Lecture 6: Energy balance and temperature (Ch 3)

1. Lecture 6: Energy balance and temperature (Ch 3) • atmospheric influences on insolation & the fate of solar radiation • interaction of terrestrial radiation with atmospheric gases • global climatology of longwave and net (allwave) radiation • convective energy transport and the surface energy balance in relation to the near-ground temperature profile

2. In Ch 2 we dealt with geometry of solar radiation: now, its interaction with atmosphere… we have to consider scattering and absorption • absorption implies energy loss from the beam and direct heating of the air • almost 100% absorption of u.v. by O3 • far greater fraction of IR is absorbed than of visible… NIR is strongly absorbed by water vapour • scattering (same as “diffuse reflection”) does not heat the air Fig. 3-2

3. Rayleigh scattering off agents with diameter d <<  is -selective but omni-directional; Mie scttrng off aerosols is less - selective but mostly forward… so polluted sky is dull Scattering out of the direct beam produces diffuse solar radiation… scattering by the air molecules is more effective in the blue than in the red, thus the blue sky Direct beam solar radiation Fig. 3-3 Total incident solar intensity at gnd K [W m-2] is sum of beam + diffuse components

4. Global-annual (climatological) disposition of solar radiation • clouds reflect 19% • atmos reflects 6% • gnd reflects 5% • sfc takes 45% • atmos takes 25% 70% 30% (= planetary albedo) (But a clear, dry atmosphere may deliver ~ 80% of solar energy to the surface) Clouds mostly scatter (little absorption) (of this, 7% is u.v. abs by O3, balance mostly NIR abs. by water vpr) You don’t have to remember all these numbers Fig. 3-7

5. Earth’s surface behaves like a black body Longwave radiation Spectral absorptivity of atmos. gases 100% • longwave rad’n emitted by sfc largely absorbed by atmos, which re-radiates (at same wavelengths) iosotropically 0% “window” 8-12 m Shading represents capacity of the atmos. to absorb at that  Fig. 3-9 Atmos gases do not have strong absorption bands in the visible

6.  abs/emit abs/emit  abs/emit emit Longwave radiation • longwave rad’n emitted by sfc largely absorbed by atmos, which re- radiates (at same wavelengths) isotropically • (i.e. in all directions equally) • thus there is an “infinite cycle of exchange” • (atmos-atmos, atmos-grnd,…) • however the atmos gases are relatively • transparent in the 8-12 m “window” • which however is “closed” by clouds, • which absorb virtually all • longwave and re-emit • as a grey body (gases emit “isotropically”, ie. equal probability in all directions, so ½ up, ½ down)

7. Global-annual (climatological) disposition of longwave radiation 4% + 66% = 70%, balancing earth’s net solar gain Fig. 3-8 • atmos emits up & down • most lngwv from sfc is absorbed

8. Global-annual (climatological) allwave (net) radiation balance You don’t have to remember all these numbers Fig. 3-10 This imbalance is rectified when we account for convective energy exchange between sfc and atmos

9. Role of (turbulent vertical) convection in local energy balance… day Daytime near-ground temperature “profile” z T=T(z) • convection: transport by virtue of bulk motion of a fluid/gas • turbulent vertical convection: vertical transport by eddies • sensible heat: ordinary thermal energy resident in kinetic energy of molecular motion (turbulent) vertical convection of (sensible) heat, or “convective heat exchange”, causes “sensible heat flux” QH [ W m-2 ] relatively cool parcel decends relatively warm parcel ascends Upward flow of sensible heat on average, QH > 0 ground

10. Convective “mixing” may be spontaneous (buoyancy-driven, “free convection”)… Fig. 3-11 … or may be “forced” Fig. 3-12

11. Night-time near-ground temperature profile z T=T(z) Role of convection in local energy balance… night In this case, forced convection causes downward flow of sensible heat relatively warm parcel decends relatively cool parcel ascends Downward flow of sensible heat on average, QH < 0 ground

12. Role of convection of latent heat in local energy balance… • latent heat: energy that is recoverable upon phase change (condensation) Near-ground humidity (v) profile Vertical convection of vapour implies a “latent heat flux” QE [ W m-2 ] relatively dry parcel decends z relatively moist parcel ascends v=v(z) Upward flow of latent heat on average, QE > 0 ground

13. Global-annual (climatological) energy balance You don’t have to remember all these numbers Fig. 3-14

14. Surface radiation budget K (not to scale) L L K K* = K - K Net shortwave Net longwave L* = L - L Q* = K* + L* = K - K + L - L Net allwave (“net radiation”) **during clear, dry skies K ~ 0.8 S0 sin  , i.e. about 80% of solar beam reaches grnd ( the solar elevation angle)