Photosynthetically-active radiation (spectral portion, CI) h h h h h h h h

Longwave Radiative Exchange The atmosphere absorbs long-wave radiation (L) from the Earth, clouds and gases at all altitudes Absorption greatest in lower portion of the atmosphere, where H 2 0 and CO 2 concentrations are highest The atmosphere absorbs effectively from  m, except in the atmospheric window (8-11  m) Most longwave loss to space occurs through this window, but clouds can partially close it

 L  =  0  (T 0 ) 4 + (1 -  0 ) L  Amount of L  reflected (slight adjustment) L  is greater in magnitude and more variable than L  L* = L  - L  (usually negative) NET ALL_WAVE RADIATION DAYTIME:Q* = K  - K  + L  - L  Q* = K* + L* NIGHT:Q* = L*

Radiation Measurements LL LL KK K  (not visible) UV-A PAR

More radiation sensors… Source: University of Colorado

K  in tropical forests of Colombia/Ecuador

Radiation Balance Components Negative in Oke

Clouds Reduce K  because of absorption and reflection from cloud tops (may eliminate S) Increase D by scattering incoming solar radiation Strongest K  under partly cloudy skies with sun in clear patch Absorb much of L  and re-emit it as L  (low cloud emits more) Reduce diurnal temperature variation

Source: NOAA Global Energy Balance (SIMPLIFIED)

Q* -positive in daytime -almost always negative at night Any Q* imbalance is accounted for by convective exchange or conduction Q* = Q H + Q E + Q G +  S where Q H = sensible heat flux Q E = latent heat flux Q G = conduction to or from ground (See Figure 1.10)

Recall the First Law of Thermodynamics ENERGY IN = ENERGY OUT Q in > Q out (flux convergence) Net storage gain leads to warming Q out > Q in (flux divergence) Net storage energy loss leads to cooling Q in = Q out No net change in energy storage

Water: H 2 O High heat capacity Exists in all states at Earth’s temperatures Heat required/released during phase changes: Latent heat of fusion (L f = MJ kg -1 ) Latent heat of vaporization (L v = 2.45 MJ kg -1 ) Latent heat of sublimation (L s = L f + L v )

Water Balance p = E +  r +  s Where p is precipitation E is evapotranspiration r is net runoff s is soil moisture storage content Q E = L v E  Q M = L f M Where E and M are in kg m -2 s -1 See Fig. 1.13

Sensible and Latent Heat Fluxes Eddy correlation (later) Sonic anemometer measurements of vertical velocity and temperature Krypton hygrometer measurements of water vapour density

Advection and Winds Air flow at local scale can affect energy balance as can air flow at scales larger than boundary layer At the micro-scale, horizontal temperature variation causes horizontal pressure differences Why ? Warm air is lighter than cold air This leads to winds (kinetic energy) Energy transferred to smaller and smaller scales before being dissipated as heat

DAYTIME: Both sides of equation are positive: surface radiative surplus Surplus partitioned into ground and atmosphere Convection is the most important means of daytime heat transport from surface Q E is greater when soil moisture is high Q H is greater when water is more restricted

NIGHT: Both sides of equation are negative: surface radiative deficit Deficit partitioned into heat gain from ground and atmosphere Q* loss is partially replenished by Q G Q E and Q H of less importance as convective exchange is dampened by the night-time temperature stratification