Radiation budget METR280 Satellite Meteorology/Climatology Professor Menglin Jin

Radiation budget b Basic definitions b Some problems with measuring radiation budget using satellites b Satellites/sensors which have been used to measure radiation budget b Solar constant b Top of atmosphere radiation budget b Surface Radiation Budget b Global-scale ERB climatologies

Problems b Problems with measuring radiation budget components Inverse problemInverse problem Diurnal problemDiurnal problem Spectral correction problemSpectral correction problem Angular dependence problemAngular dependence problem

The Nature of Electromagnetic Radiation b travels through space in the form of waves Radiation as particles "photon", no mass, occupy no space, and travel at the speed of light, X 108 m s-1.

Satellites/sensors b Satellites/sensors NOAA polar orbitersNOAA polar orbiters –Reflected SWR (  m) –LWR (TIR) (  m) Nimbus 6 and 7 (‘75-’78 and ‘78-’87)Nimbus 6 and 7 (‘75-’78 and ‘78-’87) –Earth Radiation Budget instrument –  m (SWR) and  m (broadl) –LWR = Broad - SWR Earth Radiation Budget Experiment (ERBE)Earth Radiation Budget Experiment (ERBE) –ERBS and NOAA 9 and 10

b EOS program (NASA) TERRA (EOS AM)TERRA (EOS AM) –Clouds and Earth’s Radiant Energy System (CERES) –ToA radiation budget –Cloud height, amount, particle size –Next generation ERBE –Multiangle Imaging SpectroRadiometer (MISR) –Surface planetary albedo measurements –Multiangle measurements Satellites/sensors

Satellites/sensors b Terra Moderate Resolution Imaging Spectroradiometer (MODIS)Moderate Resolution Imaging Spectroradiometer (MODIS) –Surface temperature* –Snow cover and reflectance* –Cloud cover with 250m resolution by day and 1,000m resolution at night* –Cloud properties* –Aerosol properties* –Fire occurrence, size, and temperature –Cirrus cloud cover*

b Multifrequency Imaging Microwave Radiometer (MIMR) Similar to ESMR, SMMR, SSM/ISimilar to ESMR, SMMR, SSM/I –Products –Precipitation, soil moisture* –Ice and snow cover* –SST* –Oceanic wind speed –Atmospheric cloud water content and water vapor* *Significant to radiation budget

Radiation budget b Solar constant The average annual irradiance received outside the Earth’s atmosphere on a surface normal to the incident radiation and at the Earth’s mean distance from Sun.The average annual irradiance received outside the Earth’s atmosphere on a surface normal to the incident radiation and at the Earth’s mean distance from Sun. Roughly 1370 Wm -2Roughly 1370 Wm -2 Interannual variation of 0.2 Wm -2, but annual variation of 3 Wm-2Interannual variation of 0.2 Wm -2, but annual variation of 3 Wm-2 b Top of atmosphere radiation budget We want to know the SW radiate exitance (MSW) and LW radiant exitance (MLW), a.k.a. Outgoing Longwave RadiationWe want to know the SW radiate exitance (MSW) and LW radiant exitance (MLW), a.k.a. Outgoing Longwave Radiation

Radiation balance

Satellites detect the radiation emitted by the Earth + reflected solar radiation, modified by the atmosphere TIR surface

Satellites detect the radiation emitted by the Earth + reflected solar radiation, modified by the atmosphere

Radiation budget b Surface radiation budget Must make corrections for the atmosphereMust make corrections for the atmosphere ComponentsComponents –Downwelling SWR (insolation) –Upwelling SWR (reflected) –Downwelling LWR (atmospheric emission) –Upwelling LWR (terrestrial emission) Net radiation is the sum of the componentsNet radiation is the sum of the components

Surface Albedo (example: urban)

Downwelling SWRDownwelling SWR –Three possible fates ToA insolation = reflected at top of atm. + absorbed by atm. + downwelling SWR at surface cos(  ): cosine of the solar zenith anglecos(  ): cosine of the solar zenith angle irradiance: A radiant flux density incident on some area (Wm -2 )irradiance: A radiant flux density incident on some area (Wm -2 ) We’re interested in E sfcWe’re interested in E sfc Assuming isotropic reflection (same amount of reflection in every direction)...Assuming isotropic reflection (same amount of reflection in every direction)... cos(  ) Solar irradianceToA albedo Energy absorbed by atmosphere Surface albedo Downwelling SWR irradiance at surface Solar insolationReflected radianceAtm. absorp.

Radiation budget Upwelling SWR (reflected)Upwelling SWR (reflected) –Product of surface albedo (Asfc) and the downwelling SWR at the surface (Esfc) –Surface albedo is the key –How do we account for cloud cover –Monthly minimum surface albedo

Radiation budget Downwelling LWR (atmospheric emission)Downwelling LWR (atmospheric emission) –Depends on: –Temperature profile of atmosphere –Moisture profile of atmosphere –Type and amount of cloud cover –Soundings (radiosonde or satellite sounder) Upwelling LWR (terrestrial emission)Upwelling LWR (terrestrial emission) –Little reflected, nearly all emission –Need to know surface temperature and emissivity of surface

Radiation budget Net radiationNet radiation –Can simply sum the four components –Better to retrieve directly –Visible brightness highly related to surface net radiation

b Global-scale ERB climatologies Includes effects of surface and atmosphereIncludes effects of surface and atmosphere ToA net radiation Planetary albedo Incoming solar flux Net LWR flux Fraction of energy absorbed by clouds, atm. and surface ERB climatologies

Planetary albedoPlanetary albedo –Changes in surface albedo (greening of vegetation, snow cover, sea ice) –Changes in cloud cover –0.30 (Stephens and others, 1981) –0.31 (Ohring and Gruber, 1983) LWR fluxLWR flux –Same as OLR (little incoming LWR) –Goverened by surface temperature and cloud cover Global ToA net radiation: close to 0Global ToA net radiation: close to 0

Cloud forcing b Cloud forcing Cloud are the primary moderator of the short and longwave radiation streamsCloud are the primary moderator of the short and longwave radiation streams How do changes in cloud cover affect climate?How do changes in cloud cover affect climate?

Net rad. heating Planetary albedo Incoming solar flux Net LWR emittance Fraction of energy absorbed by clouds, atm. and surface Effect of cloud forcing H under clear skies Cloud forcing

Effect of cloud forcingEffect of cloud forcing –Clear sky radiative heating (Fclr) peaks in tropics and decreases toward poles –Clear sky albedo (Aclr) peaks in tropics but also has large negative values in mid latitudes –Total cloud forcing is near 0 in tropics –Effects are greatest (and negative) with low stratus clouds off west coasts of continents –Primarily negative over most of mid to high latitudes –Effects are positive over Sahara and Sahel