Radiation budgetRadiation budget

METR280METR280Satellite Meteorology/ClimatologySatellite Meteorology/Climatology

Professor Menglin JinProfessor Menglin Jin

Radiation budgetRadiation budget Basic definitionsBasic definitions Some problems with measuring radiation Some problems with measuring radiation

budget using satellitesbudget using satellites Satellites/sensors which have been used to Satellites/sensors which have been used to

measure radiation budgetmeasure radiation budget Solar constantSolar constant Top of atmosphere radiation budgetTop of atmosphere radiation budget Surface Radiation BudgetSurface Radiation Budget Global-scale ERB climatologiesGlobal-scale ERB climatologies

ProblemsProblems Problems with measuring radiation Problems with measuring radiation

budget componentsbudget components• Inverse problemInverse problem• Diurnal problemDiurnal problem• Spectral correction problemSpectral correction problem• Angular dependence problemAngular dependence problem

The Nature of The Nature of Electromagnetic RadiationElectromagnetic Radiation

travels through space in the form of travels through space in the form of waves waves

Radiation as particles"photon", no mass, occupy no space, and travel at the speed of light, 2.9998 X 108 m s-1.

Satellites/sensorsSatellites/sensors Satellites/sensorsSatellites/sensors

• NOAA polar orbitersNOAA polar orbiters– Reflected SWR (0.5-0.7 Reflected SWR (0.5-0.7 m)m)– LWR (TIR) (10.5-12.5 LWR (TIR) (10.5-12.5 m)m)

• Nimbus 6 and 7 (‘75-’78 and ‘78-’87)Nimbus 6 and 7 (‘75-’78 and ‘78-’87)– Earth Radiation Budget instrumentEarth Radiation Budget instrument– 0.2-3.8 0.2-3.8 m (SWR) and 0.2-50 m (SWR) and 0.2-50 m (broadl)m (broadl)– LWR = Broad - SWRLWR = Broad - SWR

• Earth Radiation Budget Experiment (ERBE)Earth Radiation Budget Experiment (ERBE)– ERBS and NOAA 9 and 10ERBS and NOAA 9 and 10

EOS program (NASA)EOS program (NASA)• TERRA (EOS AM)TERRA (EOS AM)

– Clouds and Earth’s Radiant Energy System Clouds and Earth’s Radiant Energy System (CERES)(CERES)

– ToA radiation budgetToA radiation budget– Cloud height, amount, particle sizeCloud height, amount, particle size– Next generation ERBENext generation ERBE

– Multiangle Imaging SpectroRadiometer (MISR)Multiangle Imaging SpectroRadiometer (MISR)– Surface planetary albedo measurementsSurface planetary albedo measurements– Multiangle measurementsMultiangle measurements

Satellites/sensorsSatellites/sensors

Satellites/sensorsSatellites/sensors TerraTerra

• Moderate Resolution Imaging Spectroradiometer Moderate Resolution Imaging Spectroradiometer (MODIS)(MODIS)– Surface temperature*Surface temperature*– Snow cover and reflectance*Snow cover and reflectance*– Cloud cover with 250m resolution by day and Cloud cover with 250m resolution by day and

1,000m resolution at night*1,000m resolution at night*– Cloud properties*Cloud properties*– Aerosol properties* Aerosol properties* – Fire occurrence, size, and temperatureFire occurrence, size, and temperature– Cirrus cloud cover*Cirrus cloud cover*

Multifrequency Imaging Microwave Multifrequency Imaging Microwave Radiometer (MIMR)Radiometer (MIMR)• Similar to ESMR, SMMR, SSM/ISimilar to ESMR, SMMR, SSM/I

– ProductsProducts– Precipitation, soil moisture*Precipitation, soil moisture*– Ice and snow cover*Ice and snow cover*– SST*SST*– Oceanic wind speedOceanic wind speed– Atmospheric cloud water content and water vapor*Atmospheric cloud water content and water vapor*

*Significant to radiation budget*Significant to radiation budget

Radiation budgetRadiation budget Solar constantSolar constant

• The average annual irradiance received outside The average annual irradiance received outside the Earth’s atmosphere on a surface normal to the Earth’s atmosphere on a surface normal to the incident radiation and at the Earth’s mean the incident radiation and at the Earth’s mean distance from Sun.distance from Sun.

• Roughly 1370 WmRoughly 1370 Wm-2-2

• Interannual variation of 0.2 WmInterannual variation of 0.2 Wm-2-2, but annual , but annual variation of 3 Wm-2variation of 3 Wm-2

Top of atmosphere radiation budgetTop of atmosphere radiation budget• We want to know the SW radiate exitance (MSW) We want to know the SW radiate exitance (MSW)

and LW radiant exitance (MLW), a.k.a. Outgoing and LW radiant exitance (MLW), a.k.a. Outgoing Longwave RadiationLongwave 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 budgetRadiation budget Surface radiation budgetSurface radiation budget

• Must make corrections for the atmosphereMust make corrections for the atmosphere• ComponentsComponents

– Downwelling SWR (insolation)Downwelling SWR (insolation)– Upwelling SWR (reflected)Upwelling SWR (reflected)– Downwelling LWR (atmospheric emission)Downwelling LWR (atmospheric emission)– Upwelling LWR (terrestrial emission)Upwelling LWR (terrestrial emission)

• Net radiation is the sum of the componentsNet radiation is the sum of the components

Surface Albedo (example: Surface Albedo (example: urban)urban)

• Downwelling SWRDownwelling SWR– Three possible fatesThree possible fates

ToA insolation = reflected at top of atm. + absorbed by atm. + downwelling ToA insolation = reflected at top of atm. + absorbed by atm. + downwelling SWR at surfaceSWR at surface

• cos(cos()): cosine of the solar zenith angle: cosine of the solar zenith angle• irradianceirradiance: A radiant flux density incident on some area (Wm: A radiant flux density incident on some area (Wm -2-2))• We’re interested in EWe’re interested in Esfcsfc

• Assuming isotropic reflection (same amount of reflection in every Assuming isotropic reflection (same amount of reflection in every direction)...direction)...

sunEsun sunEsunAEatm (1 Asfc )Esfc

cos()

Solar irradiance ToA albedo

Energy absorbed by atmosphere

Surface albedo

Downwelling SWR irradiance at surface

Esfc sunEsun L Eatm

1 Asfc

Solar insolation Reflected radiance Atm. absorp.

Radiation budgetRadiation budget• Upwelling SWR (reflected)Upwelling SWR (reflected)

– Product of surface albedo (Asfc) and the Product of surface albedo (Asfc) and the downwelling SWR at the surface (Esfc)downwelling SWR at the surface (Esfc)

– Surface albedo is the keySurface albedo is the key– How do we account for cloud coverHow do we account for cloud cover

– Monthly minimum surface albedoMonthly minimum surface albedo

Radiation budgetRadiation budget• Downwelling LWR (atmospheric emission)Downwelling LWR (atmospheric emission)

– Depends on:Depends on:– Temperature profile of atmosphereTemperature profile of atmosphere– Moisture profile of atmosphereMoisture profile of atmosphere– Type and amount of cloud coverType and amount of cloud cover

– Soundings (radiosonde or satellite sounder)Soundings (radiosonde or satellite sounder)• Upwelling LWR (terrestrial emission)Upwelling LWR (terrestrial emission)

– Little reflected, nearly all emissionLittle reflected, nearly all emission– Need to know surface temperature and Need to know surface temperature and

emissivity of surfaceemissivity of surface

Radiation budgetRadiation budget• Net radiationNet radiation

– Can simply sum the four componentsCan simply sum the four components– Better to retrieve directlyBetter to retrieve directly– Visible brightness highly related to Visible brightness highly related to

surface net radiationsurface net radiation

Global-scale ERB climatologiesGlobal-scale ERB climatologies• Includes effects of surface and atmosphereIncludes effects of surface and atmosphere

N(1 Ap )Q I

ToA net radiationToA net radiation Planetary albedoPlanetary albedo

Incoming solar fluxIncoming solar flux

Net LWR fluxNet LWR flux

Fraction of energyFraction of energyabsorbed by clouds,absorbed by clouds,atm. and surfaceatm. and surface

ERB climatologiesERB climatologies

ERB climatologiesERB climatologies• Planetary albedoPlanetary albedo

– Changes in surface albedo (greening of Changes in surface albedo (greening of vegetation, snow cover, sea ice)vegetation, snow cover, sea ice)

– Changes in cloud coverChanges in cloud cover– 0.30 (Stephens and others, 1981)0.30 (Stephens and others, 1981)– 0.31 (Ohring and Gruber, 1983)0.31 (Ohring and Gruber, 1983)

• LWR fluxLWR flux– Same as OLR (little incoming LWR)Same as OLR (little incoming LWR)– Goverened by surface temperature and cloud Goverened by surface temperature and cloud

covercover• Global ToA net radiation: close to 0Global ToA net radiation: close to 0

Cloud forcingCloud forcing Cloud forcingCloud forcing

• Cloud are the primary moderator of the Cloud are the primary moderator of the short and longwave radiation streamsshort and longwave radiation streams

• How do changes in cloud cover affect How do changes in cloud cover affect climate?climate?

H(1 Ap )Q F

Net rad. heatingNet rad. heating Planetary albedoPlanetary albedo

Incoming solar fluxIncoming solar flux

Net LWR Net LWR emittanceemittance

Fraction of energyFraction of energyabsorbed by clouds,absorbed by clouds,atm. and surfaceatm. and surface

CH HclrCQ (Aclr A) (Fclr F)

Effect of cloud forcingEffect of cloud forcing

H under clear skiesH under clear skies

Cloud forcingCloud forcing

Cloud forcingCloud forcing• Effect of cloud forcingEffect of cloud forcing

– Clear sky radiative heating (Fclr) peaks in Clear sky radiative heating (Fclr) peaks in tropics and decreases toward polestropics and decreases toward poles

– Clear sky albedo (Aclr) peaks in tropics but also Clear sky albedo (Aclr) peaks in tropics but also has large negative values in mid latitudeshas large negative values in mid latitudes

– Total cloud forcing is near 0 in tropicsTotal cloud forcing is near 0 in tropics– Effects are greatest (and negative) with low Effects are greatest (and negative) with low

stratus clouds off west coasts of continentsstratus clouds off west coasts of continents– Primarily negative over most of mid to high Primarily negative over most of mid to high

latitudeslatitudes– Effects are positive over Sahara and Sahel Effects are positive over Sahara and Sahel