1. The General Circulation of the Atmosphere

2. 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; comes from the west
Southwest winds, comes from the south west

3. 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-West
North-south

4. Atmospheric Scales of Motion
Scale Time Scale Distance Scale Examples
Macroscale
-Planetary Weeks to years Westerlies, trade winds
-Synoptic Days 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

5. Atmospheric Scales of Motion
Scale Time Scale Distance Scale Examples
Macroscale
-Planetary Weeks to years Westerlies, trade winds
-Synoptic Days 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 miles
50 – 3,000 miles
1 – 50 miles
< 1 mile

7. 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.

9. Single Cell Model Early description of general circulation
George Hadley ( ) developed this model
Assumptions:
Surface heat imbalance produces air movement to balance.

10. Single Cell Model Early description of general circulation
George Hadley ( ) 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

12. Three Cell Model
Proposed to explain how the Earth’s heat balance is maintained
Terms:
Hadley Cell: The tropical circulation

13. 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:

16. 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!

18. 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

22. 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 sun
To the north in June
To the south in December

23. Positions of intertropical convergence zone in January and July

24. Positions of intertropical convergence zone in January and July

25. Westerly winds in the upper atmosphere
Higher heights (of pressure levels) exist in the tropics.
Pressure gradient exists across middle latitudes
Pressure gradient force is stronger in winter than summer
Upper atmosphere winds are predominately

26. 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.

27. 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

31. • A
• C
• B