气象学与气候学课件版第五章降水

Chapter5Asweallknow,cloudyweatherdoesnotnecessarilymeanthatitwillrainorsnow.Infact,cloudsmayform,lingerformanydays,andneverproduceprecipitationHOWDOCLOUDDROPLETSGROWAnordinaryclouddropletisextremelysmall,havinganaveragediameterof20μmNoticeintheFigurethatatypicalclouddropletis100timessmallerthanatypicalIfaclouddropletisinequilibriumwithitssurroundingsthesizeofthedropletdoesnotchangebecausethewatermolecules(水分子)condensing(凝结)ontothedropletwillbeexactlybalancedbythoseevaporatingfromit.Ifhoweveritisnotinequilibrium,thedropletsizewilleitherincreaseordecrease,dependingonwhethercondensationorevaporationpredominates（主导）Becausemorevapormoleculessurroundthedroplet,ithasagreaterequilibriumvaporpressure.Thereasonforthisfactisthatwatermoleculesarelessstronglyattachedtoacurved(convex)watersurface;hence,theyevaporatemorereadily.Tokeepthedropletinequilibrium,morevapormoleculesareneededaroundittore cethosemoleculesthatareconstantlyevaporatingfromitssurface.Smallerclouddropletsexhibitagreatercurvature,whichcausesamorerapidrateofevaporation.Asaresultofthisprocess(calledthecurvatureeffect),smallerdropletsrequireanevengreatervaporpressuretokeepthemfromevaporatingaway.Therefore,whenairissaturated（饱和）withrespecttoaflatsurfaceitisunsaturatedwithrespecttoacurved（弯曲的）dropletofpurewater,andthedropletevaporates.So,tokeeptinyclouddropletsinequilibriumwiththesurroundingairtheairmustbesupersaturated（过饱和）thatistherelativehumiditymustbegreaterthan100percentThesmallerthedropletthegreateritscurvatureandthehigherthesupersaturationneededtokeepthedropletinequilibrium.Thecurvedlinerepresentstherelativehumidityneededtokeepadropletinequilibriumwithitsenvironment.Foragivendropletsize,thedropletwillevaporateandshrinkwhentherelativehumidityislessthanthatgivenbythecurve.Thedropletwillgrowbycondensationwhentherelativehumidityisgreaterthanthevalueonthecurve.Wenowhaveacloudcomposedofmanysmalldroplets—toosmalltofallasrain.Theseminutedropletsrequireonlyslightupwardaircurrentstokeepthemsuspended.Thosedropletsthatdofalldescendslowlyandevaporateinthedrierairbeneaththecloud.Itisevident,then,thatmostcloudscannotproduceHowever,observationsshowthatcloudscandevelopandbegintoproduceraininlessthananhour.Sinceittakesabout1millionaveragesize(20μm)clouddropletstomakeanaveragesize(2000μm)raindrop,theremustbesomeotherprocessbywhichclouddropletsgrowlargeandheavyenoughtofallasEventhoughalloftheintricaciesofhowrainisproducedarenotyetfullyunderstood,twoimportantprocessesstandout:(1)thecollision-coalescenceprocess重力冲并(2)theice-crystal(Bergeron)process（冰晶效应COLLISIONANDCOALESCENCEthecollision-processcan asignificantroleinproducingThespeedofthefallingdropincreasesuntiltheairequalsthepullofgravity.Atthispoint,thedropcontinuestofallbutataconstantspeedwhichiscalleditsterminalvelocity（自由沉降速度）.Becauselargerdropshaveasmallersurface-area-to-weightratio（面积重量比）theymustfallfasterbeforereachingtheirterminalvelocityThuslargerdropsfallfasterthansmallerdropsIncalmair,atypicalraindropfallsover600timesfasterthanatypicalclouddroplet! 水滴下落末速和碰半径末速K重力冲并过程增长均很低。大云滴？破碎-；暖云起伏增长300-350μm，这是因为大气具有云降水，这时碰并作用更为重要。Theice-crystal(orBergeron)processofrainformationisextremelyimportantinmiddleandhighlatitudes,wherecloudsextendupwardintoregionswheretheairtemperatureiswell zing.SuchcloudsarecalledcoldNotuntilwereachanelevationof7600m(25,000ft),wheretemperaturesdropbelow40°C(also40°F),dowefindonlyicecrystals.Theregionofacloudwhereonlyiceparticlesexistiscalledglaciated.Whyaretheresofewicecrystalsinthemiddleofthecloud,eventhoughtemperaturestherearewellbelow Laboratorystudiesrevealthatthesmallertheamountofpurewater,thelowerthetemperatureatwhichwaterInacoldcloud,theremaybeseveraltypesofice-formingnucleiThedifferenceinsaturationvaporpressurebetweensupercooledwaterandiceatdifferentThedifferenceinsaturationvaporpressurebetweensupercooledwaterandiceatdifferent 冰晶过冷水碰，接触粘贴冻，生成雹。这种物质下时与云滴过冷水撞击时可 成许多冰。这些冰晶在过冷水作用而结成新软生 冰种程是式即是结凇过程。冰晶下落时，可彼此碰撞粘贴，这是聚合过程，最终形成雪。如果雪花落地前融就成为滴在夏天纬度地的很多水属于冷降，从云落下时雪在大气经过时化落地就。 人工影响冷云降人工影响暖云降带，地面用火箭和高射发射等方法都可以因地制宜地应用。飞PrecipitationUptonow,wehaveseenhowclouddropletsareabletogrowlargeenoughtofalltothegroundasrainorsnow.Whilefalling,raindropsandsnowflakesmaybealteredbyatmosphericconditionsencounteredbeneaththecloudandtransformedinto