Atul Kapurakapur@rsmas.miami.edu

from a paper by

S. Manabe and Richard T. Wetherald

1966

No atmosphere

Fixed emissivity and absorptivity at a given layer

But in reality Absolute

Humidity is a strong

function of temperature?

Fixed distribution of absorbers

including water vapor (or Absolute humidity)

(Manabe and Srickler, 1964)

4

2

2

14 eTSAR

R

Why fixed Relative Humidity?

DJF

JJA

Zonal-mean Absolute humidity (g kg-1)

DJF

JJA

Zonal-mean Relative humidity (%) (Peixoto and Oort, 1992)

Strong seasonal variation in Absolute Humidity

Weak seasonal variation in Relative Humidity

No atmosphere

Fixed emissivity and absorptivity at each layer

Fixed distribution of absorbers including water vapor (or Absolute humidity)

4

2

2

14 eTSAR

R

But Absolute Humidity is a

strong function of temperature?

(Manabe and Srickler, 1964)

Fixed distribution of Relative humidity

(Telegadasand London, 1954)

(Manabe and Wetherald, 1967)

Mixing ratio is now allowed to change with change in temperature

Another degree of freedom PARTIALLY released

Constrained by the condition of fixed Relative Humidity

TwRHw s

Mixing ratio of CO2 is assumed to be constant (300 ppm by volume)

(Herringand Borden, 1965)

Ozone

(London, 1956)

Cloud characteristics

Radiative(Fixed abs. humidity)

Radiative(Fixed relative humidity)

Radiative-Convective(Fixed relative humidity)

Cooler Atmospher

ic Temperatu

re+

Fixed relative humidity

Less moistur

e

Less greenhouse effect

Further temperature drop at

the surface

(Hergesell, 1919)

Self Amplification effect

Increase in

moisture in a given volume of

air

t

w

C

LCC

ppp 1

Warmer Atmospher

ic Temperatu

re+

Fixed relative humidity

4zT

OLR4zT

4cT

OLR4cT

Increase in height

of effective source of

OLR

OLR lesser (than in case

of fixed absolute humidity)

Slower approach towards equilibri

um

Lesser Radiative cooling

Increase in latent energy of

airIncrease in effective

heat capacity

Slower Approach towards

equilibrium

Approach of mean temperature towards equilibrium

Fixed abs. humidity (I)

Fixed RH with Cp dry (II)

Fixed RH with effective Cp (III)

4zT

OLR4zT

4cT

OLR4cT

Higher value of

Solar constant

+Fixed

relative humidity

Increase in temp.

+But OLR less than expected

Further inc. in temp. to

increase OLR at top

of atmosphere

Higher sensitivit

y of temperature upon

Solar constant

Fixed abs. humidity

Fixed RH

Solar constant

Surf Temp.

For fixed RH, equilibrium temp. is almost twice as sensitive as for fixed absolute humidity

Difference in sensitivity decreases with temp.

Mixing ratio of water vapor is lower at lower temperatures

Self-amplification

effect

More CO2 results in warmer troposphere and warmer surface

More CO2 results in colder stratosphere

Stratospheric temp. much more sensitive to CO2 than troposphere.

Inc. in CO2

+Fixed

relative humidity

Increase in temp.

+OLR less

than expected

Increased sensitivity to CO2

Sensitivity of temperature upon CO2

almost double as compared to fixed absolute humidity

Larger albedo colder the temperature

Influence of surface albedo vanishes with height

Sensitivity upon surface albedo

almost double as compared to fixed absolute humidity

Time required for radiation-condensation relaxation is almost double than that required for radiation relaxation.

For fixed RH, sensitivity of surface temp. upon solar constant, cloudiness, surface albedo, and CO2 content is almost twice as compared to that for fixed absolute humidity

Doubling of CO2 with fixed relative humidity increases surface temp. by about 2.3°C.