Lecture 3 read Hartmann Ch.2 and A&K Ch.2 Brief review of blackbody radiation Earth’s energy balance TOA: top-of-atmosphere –Total flux in (solar or SW)= Total flux out (LW) Greenhouse effect The global energy balance Poleward energy flux

Earth’s energy balance - emission temperature Solar luminosity: energy flux from sun 3.9e26 W Flux density a distance d away from sun S0= 1367 W/m2 d=1.5e11 m (Earth to sun) Only area that the planet sweeps out of the beam may get absorbed (shadow area = pi x r2, area of sphere=4xpixr2) Not all the energy that gets to TOA is absorbed, some gets reflected back to space (planetary albedo, alpha) Absorbed solar radiation: S0(1-alpha) pi x r2 The same amount must be returned to space by terrestrial radiation. Emitted terrestrial radiation=sigma x T4 x 4 x pi x r2

Earth’s emission temperature It is the blackbody temperature with which it needs to emit in order to achieve energy balance Solar radiation absorbed = radiation emitted S0/4 (1-alpha) = sigma x T4 Earth’s T=255K = -18 deg C Global mean surface temperature = 288K =15deg Why the difference? Atmosphere is almost transparent to SW radiation but absorbs and emits IR ( or LW) radiation – greenhouse effect

Absorption of shortwave and longwave radiation by the atmosphere The atmosphere is a highly selective absorber Note the Atmospheric window

The greenhouse effect The atmosphere is rather transparent to solar radiation It is efficient at absorbing longwave (terrestrial) radiation. When terrestrial radiation is absorbed in the atmosphere it then gets re-emitted, resulting in some of it heading back to the surface where it may be absorbed and re-radiated out……. Let’s go to the board….

The Earth orbits the sun once per year with its axis of rotation tilted -  seasons

The seasons: spring/fall equinox, winter/summer solstice

Solar zenith angle

Solar energy at the top of the atmosphere

Annual average global energy balance of Earth

The energy balance at TOA (Top of the atmosphere). The Heat Budget There has to be a balance between the globally averaged solar radiation that is absorbed in the Earth system annually and the outgoing longwave radiation (OLR) emitted by the Earth system. Why? Albedo is measured  ASR (absorbed solar radiation) since total incoming solar is measured OLR is measured.

Latitudinal heat balance Averaged over the year, latitudes equatorward of ~36 deg latitude receive more solar radiation than they lose in the form of terrestrial radiation. The opposite is true poleward of ~36 deg. Are the tropics getting hotter, the poles getting colder? The atmosphere and ocean transport energy poleward.

Fig. 2.21

Poleward energy transport Albedo increases with latitude because solar zenith angle, cloud cover and snow cover increase OLR does not decrease with latitude as rapidly as the ASR Atmosphere & ocean transport heat poleward to make up for the difference

P lanetary albedo a)Annual mean b)JJA c)DJF

Outgoing long- Wave radiation (OLR) a)Annual mean b)JJA c)DJF

Net incoming radiation at the TOA a)Annual mean b)JJA c)DJF

Fig. 2.21

Box 2.2