Radiative-Convective Models Manabe and Strickler (1964)

Course Notes chapter 5.1

The Hydrological CycleHadley Circulation

Climate ModelingLecture 8

Prepare for Mid-Term (Friday 9 am)

• Review Course Notes chapters 1-2.6, 5.1

• Review Textbook chapters 1, 2.1, 3.1, 3.2., 3.4, 4.1, 4.2

• Review lectures (including this one)

• Come with questions to Wednesday’s review session

Manabe and Strickler (1964)

Radiative transfer models resolvefrequency bands.

Radiative fluxes only (pure radiative equil.) gives very high surface temps.

This leads to low densities and instability, which will cause convection.

Convective overturning, in the presence of liquid water at the surface (ocean), will lead to moist adiabatic lapse rate.

Manabe and Strickler (1964)

CO2=290 ppmv constant with height

Manabe and Strickler (1964)

Ozone leads to warming in stratosphere, but not at surface.

CO2 leads to surface warming of ~10 K.

Manabe and Strickler (1964)

Ozone absorbs sunlight in stratosphere, which leads to warming.

Stratosphere is cooled mainly by long wave radiation due to CO2.

Long wave radiation by H2O and CO2 cool the troposphere.

Convective fluxes heat the troposphere by transporting heat from the ground upwards.

Manabe and Strickler (1964)

Effect of Clouds

Low and mid level clouds cool the surface and troposphere.

High clouds can heat the surface.

Average effect of clouds is to cool the surface and troposphere.

Cloud radiative forcing ΔRTOA as a function of change in albedo and cloud top altitude. Negative values are show as dashed lines. S = 342 Wm-2, FLWclear = 265 Wm-2, Ts = 288 K, Γ = 6.5 K/km. From Hartmann (1994).

Conclusions RCMs

• Radiative transfer heats the surface

• Convection leads to upward heat transport causing temperature in the troposphere to follow the moist adiabatic lapse rate

• Absorption of shortwave radiation by ozone in the upper atmosphere leads to the temperature increase in the stratosphere

Reading• Review tomorrow

• Mid-Term Friday

• Monday:

‣ chapter 5.3 in script “General Circulation Models”

‣ chapters 3.2.3 and 3.3.1 in textbook

Specific Humidity & Clausius-Clapeyron Equation

• specific humidity q is the ratio of the mass of water vapor over the mass of moist air

• the saturation specific humidity qsat is the specific humidity at which the air parcel is saturated in moisture

• qsat depends exponentially on temperature (see graph)

• the relative humidity RH = q/qsat

qsat

Evaporation and Condensation

• Evaporation is the transition from the liquid to the vapor phase

• It occurs when the relative humidity of the air is < 100% (the lower the RH the more evaporation; E ~ qsat - q)

• Evaporation leads to cooling of the surface because energy is put into latent heat (this is why we sweat)

• Condensation is the transition from the vapor to the liquid phase

• It occurs when the air is at 100% RH and when cloud condensation nuclei are available

• Condensation leads to warming due to the release of latent heat

• Water vapor transport = latent heat transport

Remember: Latent heat of vaporization/

condensation = 2300 J/g

(see experiment)

(blowing over your coffee leads to faster cooling: wind increases

evaporation)

Experiment

• Pump air into the bottle until the lid pops. What happens ?

• Now light a match and blow a little smoke into the bottle. Repeat the experiment. What happens ?

The Global Water Cycle

Trenberth et al. (2007) J. Hydrometeor.

Evap

orat

ion

Prec

ipita

tion

ERA15 1979-1993European Reanalysis

ERA401959-2002

Meridional Water Vapor Flux

FWV = E − Pdxdy 'y '=90S

y

∫x=0

360

∫

Trenberth et al. (2007)

‣ Simple Model by Held and Hou (1980)‣ based on conservation of angular momentum in a two layer

model

Hadley Circulation Script chapter 5.2

Angular Momentum M=ru

no friction

friction

zonal velocity

At equator:(assume no zonal flow)

At latitude Φ:

Φ=30°N: uM = 110 m/s

Assume heating:

Calculate width of Hadley Cell:

Vertical shear:

Thermal wind balance:

3000 km or 30°

Width is in good agreement with observations but circulation is much too slow.

‣ Lindzen and Hou (1988) showed that the circulation is improved by considering the seasonal cycle

‣ annual mean conditions lead to weak heating

‣ seasonal heating is much stronger if maximum heating is shifted slightly away from equator

Hadley Circulation and Zonal Wind

Schmittner et al. (2011)

Reanalysis

GC

M