Download Met 10 - De Anza College

MET 10
The General Circulation of the
Atmosphere
1
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
3
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
5
MET 10
6
Conservation of Angular Momentum
MET 10
 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.
7
MET 10
8
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
10
MET 10
11
Three Cell Model
MET 10
 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:
13
MET 10
14
MET 10
15
Global wind patterns
MET 10
 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!
16
MET 10
17
Semi-permanent pressure systems
MET 10
 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
18
MET 10
19
MET 10
20
MET 10
21
ITCZ Seasonal variations
•
•
•
MET 10
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
22
Positions of intertropical convergence zone in January and July
Positions of intertropical convergence zone in January and July
Westerly winds in the upper atmosphere
MET 10
 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.
26
The Jet Stream
MET 10
 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
27
MET 10
29
MET 10
30
•A
•B
•C