Annual-mean TOA radiation (ERBE, W/m2)

Absorbed SW

Outgoing LW

Surface temp (NCEP, oC)

January

July

Surface wind (NCEP, m/s)

January

July

Temperature (oC) and zonal wind (m/s) (NCEP)

January

July

km

10

4

–2

–8

–14

14

8

2

–4

–10

16 20

20

24

28

32

36

24

28

32

36

40

16 20

1

2

3

4

5

T

decreases by 6oC/km increases by 4oC/km

Temperature and potential temperature surfaces

Potential temperature (K) and zonal wind (m/s) (NCEP)

January

July

The “bare rock” temperature

In steady state,

Ein = Eout

Ein = π R2 S (1–)

Eout= 4π R2 T4

R =radius =albedoPutting it all together:

Radiative-convective equilibrium(Manabe & Strickler, 1964)

Cloud–SW interaction

Cloud–LW interaction

Net cloud forcing from simple model(Hartmann, p. 74)

Annual-mean cloud water path (g m–

2)

Annual-mean total cloud amount (%)

Annual mean low cloud amount

Annual mean middle cloud amount

Annual mean high cloud amount

Annual mean SW cloud forcing (ERBE, W m–2)

Annual mean LW cloud forcing (ERBE, W m–2)

Annual mean net cloud forcing (ERBE, W m–2)

Climate feedback: the general ideaWhat happens if we perturb the climate away from its

equilibrium,for instance by increasing CO2 concentration?

+ + +CO2 positive

feedback

T X

+

CO2 negativefeedback

T Y

–

+ + +

• In the real world, both positive and negative feedbacks act simultaneously

• Overall, the negative feedbacks win: otherwise temperature would run away to very high values (runaway greenhouse)

• However, the presence of positive feedbacks means that the temperature increase we get for a given increase in CO2 is greater than we would get in their absence: more bang for the buck

• Dominant feedbacks:– Negative:

• Planck (radiative) feedback• Lapse rate feedback

– Positive :• Surface albedo feedback• Water vapour feedback• Cloud feedback?

Ice albedo feedback

• As surface temperature increases, some of the ice in the polar ice caps melts, exposing ocean or boreal forest

• Ice is much more reflective to sunlight than ocean or forest

• So the feedback goes like this:Increased CO2

—› raises T—› melts some ice—› decreases reflectivity—› more insolation is absorbed —› raises T even further

Water vapour feedback

• Relative humidity stays roughly constant as climate warms

• Since RH = w/ws(T) and ws(T) increases exponentially with T, then humidity increases rapidly with T

• So the feedback goes like this:Increased CO2

—› raises emission level —› raises T —› raises humidity—› raises emission level even further —› raises T even further

Feedback strengths in climate models (Soden&Held, 2006)

Insolation at TOA Absorbed insolation

Insolation at TOA Absorbed insolation

SHF into ground!

The moist equations of motion

latentheatevap

condensation

evaporation

watervapourmixing

temperaturemixing

momentummixing

(friction)

Energetics

Total energy per unit mass:

Rate of change of:

• kinetic energy:

• potential energy:

• thermal internal energy:

• latent internal energy:

Energetics Rate of change of total energy per unit volume:

but:

Finally:

Note that:

moist static energy small

A B

A

B

Seasonal cycle of surface temperature

Tem

p (

oC

)

Amplitude of seasonalcycle (oC)