MET4410RemoteSensing:RadarandSatelliteMeteorologyMET5412RemoteSensinginMeteorology

Lecture4:GlobalInsolation&EMSpectrum

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EnergyInputfromtheSun� Offundamentalimportancetotheglobalclimateistheinputofenergyfromthesunanditsspatialandtemporaldistribution.Thisinputisafunctionoftwovariables:◦ 1)thefluxofsolarradiationincidentonthetopoftheatmosphere,and◦ 2)thefractionofthatfluxthatisabsorbedbyeitherthesurfaceortheatmosphereateachpointintheearth-atmospheresystem.

� Theseconddependsinacomplexwayondistributionsofcloudsandabsorbinggasesintheatmosphere,aswellasontheabsorbingpropertiesofthesurface.

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GlobalInsolation� Howmuchtotalsolarradiationisincidentontheearth’satmosphere,onaverage?

� Thesun’sincidentflux(fluxdensity)atEarth’smeandistancefromtheSunisS0=1370Wm-2

� Thus,thetotalfluxofsolarradiationinterceptedbytheearthis𝜱=S0πRE2=1.74X1017 W

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RegionalDependenceofInsolation� Theflux(density)ofsolarradiationmeasuredonaunithorizontalareaatthetopoftheatmospheredependsontheangleofincidenceofthesun(solarzenithangle):

F=S0cosѲs

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TotalInsolationofaSingleLocation� Totalinsolation[energyperunitarea]atthetopoftheatmosphereatasinglelocationoverthecourseofa24-hourperiod.Thisinsolationisgivenby

� Wdependsontworeadilyidentifiablefactors:(1)thelengthofthedaytsunset −tsunrise ,and(2)theaveragevalueofcosθs(t)duringthetimethesunisup.

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ThechangingseasonisduetothetiltofEarth’sspinaxis

WhatcausesEarth’sSeasons?

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SeasonalVariationofTotalInsolation� Atequator,thelengthofadayis12hoursyear-round,butthemaximum

elevation(zenithangle)thesunreachesinthecourseofthedayvarieswiththetimeofyear.Twiceayear,atthetimeofthevernalandautumnalequinox(approximatelyMarch21andSeptember21,respectively),thesunpassesdirectlyoverheadatnoon(zenithangle=0).Atothertimesofyear,theminimumzenithangleachievedinthecourseofthedayisequaltotheangleoftiltoftheearth’saxistowardorawayfromthesun,uptoamaximumof23◦ atthetimeofthesummerandwintersolstices(June21andDecember21,respectively).

� AtAtlatitudespoleward of23◦,thesunisneverdirectlyoverhead,andtheminimumzenithangleisalwaysgreaterthanzero.Dur-ing thesummerseason,thesuncanreachapointfairlyhighinthesky,whereasinthewinterseason,themaximumelevationangleismuchlower.Moreover,thedaysarelongerinthesummerhemi-spherethaninthewinterhemisphere.

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RadiationDailyaveragesolarfluxatTOPasafunctionoflatitudeandtimeofyear

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DailyaveragesolarfluxatTOPasafunctionoflatitudeforthetwosolsticedatesandaveragedoverayear

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TheElectromagneticSpectrum

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Whyisaspecificspectralbandinterestingtoatmosphericscientists?

� Diabatic heating/cooling:heatexchangebetweenearthandtherestoftheuniverse.

� Photochemistry:chemicalreactionsintheatmospherearedrivenbysunlight..

� Remotesensing:Anyfrequencyofradiationthatisabsorbed,scatteredoremittedbytheatmospherecanpotentiallybeexploitedforsatellite- orground-basedmeasurementsofatmosphereproperties.

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GammaRaysandX-Rays�

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Ultraviolet(UV)Band�

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VisibleBand�

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SolarRadiationatTOP

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Infrared(IR)Band� 0.7 µm< 𝝺<1000 µm(1mm)� IRbandisveryimportanttobothdiabatic heating/cooling&remotesensingbecause:◦ EnergyexchangethroughIRbandbetweenlowerandupperlevelsoftheatmosphereandbetweenearth-atmospheresystemandouterspace.◦ MajorandminorgasspeciesoftheatmospherehavedistinctiveabsorptionfeaturesintheIRband,whichallowustoexploitthisbandforremotesensingoftemperature,watervapor,andtracegases.◦ Greenhousegases:IRabsorbingtracegases.

� Sub-bands:◦ NearIR:0.7 µm< 𝝺<4 µm,52%ofsolaroutputisinthisband.Atmosphereisnotuniformlytransparent.Thereareseveralabsorptionlines.Forexample,𝝺=1.3 µm.◦ ThermalIR:4 µm< 𝝺<50 µm,0.99%oftotalsolaroutput.Veryimportantforenergyexchangeandatmosphericabsorption.◦ FarIR:50µm< 𝝺<1000 µm,energytransferisinsignificant.Potentialapplicationstoremotesensingofcirrusclouds. 18

AbsorptionSpectrumoftheCloudFreeAtmosphere

• Solar/shortwave radiation: 0.1 µm < 𝝺<4 µm, UV, visible, near IR. 99% of solar output.

• Terristrial/longwave : radiation: 4 µm < 𝝺<50 µm, thermal IR. 99% of earth-atmosphere’s output radiation. 19

Greenhouse effect: shortwave solar radiation is nearly transparent to the atmosphere, but longwaveterrestrial radiation is trapped by greenhouse gases,causing the increase of surface temperature.

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Ozone layer and climatePhotodissociation by

ultraviolet

Solar ultraviolet photon

Ozone is a loosely bonded moleculeand can be easily dissociated . 21

Formation of Ozone

Ozone

Sustaining Ozone

Depletion of Ozone

3CFCl assuch CFCs

10-50 km (stratosphere)

23 CFClCl UVCFCl +→+

ClO O Cl →+

23 O2Cl O ClO +→+

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The Ozone Hole

Ozone concentration drops sharply over Antarctica

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The Ozone Hole

Polar vortex

Cold air-80C

2. As the cold temperatures persist over the polar, polar stratospheric clouds form. 3. Chlorine reservoir species HCl and ClONO2 become very active on the surface of polar stratospheric clouds.

232 ClNHO ClONO HCl +→+

22322 ClOHHClHOCl HOCl;NHO OH ClONO +→++→+

ClCl Cl2 +→+ νh

1. Polar winter leading to the stronger circumpolar wind belt (polar vortex) to isolate the cold air within it.

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MicrowaveBand� 1mm< 𝝺<10 cmor3GHz< f<300 GHz� Noenergytransferinthisbandintheatmosphere.� Microwavecircuitshaveaquasi-opticalcharacter:waveguides,feedhorns,parabolicreflectors.

� Veryusefulforradar&passivemicrowaveradiometer.◦ Interactionswithprecipitation-sizedparticles(10-85GHz):monitoringsevereweather◦ Relativetransparencyofcloudswhenfrequency<100GHz,allowtodetectthepropertiesofsurfaceandofthetotalatmospherecolumnunderallweatherconditionsexceptrainfall.

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RadioBand� 𝝺>10 cmorf<3GHz� Frequenciesarelowenoughtobeamenabletogeneration,amplification,anddetectionusingtraditionalelectroniccomponentsandcircuits.

� Radiobandradiationtendstointeractveryweaklywiththeatmosphereandthereforehasonlylimitedapplicabilitytoatmosphereremotesensing.Twoexamples:◦ 915MHzground-basedDopplerwindprofiler.Observingscatteringfromturbulence-inducedfluctuationsinatmospheredensityandhumidity.◦ Lightningdetectionsystems:detectlowfrequency“static”emittedbylightningdischarges.

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EMSpectrumforRadar

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Summary

� Gamma&X-rays:unimportant.� UV:importanttophotochemistry,remotesensingofozone.

� Visible&IRband:veryimportanttobothenergytransferandremotesensing

� Microwaveband:remotesensingonly� Radioband:onlyafewremotesensingapplications

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