increased cloud droplet concentration leads to longer subtropical...

IncreasedclouddropletconcentrationleadstolongersubtropicalstratocumuluslifetimesandadeeperPBLfromaLagrangianperspective

RyanEastman,RobertWood,UniversityofWashington,DepartmentofAtmosphericSciences

I.ALagrangianperspective

Tobettercapturethecloudcover(CC)responsetoenvironmentalperturbationswesamplethesamecloudyparcelasitevolvesintimeandspace.Wecompute~169,000trajectoriesinfoursubtropicaloceanbasinsat925hPa,allowingustofollowandcompareparcels.Lagrangiansamplingofcloudvariables(fromtheA-Train)andenvironmentalconditions(fromERA-Interim)isdoneateveryA-Trainoverpassat12-hourintervals1. WinddataaresourcedfromtheERAInterimreanalysisdataat0.75˚resolution.

II.Cloudcoverinthesubtropics

Regionsarecharacterizedbylow,thickstratocumulus(Sc)cloudsformingnearthecoastandadvecting offshore.Cloudseventuallydeepenandgivewaytotradecumulus.Thesethick,warm,brightcloudsacttocoolthesubtropics,soitisofgreatvaluetoknowthecontrolsontheirextent.CloudcoverisestimatedusingtheMODIScloudmaskproductobservedfromtheAquasatelliteinthesefourregionsshownbytheredboxes. RegionsencompassSc maximaandthedecliningcloudcover gradientsoffshore.

Notonlyistheamountofcloudcoverchangingintime,butthedepthoftheplanetaryboundarylayer(PBL)andclouddropletconcentration(Nd)arealsoevolving.WeestimatethePBLdepthusingthe𝚫Tbetweencloudtopsandtheseasurface2.

Nd ismeasuredusingMODISretrievalsofeffectiveclouddropradiusandliquidwaterpath1.Nd declinesasthePBLdeepensoffshore.

𝜌𝜔 =Densityofliquidwater𝛤eff =𝛤ad × fad𝛤ad =adiabaticrateofincreaseinliquid

watercontentwithrespecttoheightfad =estimateofthedegreeofadiabaticityLWP =LiquidWaterPath(MODIS)re =Clouddropleteffectiveradius(MODIS)h =estimateofcloudthicknessNeff ≈Nd × kk=0.8formarineSc

IV.Comparingcloudcontrollingvariables

V.Controllingforconfoundingvariables,𝚫CC

VI.Controllingforconfoundingvariables,𝚫PBL

Manyothervariablesdrivecloudcoverchange.WecandotheanalysisfrompartIVwhileholdingthesevariablesconstant,reducingtheirconfoundinginfluence.Errorboundsrepresentthespreadinslopesafterindividuallyaccountingforconfoundingvariables.

Mainresults:MoreNdprecedessomecloudincrease,though𝚫subsidence,columnwatervapor,andinversionstrength(LTS,𝜽700- 𝜽1000)dominate.

Mainresults:ThisanalysisshowsthatanomalouslyhighNd precedesPBLdeepening.However,othervariablesshowanevenstrongerrelationship,includingcolumnwatervapor,𝚫subsidence(𝜔s,700hPa),𝚫seasurfacetemperature(SST),andInversionstrength.

References

1.Eastman,R,andR.Wood,2016:Factorscontrollinglow-cloudevolutionovertheeasternsubtropicaloceans:ALagrangianperspectiveusingtheA-TrainSatellites.J.Atmos.Sci., 73,331-351.2.Eastman,R.,R.Wood,andK.T.O,2017:Thesubtropicalstratocumulus-topped planetaryboundary layer:aclimatologyandtheLagrangianevolution.J.Atmos.Sci.,74,2633-2656.3.Eastman,R.,R.Wood,andC.S.Bretherton,2016:Timescalesofcloudsandcloud-controllingvariablesinsubtropicalstratocumulusfromaLagrangianperspective.J.Atmos.Sci.,73,3079-3091.

SupportedbyNASAgrantNNXBAQ35G

Trajectoriesareseparatedintogroupsbasedoninitialconditions.The24-hourevolutionofeachgroupiscomparedtoacontrolgroupwithanidenticalfrequencydistribution.Thedifferencebetweenthegroupanditscontrolgroupiscalledthe‘residual’change1,3.Tocomparetherelativestrengthsofseveralvariables,wecomparechangesincloudcoverandPBLdepthsforstandarddeviation(𝜎)bins2ofourcloudcontrollingvariables.Variablesareconvertedtoanomaliesrelativetotheir100-dayrunningmean.

III.Evolvingclouddecks

𝑁𝑒𝑓𝑓 = 234𝜋𝜌𝜔Γ𝑒𝑓𝑓

1/2 𝐿𝑊𝑃1/2

𝑟𝑒(ℎ)3