《大气科学基础》地面和大气辐射

?大气科学根底?课件§3地面和大气辐射〔1〕地面和大气的长波辐射地面辐射，由地面发射的指向大气的电磁波辐射。它大局部被大气吸收，只有小局部直达宇宙空间。大气辐射，大气发射的长波辐射，它一局部向下到达地面，一局部被周围大气吸收，只有小局部到达宇宙空间。式中，δ和δ′分别地面和大气的相对辐射率，又称为比辐射率。因为地球外表和大气不是严格的黑体，因此要乘上δ和δ′这两个系数。Stefan-BoltzmannLaw二氧化碳有两个吸收带，中心分别位于4.3微米和14.7微米。第一个吸收带位于温度为200～300K的绝对黑体放射能量曲线的末端，它的作用不大，第二个吸收带从～17.1微米，比较重要；臭氧在9—10微米之间有一狭窄的吸收带；大气在整个长波段，除8～12μm一段处之外，其余的透射率近于零即吸收率近于1。8～12μm吸收率最小，透明度最大，称为“大气之窗〞。这个波段的辐射，正好位于地面辐射能力最强处，所以地面辐射有20％的能量透过这一窗口射向空间。大气中长波辐射过程的特点与短波辐射相比，长波辐射的差异在于：不是平行辐射，是漫射辐射；大气中长波辐射的过程不仅要考虑吸收过程而且要考虑大气本身对长波辐射的发射过程；散射过程可以忽略不计。因为空气分子的尺度远比长波波长小。〔3〕大气逆辐射和地面有效辐射

大气逆辐射和大气保温效应大气逆辐射，大气辐射指向地面局部。大气一方面吸收大局部的地面长波辐射；另一方面又以逆辐射的形式使局部能量返回地面，从而减少了地球辐射能损耗。椐估计，如果没有大气，地球外表的平均温度为255K，而实际地表平均温度为288K。大气的这种效应称为大气的保温效应。地面的有效辐射地面有效辐射，地面发射的辐射与与地面吸收的大气逆辐射之差。影响地面有效辐射的因子

地-气间的温度差：通常，地面温度高于大气，地面有效辐射为正，地面失去热量；如果近地层有很强的逆温出现，地面有效辐射为负，地面得到热量；湿度：地面有效辐射在湿热天气情况下比干冷天气小；

云：有云地面有效辐射小，无云地面有效辐射大；

地表性质：有植被，地面有效辐射小；无植被，地面有效辐射大。〔4〕射出长波辐射射出长波辐射射出长波辐射，地面直接射向宇宙空间的长波辐射与大气和云层向外发射的长波辐射的和；射出长波辐射，亦称为外逸长波辐射，英文为OutgoingLongwaveRadiation〔简称OLR〕，是地气系统进入宇宙空间的热辐射。因为OLR与〔地球外表或者云顶的〕温度有关，所以OLR在大气科学研究中有着其特殊的用途。Regionscoloredinredandpurpledefineregionsofhighamountofinfraredradiationleavingtheearth,andgreenandbluecolorsdenotelowIRvalues.FromtheStefan-Boltzmannequation,wecanalsosaythattheradiatingregionscoloredredandpurplearewarmerthanthosecoloredgreenandblue.Asexpected,thetropicalandsubtropicalregionshavethehighestoutgoingradiation(andtemperature)andpolarregionshavelowestvalues.TheleftFducedbytheEarthRadiationBudgetExperiment(ERBE)programoftheNationalOceanicandAtmosphericAdministration(NOAA)(Harrisonetal,1988),showsamapofoutgoinglongwaveradiation,inWm-2,forthemonthofApril1985underclear-skyconditions.ThisFig.isalsofromHarrisonetal,1988,depictsthediurnalrange,

thatistheday-to-nightchanges,inamountofradiatedenergyinWm-2undercloud-freeskiesforApril1985.NotethattherangeofvaluesismuchlowerthanforthepreviousphotographRegionshavingthelargestdiurnalvariationaregenerallydesertsinthesubtropicalzones.Havingfewcloudsandlowhumidity(i.e.,verylittlewatervaporforgreenhousegasabsorption)intheoverlyingatmosphere,theseregionsradiatetoouterspacedirectlyfromtheirsurfaces,whichrangeintemperaturefromover600C(333K)duringthedaytonear100C(283K)atnight.Notethatmostoceanregionshaveverylowchangesinoutgoingradiation(and,therefore,temperature)fromdaytonight.Nowifweconsidertheeffectofclouds,wegetaquitedifferentpicture.Figureshowsoutgoingradiation,includingeffectsofclouds,averagedovertheentiremonthofApril1985.Comparingthiswiththeclear-skyphotographshownabove,youseethatthetropicalareashaveamuchloweroutgoinglongwaveradiation.InfactsomeareasoverIndonesia,SouthAmerica,andAfricaontheEquatorhavetemperaturescomparablewithpolarregions.Howcanthisbe?Areviewofthetemperaturestructureoftheatmosphereandyourobservationsofcloudpatternsfromsatellitephotographsfromthethirdunitwillhelpanswerthisquestion.Verystrongsurfaceheatinginthetropicalregionsgivesstrongconvectionthatcreatesverydeepcloudlayers,thetopsofwhichareveryhighandthereforeverycold.Thisshowstheeffectofcloudsinreducingthediurnalvariation.Note,forinstance,thataroundthemarginsoftheSaharaDesertinNorthernAfricatheareaofhighdiurnalrangeshrinkswhencloudsarepresent.Cloudstendtokeepdaytimetemperatureslowerandnighttimetemperatureshigher,therebyreducingthediurnalrangeintwoways.Thisfig.showsacompositeoftheoutgoinglongwaveradiationfor10Januarys(Bessetal,1989).Anotablefeatureofthisplotisthat,whiletheSouthAmericanandAfricanminimainoutgoinglongwaveradiationareconfinedtothecontinentalborders,thelongitudinallyextendedminimuminoutgoinglongwaveradiationoverIndonesiaismuchlargerandspansalargeareaofocean.ThisparticularregionofenhancedamountofdeepcloudinesswillbediscussedwhenwediscusstheSouthernOscillationandElNinoeffects.Thisfig.isforacompositeof10Julysshowageneralnorthwardseasonalshift,reflectingsummerintheNorthernHemisphereandwinterintheSouthernHemisphere,andmarkedreductionoftheSouthAmericanandAfricancloudinesspatterns.TheIndonesianpatternhasshiftednorthwardandwestwardtoencompasstheIndianMonsoonphenomenon.TheSouthAmericanpatternalsohasevolvedintowhatisknownasth