Week 3 - World Climate and Solar Energy; Patters of Global Radiation.

SOLAR RADIATION

Sun - photosphere - outer radius nearly 7x10^5 km - surface temp ~5760 K

  • sun virtual black body, absorbs all energy and emits energy at max rate possible for temp
  • total energy emitted (equation in ppt)
  • solar radiation peaks in visible part of spectrum at 0.5um
  • solar constant (S): energy recieved at top of atmosphere on surface perpendicular to solar beam for mean solar distance; ~1368 Wm-^2
  • geothermal flux nearly 4 orders magnitude < S or ~0.5 Wm-2 averaged over earths surface

FACTORS AFFECTING AMOUNT OF SOLAR ENERGY REACHING EARTHS SURFACE

Solar output, sun earth distance, latitude time and season, atmosphere and cloud cover, land and sea, elevation and aspect -> albedo

SURFACE RECEIPT OF SOLAR RADIATION AND ITS EFFECTS

Average rate of input of solar energy per unit horizontal area at top of atmosphere is 1/4 S as surface area of spherical Earth is 4x its cross sectional area.


Atmosphere affects transfer of solar energy to earths surface


clear sky modifies solar spectrum quite significantly by selective absorption and scattering


clouds much less wave-length dependant but backscatter/reduce solar power reaching surface by up to 90%.

EFFECT OF ATMOSPHERE

  • Solar wavelengths >0.295um reach surface (shorterr wavelengths scarcely penetrate <20-km altitude).


  • -30% incoming solar radiation immediately reflected back to space by atmosphere (including clouds) and earths surface


  • 70% heats the earth and atmosphere


  • surface absorbs nearly half incoming solar energy available top of atmosphere and re-radiates it out as infrared wavelengths. Much of this re-radiated terrestrial energy is absorbed by H2O, CO2 and O3, warming earths surface.

EFFECTS OF LAND AND SEA

  • calm sea surface has very low altitude for solar elevation >60º


  • for land surfaces a ~0.08-0.40


  • water tends to store heat, whereas land quickly returns it to atmosphere. Water has high specific heat capacity, so must absorb 5x as much heat energy to raise its temp. By same amount as comparable mass of dry soil. So if water is cooled/evaporates it releases/expends a large amount of heat.

TERRESTRIAL IR RADIATION

  • assume earth emits terrestrial radiation as uniform spherical BB whose size and surface temperature determine total power output.


  • terrestrial radiation ranges 4-100um and peaks at ~12um. Its spectrum is much flatter and more spread out in wavelength than solar irradiance spectrum.

BALANCING SOLAR RADIATION - can be found on lecture slides week 3 page 15-16

EARTHS NATURAL GREENHOUSE EFFECT

  • strong absorption by H2O between 5-8um and again 14um by H2O and CO2.
  • Kirchhoffs Law, H2O and CO2 must emit these heavily absorbed wavelengths with the same efficiency as they absorb them. But unlike the surface, the air emits both upwards and downwards
  • to sustain net upward flux of terrestrial radiation through opaque atmosphere, emissions of terrestrial radiation through opaque atmospheric layers must decrease with increasing altitude.
  • since decreasing emissions require decreasing temp (Stefans law) there must be continuing temperature lapse from max temp at surface to min temp at highest opaque layer.

EARTHS NATURAL GREENHOUSE EFFECT

Schematic picture of vertical fluxes of terrestrial radiation in a 3-layer model atmosphere in purely radiative equilibrium (After Mcllveen, 1992).

  • surface absorbs nearly half of incoming SOLAR energy available @ top of atmosphere, and re-radiates it out as infrared (>3um). Much of this re-radiated terrestrial energy is absorbed by H2O, CO2 and O3, warming earths surface.

Terrestrial radiation emitted to space is mainly (~80%) from highest opaque layer in atmosphere, i.e. in upper troposphere, where temp is usually >40k < underlying surface temp.


This plus tropospheric temp lapse rate explains low Te (225k) of terrestrial BB and elevated surface temp, i.e. Greehouse Effect.


Term misleading

VENUS - atmospheric mass -100x earths mainly CO2 with H2SO4 (sulfuric acid) huge GE.


its Te is only 245k, yet its S is >2x for Earth. Surface temp. ~730k GE nearly 400k.


Carbon in carbonate rocks degassed into atmosphere as CO2 during runaway GE.

H2O = ~21k, CO2 = ~7k, O3 = ~2k, other trace gases = ~3k total = ~33k


H2O is main source of long-wave infrared radiation in atmosphere