1. EVAT 554 OCEAN-ATMOSPHERE DYNAMICS
LECTURE 8
LARGE-SCALE ATMOSPHERIC CIRCULATION
(Reference: Peixoto & Oort, Chapter 3,7)

2. LARGE-SCALE ATMOSPHERIC CIRCULATION

3. IDEALIZED ATMOSPHERIC CIRCULATION
THERMALLY-DIRECT CIRCULATION FOR AN IDEALIZED
NON-ROTATING EARTH

4. Vertical temperature profile
THERMAL PROFILE
Vertical temperature profile
Temperatures increase with altitude in the stratosphere ( km) due to absorption of UV light by ozone (O3).
There is limited exchange of material between the troposphere and the stratosphere (i.e., across the tropopause)

5. THERMAL PROFILE
Simplest possible reasonable model for surface temperature
steady
no motion
no thermal diffusion
no water
Ignore GHGs
Consider global mean surface temperature, Ts
esTS4 =(1- a0)S0/4
TS4 =(1- a0)S0/4es
Tearth = [( 0.7)(343 W m-2)/(1)(5.67 x 10-8 W m-2 K-4)]1/4
= 255 K (-18 C)
+33K (GHG) = 288K
Earth
S0 = 1370 W m-2

6. THERMAL PROFILE
Simplest possible reasonable model for surface temperature
steady
no motion
no thermal diffusion
no water
Ignore GHGs
Consider surface temperature as a function of latitude
esT4 =(1- a)S/4
T4 =(1- a)S/4es
Assume small latitudinal variations about the global mean temperature TS
linearization
T4 =(1- a)S/4es
4T3 DT =[(1- a)/4es] DS
T3 ~TS3
DT =[1- a(f)][S0 /16es TS3] (cosf-2/p)
Where:
DS~S0[cosf- 2/p]

7. DT =[1- a(f)][S0 /16es TS3] (cosf-2/p)
THERMAL PROFILE TS
DT =[1- a(f)][S0 /16es TS3] (cosf-2/p)

8. DT =[1- a(f)][S0 /16es TS3] (cosf-2/p)
THERMAL PROFILE TS
DT =[1- a(f)][S0 /16es TS3] (cosf-2/p)

9. GLOBAL ENERGY BALANCE
343 W/m2

10. EIN =S( )  (S/4)(1- a)cosf
GLOBAL ENERGY BALANCE
EOUT = esT4
EIN =S( )  (S/4)(1- a)cosf

11. GLOBAL ENERGY BALANCE
Why don’t the tropics continue to heat up and the poles continue to cool?

12. GLOBAL ENERGY BALANCE
Eout
Ein

13. Advective heat transport
GLOBAL ENERGY BALANCE
Eout
Ein
steady
no motion
no thermal diffusion
no water
Advective heat transport

14. THERMALLY-DRIVEN CIRCULATION

15. THERMALLY-DRIVEN CIRCULATION
Approximate circulation as steady, linear, two-dimensional (meridional-vertical plane)
We will derive the circulation as a perturbation that arises in response to a mean imposed thermal gradient
THERMALLY-DIRECT CIRCULATION FOR AN IDEALIZED
NON-ROTATING EARTH

16. THERMALLY-DRIVEN CIRCULATION
Approximate circulation as steady, linear, two-dimensional (meridional-vertical plane)
Vertical momentum balance
We will derive the circulation as a perturbation that arises in response to a mean imposed thermal gradient
Boundary conditions:
Start out assuming horizontally uniform surface pressure
(1) no slip
(2) no normal flow
v’=w’=0
Pole Equator
Meridional momentum balance
v’=w’=0
v’=w’=0
v’=w’=0
THERMALLY-DIRECT CIRCULATION FOR AN IDEALIZED
NON-ROTATING EARTH

17. THERMALLY-DRIVEN CIRCULATION
Approximate circulation as steady, linear, two-dimensional (meridional-vertical plane)
Continuity equation
Not zero for compressible fluid!
Boundary conditions:
(1) no slip
(2) no normal flow
(linearized)
v’=w’=0
Pole Equator
v’=w’=0
v’=w’=0
zero near boundaries
v’=w’=0
Implies the circulation pattern shown!
THERMALLY-DIRECT CIRCULATION FOR AN IDEALIZED
NON-ROTATING EARTH

18. HADLEY CELL CIRCULATION
THERMALLY-DRIVEN CIRCULATION
Approximate circulation as steady, linear, two-dimensional (meridional-vertical plane)
HADLEY CELL CIRCULATION
v’=w’=0
v’=w’=0
v’=w’=0
v’=w’=0
Pole Equator
THERMALLY-DIRECT CIRCULATION FOR AN IDEALIZED
NON-ROTATING EARTH

19. HADLEY CELL CIRCULATION
Hadley Cell Circulation is a thermally direct circulation that transports sensible heat poleward N
Equator S

20. HADLEY CELL CIRCULATION
Hadley Cell Circulation also transports latent heat owing to different adiabatic lapse rates for dry and moist air
Dry/Cold
Dry Adiabatic warming (rapid)
Moist Adiabatic cooling (gradual)
Dry/Warm
Moist/Hot N
Equator S