Adv. Space Res. Vol. 11, No. 3, pp. (3)45—(3)49, 1991 0273—1177/91 $0.00 + .50Printedin GreatBritain. All rights reserved. Copyright© 1991 COSP4R

THIRTY-YEAR TREND OF OBSERVEDGREENHOUSECLOUDS OVER THETROPICAL OCEANS

Julius London,* Stephen G. Warren,** and Carole J. Hahn****Departmentof Astrophysical, Planetary and AtmosphericSciences,

University of Colorado, Boulder, CO 80309—0391, U.S.A.**Depar~entof AtmosphericSciences,University of Washington, Seattle,WA 98195, U.S.A.***Cooperative Institute for Researchin EnvironmentalSciences,University of Colorado, Boulder, CO 80309—0216,U.S.A.

ABSTRACT

Analysis of the30-yeartrend(1952—1981)of theamountsof different cloud types,asobservedfrom shipsin the tropical oceans,hasshown that, overlatitudes 20°N—20°Sduring the time period studied, cirrus(Ci) andcumulonimbus(Cb) typeshaveincreasedwhile cumulus (Cu) andstratus(St) typesdecreasedorremainednearly constant.Thegreenhouseimplicationsof thesecloudtrendsarediscussed.

INTRODUCTION

Clouds representthemostimportant single variableaffectingtheoverall net radiativeheatingor coolingof theearth-atmospheresystem. For clear skies, incoming solar radiationis largely absorbedin theloweratmosphereandat theEarth’ssurface. This energy gainedis re-emittedto spacepartially by thegroundandby theso-calledgreenhousegases.If theconcentrationof thesegasesincreases,theatmospherebecomesmoreopaqueto infrared radiation, andtheatmosphericemissioncomesfrom ahigher(colder) layer thatthenrequiresawarmersurfacetemperatureto balancetherelatively unaffectedincomingsolar radiation.

Thepresenceof cloudsstrongly modifiesthis picture. Most clouds, dependingon their physicalpropertiesandthickness,significantly reflect solar radiation and thus decreasethe availableradiative input to theearth-atmospheresystem. At the sametime (with theexceptionof very thin cirrus), cloudsact as highlyefficient infrared radiators. Their emissionto spacedependson their cloud-toptemperature.The infraredemissionfrom cloudsthusdecreasesas thelevel of theaveragecloud top increases.Whethercloudsact, onaverage,to heatorcool theatmospheredependson themagnitudeof their albedoas comparedto theeffectof theheight (i.e., the temperature)of theemitting uppersurfaceof the cloud. It hasbeenshownfromresultsof satelliteobservationsthat for aglobalaverage,cloudsact to radiatively cool theearth-atmospheresystem(see,e.g., /1,2/).

Thenetcloudradiativecooling (or warming), however,is sensitiveto cloudtype. High cloudsgenerallyhavelow albedosandrepresenthighly efficient greenhouseclouds. Low cloudshavemoderatelyhigh albedosbutrelatively warm cloud-toptemperatures,thus resultingin significantcloud-effectedradiativecooling of theatmosphere.Cloudsof largeverticalextenthaveveryhigh albedosbut alsohavecold top radiatingsurfaces.They thereforetend to maintain an approximatebalancebetweennet short-waveincoming andinfraredoutgoingradiation. However,cumulonimbuscloudsprovide an important link betweensurfaceheatingandextendedcirrusclouddevelopmentoverthetropicaloceans.It is thusimportant thatcloudradiativeforcingcalculatedfor climate modelssimulatethe time-dependentvariationsof the individual cloud types. For arecentdiscussionof the effect of different cloud typeson numericalmodelsof “greenhousewarming,” see/3/.

In our analysis,we haveconsideredthe time variationsof four different cloudtypesthat representdifferentcharacteristicsof cloudradiativeforcing: Ci (cirrus,cirrostratus,andcirrocumulus),Cb,Cu, andSt (stratus,stratocumulus,andfog).

Although some cloud-typeinformation has beenreportedfrom satellite observations(see,for instance,/4/), we usesurface-basedobservationsfor our studybecause,in most cases,specificcloud typesarebestidentifiedby surface-basedobservers.In addition, long-termdataareavailableonly from thesesurfaced-basedobservations.Surface-basedcloudobservationshavebeenarchiveddatingbackto themid—nineteenthcentury. However,a reasonablyconsistentdatasetfrom which cloud-typeinformation could be derivedisavailableonly from theearly1950s. In this paper,we discusstime trendsof thefour cloudtypes,referredtoabove,as determinedfrom ship observationsovertheequatorialandtropicaloceansfor the30-yearperiod1952—1981. Efforts arecurrentlyunderwayto extendour analysisfor an additional 5-yearperiod.

DATA SOURCE

The datafor this studywere takenfrom theComprehensiveOcean-AtmosphereDataSet(COADS; /5/).

Detailsof themethodof analysisof theclouddataover theoceans—including,for instance,dataselection(3)45

(3)46 J. Londonet a!.

Table 1 Annual AverageCloud Amounts(%) over theEquatorialandTropical Oceansfrom Surface-basedObservations(1952-1981)

Latitude Ci Cb Cu St

10°—20°N 13.0 6.0 16.3 18.00°—10°N 16.0 8.4 15.8 21.80°—10°S 12.3 7.0 16.8 18.0

10°—20°S 9.5 6.3 15.6 23.9

Table2 Trend(t) andStandardDeviationabouttheTrend (a) of theAnnual AverageCloud Amounts overtheOceans(1952—1981)

(Changein Cloud Amount over29 Years)

Latitude Ci Cb Cu St0~ i U i U

10°—20°N 0.6 0.7 2.4 0.3 0.0 0.4 0.4 1.00°—10°N1.4 0.5 3.5 0.5 —1.0. 0.4 —0.4 1.20°—10°S1.1 0.4 2.9 0.6 —1.1 0.5 0.5 1.0

10°—20°S 0.6 0.4 2.5 0.6 —0.8 0.6 —0.5 1.0

andgridding, samplingbiases,anddeterminationof averagecloud amounts—aregiven in /6/. In observa-tionswheremorethanonecloudtypewasreported,cirrus-typecloudamountsweredeterminedby makinguseof the randomoverlapassumption. In casesof lower overcast,the cirrus amountwasassumedto bethesameas in the caseswhere theupper level wasvisible. We haveusedgridded 100 x 20°(latitudexlongitude)seasonalaveragesto deriveannualaveragesfor each100 zonebetween20°Nand20°S.(Thetotalarchivedgriddeddatasetis discussedby Hahnet al. /7/. A least-squareslinear trendwasthencalculatedfrom theannualzonalaverages,andthestandarddeviationof theindividual annualzonalvaluesaboutthetrend line wascomputed.

Becauseof thestrongbiasbetweendayandnight observationsof cirrus-typeclouds,only daytimevaluesof

thesecloudswere usedin our analysis.

ANALYSIS

The 30-yearaveragesof theobservedcloud-typeamountsover theoceansare given in Table 1 for each

10°latitude zonebetween20°Nand20°S.The percentcloudamountis thepercentof sky coveredby the

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YEAR YEAR

Fig. 1. Annual cirrus,cirrostratus,andcirrocumuluscloud amount: zonalaveragesandtrend(1952—1981).

Thirty-YearTrendof GreenhouseClouds (3)47

12 I I I I I I I I I I I I I I I I I I I I I I I I I I I I 12 I I I I I I I I I I I I I I I I I I I I I I I I I I I I

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2 I1I1III1II 111111111111111111 2 IIIIIIIIIiI 111111 11111 III Iii

525456585052846668707274767880 525456586062648668707274787880YEAR YEAR

Fig. 2. Annual cumulonimbuscloud amount: zonal averagesandtrend(1952—1981).

specifiedcloud type,including (for high clouds)amountshiddenby lower clouds. Cloud-typeamountsovertheoceansas functionsof all latitude zonesaregiven in /6/ for eachof the four seasons.The dominantcloud typesin thetropics arestratus,altostratus,andaltocumulus.Cirrus, cirrostratus,andcirrocumulusamountsare almost twice theamount of cumulonimbusclouds. The relativeproportionof thesecloudamountsis similar to theglobalaverageexceptthat stratus,stratocumulus,andfog (groupedas St) aremuchmoreprevalentin middle andhigh latitudesthanin the tropics.

Year-to-yearvariations of theamountsof the four different cloud types, determinedfrom ground-basedobservations,aregiven in Figures1—4 for eachof the four latitude zonesconsideredhere. Includedin the

20 I I I I I I I I I I I I I I I I I I I I I I I I I I I I 20 I I I I I I I I I I I I I I I I I I I I I I I I I I I I

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525456586062646688707274767680 525456586062646668707274767880

YEAR YEAR

Fig. 3. Annual cumuluscloud amount: zonal averagesandtrend (1952—1981).

(3)48 J. London ef a!.

30 ____________ 30 _____________1111111111111111 11111111111111111111111111FF

28 - 2826 . . 26

24 I0°~20°H 24

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tO I I I I I I I I I I I I I I I I I I I I I I I I I I I 10 I I I I I I I I I I I I I I I I I I I I I I I I I I I I525456586062646668707274767880 525456586062646668707274767880

30 ____________ 30 _____________I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I

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10 I I I I I I I I I I I I I I I I I I I I I I I I I I I I_ 10 I I I I I I I I I I I I I I I I I I I I I I I I I I I I

525456586062646668707274767880 525456586062646668707274767880

YEAR YEAR

Fig. 4. Annualstratusandstratocumuluscloudamount: zonal averagesandtrend (1952—1981).

figuresarethecomputedtrend lines for eachtypeandzone. Table 2 lists thecomputedtrend (i) andthestandarddeviationofthe individual annualvalues for eachcloud type. Figures1—4 aresimilar to diagramsin themicroficheattachmentto ouroceancloud atlas/6/.

During the30-yearperiodcoveredby thedata, thepercentchangeis positiveandgenerallysignificant atthe2o- level for Ci andCb at all latitudesbetween20°Nand20°S.Thepronouncedpositivetrend shownforcumulonimbusclouds(i.e., about25 percentof theaverageCb amountin thetropics),althoughstatisticallysignificant,seemssuspiciouslylarge. Part of this apparentchangecouldhavearisenfrom codinginstructionsandsubjectivedifferencesin observerclassifications.The percentchangeof St during this period is smallandnot significantly differentfrom zero. Thelargecomputednegativetrendsof Cu in theequatorialzonesandsoutherntropicsresult, in part, from thepositiveCu cloudanomaliesasreportedin 1952—1953. If thesetwoyearsareomitted, thenegativetrendsaresignificant(la) only neartheequator.Note alsothatduringthesefirst two years,theaveragestratusamountswere belowthe30-yearaverage.

Ground-basedobservationsshowthat over the tropical oceans,more than40 percentof reportedcumu-lonimbusare accompaniedby reportsof cirrus-typeclouds /8,9/. This associationreflectsthe fact thatin the tropics, cirrusandcirrostratusmost frequentlyoriginate from thespreadingof anvil tops of cumu-lonimbusclouds. This situation is in contrastto thenormal formationof cirrus-typecloudsassociatedwithfrontal activity alongmiddle-andhigh-latitudestormtracks.Theobservedco-occurrenceindicatedbetweencirrus-typeandstrongconvectivecloudsin thetropics would provideahigh-level basefor radiativecoolingof theair in the layerat thetop of deepconvectivecloudsandareductionof theoutgoinginfrared emissionfrom relatedspreadingcirrusclouds at the top of the troposphere.The presenceof suchcirrus clouds iscounterto thesuggestionof drying out of air nearthe tropopauseandtheimplication of convectivelypro-ducedwarmingat tropopauselevels/10/. Thelatter is also inconsistentwith the resultsof WetheraldandManabe/3/ who concludedfrom their model calculationsthat thecloud-feedbackprocessassociatedwithstrongconvectivesystemsleadsto an increasein thecloud height andcloudamountat tropopauselevels,thus reducingthe temperatureat thecloud top andconsequentlytheoutgoingradiationfrom theselevels.

SUMMARY

Clouds act as prime regulatorsof the radiationbalanceof theearth-atmospheresystem. Different cloudtypesaffect theradiativeforcing of this systemdifferently. High cloudsact to warmthesystemwhereaslowcloudsact to cool thesystem. We havefound, from analysisof ground-basedcloud observationsoverthetropical oceans,that cirrus-typecloudshaveincreasedandlow-type (cumulus,stratus,andstratocumulus)cloudshaveremainedsteadyor decreasedoverthe30-yearperiod1952—1981.Theseresultsimply apossibleincreasein greenhouseforcing by changesin different cloud-typeamountsover thetropical oceansduringthis period. However, we do not know to what extent the indicatedtrendsin cloud-typeamountshavebeenbiasedby undiscoveredchangesin observingprocedures.Additional studyis plannedto extendtheseresultsin time andto coverlandareasandmiddle andhigh latitudes.

Thirty-YearTrendofGreenhouseClouds (3)49

ACKNOWLEDGMENTSThework reportedherewas supportedby NASA GrantsNAGW-1002 andNAG1-998. We thankKristin

HoyerandMary Eberlefor programmingandtechnicalassistancein preparationof thepaper.

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