
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Atmospheric Scales of Motion
MET 10
Scale
Time ScaleDistance Scale Examples
Macroscale
-Planetary
Weeks to years
500 – 25,000 miles
Westerlies,
trade winds
-Synoptic
Days to weeks
50 – 3,000 miles
Cyclones, anticyclones
and hurricanes
Mesoscale
Minutes to days
Microscale
Seconds to minutes
1 – 50 miles
< 1 mile
Land-sea breeze,
thunderstorms and
tornadoes
Turbulence, dust
devils and gusts
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Conservation of Angular Momentum
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 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
MET 10
 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
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Three Cell Model
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 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
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 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
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 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
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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 sun
To the north in June
To the south in December
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Positions of intertropical convergence zone in January and July
Westerly winds in the upper atmosphere
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 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
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 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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