Overview of Climate

V. Ramaswamy

(“Ram”)

U.S. National Oceanic and Atmospheric Administration

Geophysical Fluid Dynamics Laboratory

Princeton University [USA]

Lecture # 1

• Energy balance of the planetary surface-atmosphere system.

• Factors governing the global-mean energy balance.

• Radiative and Radiative-Convective Equilibria.

Temperatures of Planets

Planet Dist. S0 A Te Tm Tsfc GHE from (W/ (K) (K) (K) (K) Sun m2) (AU)

VENUS 0.72 2640 0.75 232 235 730 495

EARTH 1.0 1366 0.30 255 254 288 34

MARS 1.52 570 0.15 217 218 223 5

AU = Astronomical unit = 1,5 x 108 km

S0 = Solar irradiance at planetGHE = GreenHouse Effect

Can you estimate a “WhiteHouse Effect” viz., how much the Earth is kept ‘cool’ owing to its reflecting abilities ?

Factors involved in the Global Heat Balance

• Gradients in Temperature• Amount and location of species (gases, aerosols

and clouds)• Radiative (absorption, emission, reflection)

properties of species in the electromagnetic spectrum

• Radiative properties of the surface

Convection (arising due to differential heating of surface and atmosphere)

Large-scale dynamical flows caused by planetary rotation, topography, and land-sea contrast

uv vis

near-ir longwave

Methane

Nitrous oxide

Oxygen; Ozone

Carbon dioxide

Water vapor

Solarblackbody

fn.

Earth’s “effective”

blackbody fn.

CFCs

Clouds,Aerosols

activethroughout

spectra

CFCs

CH4, N2O

CO2 (15 micronBand

Curve of growth of absorption by gases

GCM vs. AIRS – Global annual mean

spectra Clear-sky

Total-sky

Note: Radiances (represented through brightness temperatures) are in the unit of Kelvin.

[Huang et al. 2007 GRL]

E = surface emitted flux (goes as T4)F = Longwave flux at TOA

(E – F) = a measure of “greenhouse effect”

Raval and Ramanathan (1989)

CLEAR Sky (over Oceans)

Total Outgoing LW radiation ~ 240 W/m2

VIS

Near-IR

0.01

100

GasDepth

CloudSS alb.

0.999

0.9

Water clouds can usually be treated as “blackbody” radiative agents in thelongwave, just like the surface.

256

128

0

150

250

350

Reflected shortwave radiation (W m-2)

Outgoing longwave radiation (W m-2)

Aqua CERES Measurement

Global, annual-meanNet SW = Net LW = 240 W/m2

Vertical profile of temperature(RE and RCE conditions)

• SW and LW components only Radiative Equilibrium (RE), BUT this is not the real story

• Balance against the radiative cooling of atmosphere

• Considerations for the global,annual-mean• Horizontal- and time-averaging a

compensating ‘force’ acting in the vertical• This ‘force’ acts to redistribute heat in the

vertical• This ‘force’ is CONVECTION Radiative-

Convective Equilibrium (RCE)

Strictly speaking, an assumption is that contributions from large-scale dynamics is negligible.

Concept of ‘lapse rate’ the gradient of temperature with respect to height (or pressure).

Question

If the solar irradiance available to the Earth were to change by 2% from the present-day value, what would be the response in the effective planetary temperature?

[The solution is the same as that for doubling of carbon dioxide in the absence of feedbacks]

Principal Sources

• “Physics of Climate” by A. OORT and J. PEIXOTO

• “Global Physical Climatology” by D. HARTMANN

• Radiation notes [JOS LELIEVELD, MPI-Mainz]

• Atmospheric Radiation lectures [Boulder, 1986]

• Intergovernmental Panel on Climate Change, 2001 and 2007, Working Group I (The Physical Science Basis)

• Y. Huang, Ph. D. thesis (Princeton University, 2008)