Compilationsofexpertreviewers’commentsonSAP2.3ExpertReviewers:Name Affiliation Email Comments

ReceivedSundarChristopher UniversityofAlabama,

Huntsvillesundar@nsstc.uah.edu 8/25/08

SteveGhan PacificNorthwestNationalLaboratory

steve.ghan@pnl.gov 7/21/08

DanielJacob HarvardUniversity djacob@fas.harvard.edu 7/24/08JohnOgren NOAAESRL john.a.ogren@noaa.gov 8/21/08SusanSolomon NOAAESRL Susan.solomon@noaa.gov 8/25/08

Ourresponsesareundereachcommentwithtextcolored.

Overall:

DanielJacob:

Ifoundthisdocumenttobeoverallverywellinformed,objective,comprehensive,accurate,andwellwrittenforitsintendedaudience.IthinkthatitislargelyacceptableasisbutIhaveanumberofcommentsfortheauthors’consideration.

Allthecommentsareaddressed(seereplyforeachchapters).Allchaptershavebeensignificantlyrevised.

SteveGhan:

Thefocusofthereportseemstobeonresultsratherthanapproaches.Thatmakesitdifficulttoidentifyspecificrecommendationsforfutureapproachesotherthanthebroadintegrationofmodelsandmeasurementsthemeorappealsforimprovedunderstanding.Muchmorecouldbesaidaboutthepresentrepresentationofaerosolprocessesinclimatemodels,toputcallsforfutureimprovementsincontext.

Thisreport(includingallchapters)hasbeensignificantlyrevised.Specifically,nowChapter2andChapter3reviewtherecentresultsinobservationsandmodeling,respectively,anddiscusstheoutstandingissues.Chapter4nowpointsoutthewayforward.Wehavebeeninstructednottomakespecificrecommendationsforfutureapproaches,buttoshowthefuturedirections.

(SteveGhansaysallyestothesevenquestionslistedbelowinJohnOgren’sreview.)

JohnOgren:

1. Arethegoals,objectivesandintendedaudienceoftheproductclearlydescribedinthedocument?Doestheproductaddressallquestionsoutlinedintheprospectus?

Notothefirstquestion,andyestothesecond.Thegoalsandobjectivesarenotdescribed.Theprospectusliststwoobjectives,oneforalltheCCSPScienceandAssessmentProducts“promoteaconsensusabouttheknowledgebaseforclimatechangedecisionsupport,”andoneforthisparticularproduct“provideasynthesisandintegrationofthecurrentknowledgeoftheclimate‐relevantimpactsofanthropogenicaerosols”.Theseobjectivesarenotdiscussedinthereport.Theprospectusisquiteclearthattheintendedaudienceis“policymakersandpolicyanalystsbothwithinandoutsidetheUSgovernmentandworldwide,interestedinthesetwoareas[climatepredictionandremotesensing]arethemostlikelytargetaudience.”Thereportdoesnotdescribethisintendedaudience.JohnOgren,NOAA/ESRL

Nowtheobjectivesandintendedaudiencehavebeenclearlyidentifiedatthebeginningofthisreport(ExecutiveSummary).

2. Areanyfindingsand/orrecommendationsadequatelysupportedbyevidenceandanalysis?Incaseswhererecommendationsmightbebasedonexpertvaluejudgmentsorthecollectiveopinionsoftheauthors,isthisacknowledgedandsupportedbysoundreasoning?

Yes,andyes.

3. Arethedataandanalyseshandledinacompetentmanner?Arestatisticalmethodsappliedappropriately?

Yes,andyes.

4. Arethedocument'spresentation,leveloftechnicality,andorganizationeffective?Arethequestionsoutlinedintheprospectusaddressedandcommunicatedinamannerthatisappropriateandaccessiblefortheintendedaudience?

No,andno.Thedocumentdoesnotdescribetheintendedaudience,butitappearstowrittenlargelyasascientificreviewpaperwheretheaudienceisotherscientists.Thisisnotthetargetaudiencedescribedintheprospectus.

Thedocumenthasbeenverysignificantlyrevisedwithpolicymakersinmind.Thejargonsareavoided,thetermsareclearlydefinedandexplained.

5. Isthedocumentscientificallyobjectiveandpolicyneutral?Isitconsistentwiththescientificliterature?

Yes,yes,andmostly.Specificcommentsoninconsistencieswiththescientificliteraturearegivenintheline‐by‐linecommentsbelow.

6. Isthereasummarythateffectively,conciselyandaccuratelydescribesthekeyfindingsandrecommendations?Isitconsistentwithothersectionsofthedocument?

No,andyes.TheExecutiveSummaryis9pageslong,whichIconsiderexcessive.The“lookingforward”bulletsareaneffectivewaytosummarizethekeyrecommendations.

Theexecutivesummaryhasbeensignificantlyrevisedtobemoreconciseandparalleltothechapters.Eventhoughitisstillabout8pageslong,thelengthisappropriatedforprovidingnecessaryinformationandsummaryfortheentirereport.

7. Whatothersignificantimprovements,ifany,mightbemadeinthedocument?

IsuggestthatChapters2and3bereorganized.Bothchaptersdiscussmodels,measurements,andtheirsynthesis,Chapter2fromtheperspectiveofpeoplemakingmeasurementsandChapter3fromthemodellers’perspective.Thisstructuredoesnot

addresstheinformationneedsofthetargetaudience,wherethemeasurement/modelperspectiveoftheauthorsisirrelevant.Abetterstructurewouldbetocreatethreechapters,onethatdiscussesmeasurementsandmeasurement‐measurementcomparisons,onethataddressesmodelsandmodel‐modelcomparisons,andathirdthataddressesmeasurement‐modelcomparisonsandsynthesis.

WedecidedtokeepChapter2focusingonmeasurementsandChapter3onmodeling.Meanwhile,Chapter2containsagoodfractionaboutusingdatatoimprove/constrainthemodelandChapter3mentionedtheneedfordatatoevaluatethemodel.Thereforeanadditionalchaptercombiningdataandmodelisnotnecessary,buttheintegrateduseofthemhasbeendiscussedinallchapters.

Therehavebeennumerouspapersandreportsinthepastthatmaderecommendationsforfuturedirections(e.g.,theSeinfeldNRCreport,andthePARAGONpapers),andmanyofthepastrecommendationslookverysimilartowhatisbeingrecommendedinthecurrentreport.Ithinkthatitwouldbeusefulforthetargetaudiencetoseeadiscussionofwhathasbeenrecommendedinthepastvs.whatisbeingrecommendednow,includingrecommendationsfora“wayforward”thatmightleadtoanew(notrecycled)setofrecommendationsin5‐10years.

Thepreviousstudiesarenowsummarized,andthesignificantlyrevised“TheWayForward”chapterhasclearlypointedoutthedirectionsthatarefromthisreport,notrecycledfromotherreports.

Thequalityofthefiguresisuniformlypoor,andinmanycasesthetextinthefiguresisbarelyreadable.

Nowallthefiguresareofhighquality.

SundarChristopher:

Thisreportprovidesanassessmentandreviewoftheeffectofaerosolsonclimate.Thereportdiscussesthedirectandindirectaerosoleffectsandhowmeasurementsandmodelingarecontributingtoourunderstanding.Thereportalsoprovidesabriefsynopsisonhowaerosolsmayinfluenceprecipitationandweather.

Theexecutivesummaryisbrokenintoseveralcomponentsandineachsectionitoutlinesthemeasurementsandmodelingframeworksanda‘lookingforward’sectiononhowtoreducetheuncertaintiesandmovethefieldforward.Thegoalsandobjectivesareclearlyoutlinedandarecompatiblewiththequestionsoutlinedintheprospectus.TheentiredocumentreliesheavilyonvariousreviewactivitiesconductedoverthepastseveralyearsandalsoontherecentIPCCAR4report.Thereforethecurrentdocumentisconsistentwiththescientificliterature.

Thedifferentchaptersalsoprovideagoodoverviewofthetopicandtheimportantissuesthatneedstobeaddressed.

Icommendtheauthorsforreviewing,documentingandprovidingacomprehensiveassessmentofaerosolimpactsonclimatewhilealsoprovidingrecommendationsonhowtoreduceuncertainties.

WhilethedocumentreliesheavilyonCCSPactivitiesofthepastandthemorerecentIPCCreport,itwillbebeneficialtotheintendedaudienceiftheexecutivesummaryincludedintheformofatable‐thesignificantareasofprogressinmeasurementsandmodeling.EspeciallyusefulwillbeinformationonspecificimprovementsasafunctionofpreviousIPCCandCCSPefforts.

TokeeptheESshortandconcise,theprogressesinmeasurementsandmodelingaresummarizedinshort,bulletisedsentencesandparagraphs,whilethetablesarecontainedinChapters1‐3.

SusanSolomon:

Pages 4-133. I appreciate that many of the authors have worked hard, and on a limited timetable. There is much in the document that is useful and well done, particularly the sections on observations. Thanks.

Pages 4-133. Missing from the document and missing from the summary is a balanced discussion of the fact that the issue of what constitutes an indirect aerosol forcing, and what constitutes a feedback. This is an important matter and is presently unclear. It isn’t appropriate to highlight potentially important ‘forcings’ without making this issue very clear. Please make appropriate additions on page VIII, line 38 and in the main text (see comments on other chapters).

The report has been significantly revised. Now the terms are clearly defined and consistently used throughout the report. We have also added a “Glossary” to reinforce the definitions.

CommentsonExecutiveSummary:

DanielJacob:

1. p.vi:somewhereinthis‘lookingforward’section(andalsointhebodyofthedocument)itmaybeusefultoposetheproblemofhowtoassimilatesatellitemeasurementsofTOAradiancesinmodelsasconstraintsonaerosolloadings.AdifficultyinusingsatelliteAODproductstoevaluatemodelsistheinconsistencyinassumptionsofaerosolpropertiesbetweenthemodelandthesatelliteretrieval.Thedocumentshouldatleastmentionthisdifficulty,whichcanbeavoidedbyusingTOAradiances.

SentenceaddedtofirstparagraphofES3.2.ModelingAtmosphericAerosols:“InconsistencyinassumptionsofaerosolpropertiesbetweenmodelsandaerosolretrievalsmaybeavoidedbycomparingTOAradiancesinsteadofAOD.”

2. p.vii:Iwouldarguethatthelargestuncertaintyinmodelingofanthropogenic

aerosolsisintherepresentationofchemicalprocesses,notemissions.Largestuncertaintynolongerclaimedtobeemissions.NewsentenceinES3.2:“Multi‐modelexperimentsindicatethatdifferencesinthemodels’atmosphericprocessesplayamoreimportantrolethandifferencesinemissionsincreatingthediversitybetweenmodelresults.”

3. p.vii,alsointextofdocument:therearetonsoffielddataforevaluatingmodel

simulationsofwetdeposition.Whatisreallyneededinmyviewisanimprovedleveloftheoryforinclusioninmodels.Cloud‐resolvingmodelscouldbeusefulforthispurpose.

WetdepositionisnolongeraddressedspecificallyintheExecutiveSummary.Subsequentchaptersdiscussatlengthaerosol‐cloudprocessesandhowinsitudataisusedtoconstraindifferentaerosolandcloudprocesses.

4. p.viii,alsointext:tosaythat“Uncertaintieswilllikelyincreasebeforethey

decrease”mayseemwittybutisabitsilly.Newsciencedoesnotincreaseuncertainty.Itmayrevealthatprioruncertaintywasunderestimated,butthat’sadifferentstatement.

Offendingsentencedidnotmakeitintotherevision.5. p.x,alsointext:mentionicecores,lakeandoceansedimentsassourcesof

informationonpastaerosoltrends?Toodetailedforexecutivesummary.DiscussionoftrendsremovedfromExecutiveSummary.

JohnOgren:

[Note:Manyofthisreviewer’scommentswereeditorialsuggestionsthatarenotapplicableinthesubstantiallyreviseddocument.]

1. ChapterES,Pageiii,Line21:Thereshouldbeadiscussionofthedistinctionbetweenforcingsandfeedbacksinthissection.

DiscussionofforcingsandfeedbacksarediscussedintheIntroduction,butwerefelttobetoodetailedfortheExecutiveSummary.

2. ChapterES,Pageiv,Line4‐5:Aretherealternativeexplanations?Forexample,

coulddifferencesinmodeledclouddistributionsbeasimportantasdifferencesinmodeledaerosolforcings?

NolongerapplicableforES3. ChapterES,Pageiv,Line6‐7:Thisusageofthewords“extensive”and“intensive”is

jargon,andneedstobeexplained.Theterms“extensive”and“intensive”arenolongerusedinthisreport.

4. ChapterES,Pageiv,Line12:Towhatdoes"this"refer?Theprecedingsentence,or

theentireparagraph?NA.EShasbeensignificantlyrevisedbasedonallreviewers’comments.

5. ChapterES,Pageiv,Line46:TheintroductorypartoftheExecutiveSummaryends

withaparagraphondirectforcing,whichforbalanceshouldbefollowedbyaparagraphonindirectforcingsandfeedbacks.

IndirecteffectsgivenmoreweightinEStobetterbalancepresentation.6. ChapterES,Pagev,Line1:does"their"refertomeasurementsorproperties?

NA7. ChapterES,Pagev,Line9:WithinNOAA,theacronym“GMD”candenotetheGrants

ManagementDivisionortheGlobalMonitoringDivisionoftheEarthSystemResearchLaboratory(ESRL).Toavoidconfusion,Irecommendthatallreferencesto“GMD”inthisreportbechangedto“ESRL”.

TheseacronymsarenolongerusedintheES.ForotherchaptersthenameESRLisused.8. ChapterES,Pagev,Line42:Change“GMD”to“ESRL”.

NA9. ChapterES,Pagev,Line43:Change“mulit”to“multi”.

NA10. ChapterES,Pagevi,Line1:Measurementsofspectralaerosolabsorptionare

currentlybeingmadeatmostlong‐termstations.Theemphasisshouldbeonupgradingthestationswithmoreaccurateinstruments.

InSectionES2Measurement‐basedassessmentofaerosolradiativeforcing,sentencenowreads:Developmentanddeploymentofnewandenhancedinstrumentationincludingaerosolmassspectrometersabletodeterminesizedependentcompositiononfasttimescales,andmethodsfordeterminingaerosollightabsorptioncoefficientsandsinglescatteringalbedowithgreateraccuracy.

11. ChapterES,Pagevi,Line10:addasentence(orperhapsaseparatebullet?)onthe

needforsystematicintegrationofsatelliteandsub‐orbitalmeasurements,toquantifyuncertaintyofsatelliteretrievalsandenhancetheirinterpretation.

ThefinalsectionoftheESincludesthefollowingstatements:“Individualsensorsorinstrumentshavebothstrengthsandlimitations,andnosinglestrategyisadequateforcharacterizingthecomplexaerosolsystem.Thebeststrategyistomakesynergisticuseofmeasurementsfrommultipleplatforms,sensorsandinstrumentshavingcomplementarycapabilities.”ANDalso“Mostimportantly,aworkingsynergybetweendifferenttypesofmeasurements,betweendifferenttypesofmodels,andespeciallyanenhancedsynergybetweenmeasurementsandmodelsiscritical.”

12. ChapterES,Pagevi,Line13:addaclauseatendofsentence,“andatlong‐term

sites”.NA

13. ChapterES,Pagevi,Line24:change“between”to“among”.

NA14. ChapterES,Pagevi,Line27‐29:whereistheverbinthissentence?

NA15. ChapterES,Pagevi,Line34‐36:Theneedtodealwithaerosolforcingsonregional

scales,ratherthanglobalaverages,isakeypointthatshouldbeincludedinthe“lookingforward”bullets.

InES3.4thisstatementismade,“However,sinceaerosolforcingismuchmorepronouncedonregionalscalesthanontheglobalscalebecauseofthehighlyvariableaerosoldistributions,itwouldbeinsufficientorevenmisleadingtoplacetoomuchemphasisontheglobalaverage.”Thisstatementfollowsfrommanystatementsmadeinothersectionsconcerningtheregionalnatureoftheaerosolforcing.

16. ChapterES,Pagevii,Line39:Itwouldbeusefultoreconcilethevaluesinthe

precedinglineswiththeTOADCFinIPCCAR4of‐0.5(‐0.9to‐0.1).Rewritten

17. ChapterES,Pageviii,Line39:Thereshouldbesomediscussionofwhichofthe

variousindirecteffectsareobserved.Thisisdoneindetailinsubsequentchapters.IntheESwedefinethe“cloudalbedoeffect”,butavoidthedetailsandjargonoftheotherindirecteffects.TheparagraphintheESreads,“bymodifyingamountsandmicrophysicalandradiativepropertiesofclouds(the“indirecteffects”).Aerosolsinfluencecloudpropertiesthroughtheirroleascloudcondensationnuclei(CCN)and/oricenuclei(IN).IncreasesinaerosolparticleconcentrationsmayincreasetheambientconcentrationofCCNandIN,affectingcloudproperties.ACCNincreasewillleadtomoreclouddropletssothat,forafixedcloudliquidwatercontent,theclouddropletsizewilldecrease.Thateffectleadstobrighterclouds(the“cloudalbedoeffect”).Aerosolsmayalsoaffectcloudsbyabsorbingsolarenergyandalteringthe

environmentinwhichtheclouddevelops,thuschangingcloudpropertieswithoutactuallyenteringtheclouddropletsasCCN.”

18. ChapterES,Pageviii,Line43:PleasespecifywhichIPCCreport.

WenowareclearwheneverspecifyinganIPCCreport.19. ChapterES,Pagex,Line8:Inadditiontobeingamovingtarget,the“globalaerosol

system”islargelyanirrelevantconcept.Themanyregionalaerosolsystemsareamorerelevantwaytoaddressaerosol‐climateinteractions.

Wementionthe“globalaerosolsystem”onceintheES.Thistermhasbeenusedpreviouslywithinthecommunity,andanentireJGRspecialissueusedthattermforitstitle.Wefeelthatthetermisusefulinthecontextused.

20. ChapterES,Pagex,Line9‐10:WRONG.Thespatialdistributionsofsourcesand

sinksofgreenhousegasesarealsochangingondecadaltimescales.Offendingstatementremoved.

21. ChapterES,Pagex,Line17‐18:Thiswordingmakesitsoundlikeaerosolsare

responsiblefortheobserveddimming/brightening,whichhasnotbeenproven.Suggestalternatewording:"neededtoendspeculationabouttheroleofaerosolchangesintheobserveddimmingorbrightening."

Dimming/brighteningnolongeraddressedintheES.22. ChapterES,Pagex,Line27:Shouldn'tthewayforwardincludeastatementabout

testingthisassumption?TheWayForwardhasbeencompletelyrewritten.

23. ChapterES,Pagexi,Line25:“Theresponsetoforcingdependsontheforcing.”Isn’t

thissoobviousthatit’snotworthsaying?Offendingstatementremoved.

24. ChapterES,Pagexi,Line25‐28:Thisisn’tmuchofawayforward.Isthisallthat

yourwanttorecommendbedonetoimproveknowledgeoftheclimateresponsetoaerosolforcing?

TheWayForwardhasbeencompletelyrewritten.Thereviewershouldnotethatthecommitteewastaskedspecificallynottomakespecificrecommendations.A“summaryandassessment”isnotarecommendation.

25. ChapterES,Pagexi,Line32:Howisthisanimprovement?Whathasbeenbetter

quantified?Howcanyoudemonstratethatthisisthemostimportantofall?NA

26. ChapterES,Pagexi,Line40‐45:Thesetwobulletssummarizethewayforwardfor

theentirereport,notjustsection7.TheWayForwardhasbeencompletelyrewritten.

SundarChristopher:

1. Theexecutivesummaryprovidesa2pageoverviewandsevenseparatesectionsrangingfrommeasurementsofaerosolpropertiestoaerosolinteractionwithprecipitationandweather.However,the2pageoverviewmerelysummarizesthe‘accomplishments’inthedirectradiativeeffect(DRE)ofaerosols.Irecommendthatbeforethestartoftheindividualsectionsaconciseitem‐by‐itemoverallsummaryisprovidedonwhatisthecurrentstateoftheartandwhatneedstobedonetoreducetheuncertainties.Thisinformationisintheindividualsectionsbuttheintendedaudiencecanbenefitfromaconciseitemizedsummary.ItwillbeveryusefuliftheauthorsprovidedaconciseassessmentofhowtheaerosolforcingcompareswithGHGandimplicationstotemperaturesratherthancitingonlyHansenetal(2007)onpageX(Line45).

2. CitingAERONETnumbersinamodelingcontextinsection2.1maynotbeappropriatebecauseofthespatialsamplingissues.

3. Insection3.1,thereaderneedstobeinformedthattheDREattheTOAandatthesurfaceforclearskyarenotentirelyfrommeasurementsandthestandarddeviationcitedisnotanuncertaintyestimatebutitisthestandarddeviationamongthevariousmethods.Newstudieshavealsoshownthatthedefinitionofaerosolforcingisimportanttoreconcilenumbers(Bellouinetal.,2008‐JGR).

4. Section4.1needstostartwithashortdescriptiononhowtheseaerosolindirecteffects(AIE)areassessedfromanobservationalperspective.ThissectionalsoneedstodrawfromrecentworkofhowmeteorologycouldconfoundtheAIE(e.g.Yuanetal.,2008).

TheExecutiveSummaryhasbeensignificantlyrevisedwiththereviewers’commentsincorporated.Itnowparallelsthesubsequentchapters,highlightingthemostimportantpoints.Ithighlightswhatweknownowwithoutgoingintodetail,anditendswithaWayForwardthatshowswhatthenextstepsshouldbe,withoutmakingspecificrecommendations.

SusanSolomon:

1. PageVIII,line38.Pleaseaddthefollowingimportantstatement:“Recentstudieshaveraisedquestionsregardingwhichaerosolindirecttermsareappropriatetoconsiderasaforcing,andwhicharemoreproperlydealtwithasafeedback.Currentuncertaintiesinthisveryfundamentalissuemakeitdifficulttoquantitativelycompareaerosolforcingstoforcingsduetolong‐livedgreenhousegases,andalsohasimplicationsforhowthesetermsareconsideredandcomparedinclimatemodels.”Pleasealsoaddthisclearlywhereitisdiscussedinthemaintextaswell.

TheissueofcategorizingcloudeffectsaseitherforcingorfeedbackwasconsideredtobetoodetailedfortheExecutiveSummary.Adiscussionisincludedinsubsequentchapters.Itwasdecidedtodefinefeedbackasonlyresultingfromchangestosurfacetemperature.

2. PageIII,line37‐39.Hereandelsewhere,theissueofregionalforcingsishighlighted

eventhoughitiselsewhereacknowledgedthattheseprobablymatterverylittletoclimate.Deletethishere.PageXI,lines9‐11haveitrightandotherstatementsshouldnotcontradictthisorunbalanceitsmessage.

Wethinktheregionalforcingsarecriticallyimportanttobeingabletopredictthestateoftheatmosphereinachangedclimate.Spatiallyvariableforcingwillinitiateorinhibitregionalcloudpatternsandcirculations.

3. PageIII,line45andpageIV,lines1‐14.Thisisunbalancedanddoesnotbelonginanexecutivesummary,particularlynotherewhereitappearsyouaretryingtogiveoverarchingconclusions(whichoughttobesolidones).Whatishereisalsoinaccurateandispresentedinthereportinaverybiasedway.Kiehletal.examined9models;thereare23intheIPCC(2007)reportsoitisnotappropriatetogeneralizethis,anditisalsoinappropriatetostatethatthereisa‘tuning’.Thishasnotbeenproventothelevelofthoroughnessrequiredinanassessment.Delete.

TherewrittenExecutiveSummaryismuchlessdetailed.Offendingsectionhasbeenremoved.

4. PageIV,line37.ItisinappropriatetorecastanIPCCconclusionthatclearlystated‘mostofthewarmingofthelast50yearsisverylikelyduetogreenhousegaseincreases’toinsteadbeaboutglobalchange,todropthe‘mostof’andtochange50yearsto100years.Thisisincorrectandmustbecorrectedifacrediblereportistobeproduced.Youcansaythatimprovedunderstandingofaerosolshasimprovedourunderstandingofkeyaspectsofaerosolradiativeforcing,particularlythetotaldirecteffect.Pleasethenadd:‘Aerosolconcentrationsarebelievedtohavechangedverylittlesinceaboutthemid‐1970s,theperiodofwhichmostofthewarmingofthepast50yearshasoccurred,andmostofthewarmingofthepast50yearsisthereforenotverydependentuponaerosolchangesbutratherupongreenhousegasincreases’

Wefeelthatweareveryclearinthismatter.Thenetwarmingisprimarilyduetogreenhousegases.However,inordertorepresenttheobservedtemperaturetrends,aerosolforcingmustbeincorporatedintothemodels.However,becauseofthewiderangeofhowaerosolsarerepresented,predictionsremainuncertain.Thereviewer’sproposedsentenceadditionisincorrect.Wedoknowthataerosolconcentrationshavechangedsignificantlyonregionalbasessince1970,althoughitisnotclearonaglobaltrend.

5. PageVI,line15,topageXI,line44.Theorderofmuchofthismaterial(andinthemainreport)doesnotseemtometobeappropriate.Itwouldbefarbettertointroducetheconceptofforcing,thentalkaboutobservations,thentalkaboutmodels.Puttingthewaythingsarerepresentedinmodelsfirstisunbalanced,becauseitfailstofirstexplainallthemanybarrierstorepresentingthingsinmodels(i.e.,observationstellustherearemanychallengestodoingthat)butinsteadmakesitsoundasifthemodelsareallbad.Pleasereorder.Also,pleasemakeclearthroughoutthereportwhenyouaretalkingaboutaerosolmodelingandwhenyouaretalkingaboutcoupledAOGCMs.

TheEShasbeencompletelyrewrittenwithadifferentstructure.WenowtrytokeepclearthedifferencesbetweenCTMandGCMs.

6. PageVIII,lines37‐38.Isthisindirectforcingimportantforlocalclimateresponses?Clarify,please.

IntherewrittenESindirecteffectsaredescribedwithgreateremphasiswithasmuchdetailasisappropriatefortheES.

7. PageIX,line23.Pleaseaddthefollowingstatement:‘Somestudiessuggestthatsuchhighresolutionandinteractiveapproachestocalculatingfeedbackssubstantiallyreducetheaerosolindirecteffectscomparedtolessdetailedtreatments.”

IntherewrittenESindirecteffectsaredescribedwithgreateremphasiswithasmuchdetailasisappropriatefortheES.

CommentsonChapter1,Introduction:Thischapterhasbeenalmostcompletelyrewritteninresponsetoreviewercomments.Indeed,oneofthereviewers(RalphKahn)hasservedasleadauthorfortherevisedtext.Assuch,essentiallyallthepointsraisedhavebeenaddressed.DanielJacob:

1. Page9,lastparagraph.“…forthenextsetofIPCCsimulationstheprimemodeofoperationmaycontinuetobeoff‐linesimulations…”Thiswillnotbegoodenoughforaerosolindirecteffects.Onlinesimulationsarenecessaryforatleastonesimulationperemissionscenario.Seecommentsregardingpage132.

Theentiresectioncoveringthecontentsofthereporthasbeenrewritten.2. Page10,inset1,secondparagraph:muchattentionwaspaidtothiseffectofaerosol

inthemid‐to‐late‐seventieseightiesinstudyingtheconsequencesofanuclearwar.Inset1wasentirelyeliminated,asthetechnicaldetailspresentedarenotdirectlyrelevanttothematerialinthereport.JohnOgren:

1. Chapter1,Page1,Line30‐31:Thisuseoftheword"aerosol"isinconsistentwithitsdefinitionas"asuspensionofparticlesinair".Aerosolscontainparticles,buttheyarenotparticles.Cloudscanalsobeconsideredtobeanaerosol.

Aerosolsarenowdefinedassuspendedparticleswithinacertainsizerange,distinguishingthemfromclouddroplets.

2. Chapter1,Page1,Line42:“thoughttobe”‐whysuchweakwording?IPCCAR4states“itisvirtuallycertainthatanthropogenicaerosolsproduceanetnegativeradiativeforcing.”

TheconclusionsofIPPCAR4arenowdescribedaccurately,andtheirownterminologyisusedandexplainedaspartofthediscussionofFigure1.3.

3. Chapter1,Page3,Line8:Misleading.63%isremovedwithin3000km,37%istransportedfurther.

Thishasbeenrewritten.

4. Chapter1,Page3,Line22:Weakwordingagain,whereIPCCAR4uses“virtuallycertain.”

TherelevantIPCCterminologyisnowusedexplicitlyandexplained.

5. Chapter1,Page3,Line24:Comparableonaglobalscale,butaerosolforcingcanbemuchlargeronregionalscales.

Right.Theregionalnatureofaerosolimpactsisnowamajorthemeofthischapter.

6. Chapter1,Page3,Line39:WRONG.Greenhousegasesdonothaveuniformdistributions.Troposphericozonedistributionsarefarfromuniform,andCO2andmethanedistributionsshowhemisphericdifferences.

Yes.Long‐livedGHGsarenowspecifiedexplicitly.

7. Chapter1,Page4,Line41:“firstidentified”‐TheIPCCreportswerenotthefirst.Thishasbeencompletelyrewrittenandthehistorycorrected.

8. Chapter1,Page5,Line34:Not"farmoreofanoffset",rather"canexceedthemagnitudeoftheGHGforcing,leadingtoanetnegativeTOAforcingfortheregion".Probablyshouldcautionagainaboutuncertaintyofaddingforcings.

SeeanswertoPoint5above.

9. Chapter1,Page6,Line19:ShouldincludeareferencetotheSeinfeldNRC(1996)report“AerosolRadiativeForcingandClimateChange”.

Done.Points10‐15.Thistexthasbeenentirelyrewritten,sothesuggestedcopyeditsnolongerapply.

10. Chapter1,Page6,Line20:Change“to”to“for”.11. Chapter1,Page7,Line18:deletecomma.12. Chapter1,Page7,Line19:change“develop”to“estimate”or“evaluate”13. Chapter1,Page7,Line20:change“are”to“is”14. Chapter1,Page8,Line18:ConfusingreferencestoIPCCreports,sometimetheyear

isusedandsometimesthereportname(“AR4”)isused.Pickasinglestyleandstickwithit.

15. Chapter1,Page9,Line23:fixspellingof“distributions”.Points16‐24.ThethreeInserts,formerlyonpages10‐12ofthechapter,wereeliminatedintherevisedversion(seeresponsetootherreviewers).

16. Chapter1,Page10‐11,Line1‐44:It’snotclearwhytheseinsetsareevenincluded.Whoisthetargetaudience,andisthisinformationthattheyneedanddon’talreadyknow?

17. Chapter1,Page12,Line3:Itisappallingtoseeanincorrectdefinitiongivenfortheaerosolextinctioncoefficient.Itisadimensionedquantity,notafraction!

18. Chapter1,Page12,Line9:Itisequallyappallingtoseeanincorrectdefinitiongivenfortheaerosolopticaldepth.Itisadimensionlessquantity,notalength!

19. Chapter1,Page12,Line11:BeconsistentthroughoutthereportofthespellingofÅngström.Itisapropernoun,andthediacriticalmarksoverthe“A”and“o”arebothrequired.Checktheentirereport.

20. Chapter1,Page12,Line27‐28:Anindividualparticledoesnothaveasizedistribution

21. Chapter1,Page12,Line29:addqualifier“allelsebeingequal(compositionandsizedistribution)”

22. Chapter1,Page12,Line33:whatdoes“aerosolsbecome..morescattered”mean?

23. Chapter1,Page12,Line33:change“moreclouddropletnumber”to“higherclouddropletnumberconcentrations”

24. Chapter1,Page12,Line44:shoulddefineTOAintheinsetSundarChristopher:

1. Chapter1–Introductionisconcisebutitlackssomerelevantdetailsthatwillbenefittheintendedaudience.Forexample,thechaptershouldstateclearlythecurrentunderstanding.ItalsorequiresaTableofaerosols,relevantprocessesthatgeneratestheseaerosolsandtheaveragelifetimes.Section1.2.2willbenefitifadiscussiononhowourunderstandingonaerosolshasimprovedfromtheprevioustothecurrentIPCCreportsisincluded.Itwillbeusefultoincludequantitativenumbersthroughoutthissection(e.g.Line28,Page5,howmuchis‘muchgreater’).Page7,Line4,howmuchisquitewellknown.Page7,Line14‐17needsreferences.Page7,Line25/26,howmuchis‘shorttime’?Page9,Line45,Reword‘savingofaerosoldistributions’.

Currentunderstandinginnowmorecompletelycoveredinthischapter,therequestedtablehasbeenadded,aerosollifecycleprocessesarenowdescribed,asummaryofrelevantIPCCreportresultsisgiven,andnumbersareprovidedinSection1.2forradiativeforcing.Thespecificeditsrequiredarenolongerrelevant,astheentiresectionhasbeenrewritten.

2. Inset1onpage10isdifficulttoreadsincetherearenumerousparentheses.Pleaserestructuresentences.

Inset1wasentirelyeliminated,asthetechnicaldetailspresentedarenotdirectlyrelevanttothematerialinthereport.

3. Page12,Line30.Dustcould‘become’hydrophilicifmixedwithotheraerosoltypes.Inset3wasreplacedwithaGlossaryattheendoftheoveralldocument,andthecontentsentirelyrewritten.

CommentsonChapter2,Measurements:DanielJacob:

1. p.18:thetextaboutCTMsbeingusedto“interpolateandsupplement”fieldobservationsseemswaytoooptimistic.Wearenotatthatlevelintermsofobservationalqualityorcoverage.

Wehaverewordedthesentence.

2. p.20:IMPROVEshouldnotbeincludedinTable.Extinctionisnotmeasured,just

chemicalcomposition.IfyouhaveIMPROVEthenyoushouldalsoincludeEMEPandotherairqualitynetworks.

According to their web site, IMPROVE measures light scattering coefficients with nephelometers at a number of sites (http://www.nature.nps.gov/air/monitoring/MonHist/parameter.cfm). In addition, we now mention the EMEP and WMO GAW networks in the text.

3. p.27:somementionofLITEinbox2.4?Yes,ithasbeenspelledoutandreferenced.

4. p.27,captionofFig2,2:A‐trainisinpresent,notfuture.Fixed.

5. p.29:thecommentabout30%increaseinDREandDCFrelativetoaprioriobviously

dependsonwhataprioriisused.Notveryhelpful.Deleted.

6. p.30‐35,alsop.53:section2.3.1isjustalengthysummaryoftheBatesetal.paper.

Suchparochialismdoesn’tseemappropriateforareviewdocument.This section has been changed substantially. The detailed description of the Bates et al. paper has been removed with only fundamental results described. Major results of aerosol characterization from NEAQS, MILAGRO, and ICARTT have been included. References to other work (e.g., Koch et al., 2007; Shindell et al., 2008) have been added. The section now reports on three ways in situ measurements can be used to improve models: validation of CTMs with measured in situ aerosol properties, use of measured optical properties in RTMs, and development of simplifying parameterizations for use by RTMs based on field observations.

7. p.37:infigure2.12,definealpha.Reflectivity?Alphaisbroadbandsurfacealbedo.Toavoidconfusionwith“alpha”foraerosolmassextinctionefficiency,wearenowusingAforthesurfacealbedo.AlsothisfigurehasbeenmovedtoChapter1introduction(i.e.,Figure1.4)todiscusssensitivityofaerosoldirectforcingtoAODandthemeasurementrequirement.

8. p.40:figure2.14ismessyandfigure2.15isnothelpful.Bothfigureshavebeendeleted.

9. p.43:MODISdataoverlandhavebiggerproblemsthan10‐15%.Reworded.

10. p.48:section2.4.1isweak.Itdoesn’ttellmewhyresolvingverticaldistributionis

critical.Thesectionisre‐structuredandimportanceofresolvingaerosolverticaldistributionisnowdiscussedinsection2.4.3.

SteveGhan:

1. Page29,secondparagraph.Constrainingmodelswithin­situmeasurements.ShouldalsomentionsthatmeasurementsofextensivepropertiesofaerosolcanbeusedeffectivelytoevaluateCTMsonaregionalbasis,whichcansuggestimprovementsinsuchuncertainmodelparametersasemissionsfactorsandscavengingcoefficients,thusincreasingconfidenceinregionsandperiodswhereandwhenobservationsarenotavailable.

Added a paragraph that reads “Short-term, focused measurements of extensive aerosol properties (e.g., aerosol concentration and AOD) also can be used to evaluate CTM parameterizations on a regional basis in order to suggest improvements in such uncertain model parameters as emission factors and scavenging coefficients (e.g., Koch et al., 2007). Improvements in these parameterizations using observations yields increasing confidence in regions and periods where and when measurements are not available. To evaluate extensive properties generated by CTMs on broader scales in space and time, satellite observations and long-term in situ measurements are required.”

2. Page29,thirdparagraph,Integrationofsatellitemeasurementsintomodelsimulations.ShouldnotethatGCMsusedtosimulatethepre‐andpost‐satelliteerascannotassimilatesatellitemeasurements.Thus,avaluableroleforsatelliteretrievalsistoevaluatetheGCMsimulationduringthesatelliteera.Modelrefinementsguidedbysuchevaluationscanthenbeusedinimprovesimulationsofthepre‐andpost‐satelliteeras.

We agree. The paragraph now reads: “Global measurements of aerosols from satellites (mainly AOD) with well-defined accuracies offer an opportunity to evaluate model simulations at large spatial and temporal scales. The satellite measurements can also be used to constrain aerosol model simulations and hence the assessment of aerosol direct radiative effect through data assimilation or objective analysis process …….. Recent efforts have also focused on retrieving global sources of aerosol from satellite observations using inverse modeling, which may be potentially valuable for reducing large aerosol simulation

uncertainties (Dubovik et al., 2007). Model refinements guided by the model evaluation and integration practices with satellite retrievals can then be used to improve aerosol simulations of the pre- and post-satellite eras”. 3. Page53,lastsentence.IfBatesetalshowedthattheestimatedDREusingmeasured

opticalpropertiesandsimulatedaerosolmassisgreaterby30%andthatestimatedusingaprioriopticalproperties,thenthatbiascouldpartiallyexplainthedifferencebetweentheAODretrievedbysatelliteandestimatedbymodels.

As pointed out by other reviewer, the 30% increase in DRE and DCF relative to a priori obviously depends on what a priori is used. Note that the satellite-model differences were examined using models that are not the same as those used in Bates et al. (2006). So we are hesitated to make the suggested claim.

JohnOgren:

1. Chapter2,Page20,Line1‐27:Itwouldbehelpfultoprovideacitationand/orURLforeachnetwork

Done.2. Chapter2,Page20,Line7:ARMonlyhasonemobilefacility.Ithasnositesin

EuropeorAsia,althoughthemobilefacilityhasbeendeployedonthosecontinents.Fixed.3. Chapter2,Page20,Line10:Change“GMD”to“ESRL”.Done.4. Chapter2,Page20,Line11‐15:TheNOAA/ESRLnetworkmeasuresthe

hemisphericbackscatterfraction,nottheupscatterfraction.TherearefivebaselinestationsintheNOAA/ESRLnetwork.

Fixed.5. Chapter2,Page21,Line1:TheARMAerosolIOPisnotlistedinTable2.1.Nowlisted.6. Chapter2,Page21,Line2:Badgrammar,“thattargetedatcharacterizing”Fixed.7. Chapter2,Page21,Line40:Beconsistentwithnotation,chapter3usesadifferent

symboltodenotesingle‐scatteringalbedoFixed.8. Chapter2,Page22,Line22‐23:Change“GMD”to“ESRL”.Thecitationsrefertothe

in‐situmeasurementsbythisnetwork,whichdon’tbelonginasectiononremotesensing.

Fixed.

9. Chapter2,Page22,Line37‐43:TheWMOGlobalAtmosphericWatchshouldbementionedinthissection.Theparagraphiswrittenasiftheroleoftheground‐basednetworksistosupportintensivefieldcampaigns.Theyhaveagreaterrolethanthat,forexample,determiningtrends,providingclimatologiesofaerosolradiativepropertiesusedbythemodels,andprovidinglong‐termdatasetsfortestingmodels.

We now mention the WMO GAW network as well as other national and international networks in Section 2.2.2. We also have added text describing more fully the information obtained from long-term surface measurements as suggested by the reviewer.

10. Chapter2,Page22,Line39:Change“GMD”to“ESRL”.Done.11. Chapter2,Page22,Line46:Lateroninthisreport,theneedforcharacterizationof

aerosolpropertiesinthecolumnisdiscussed.Chapter2.2wouldbeagoodplacetodiscussin‐situaerosolprofilingprograms(DOE/ARM,NOAA/ESRL,UCSD/Scripps).

We have added a section (2.2.3) describing the regular flights over Bondville and Southern Great Plains.

12. Chapter2,Page28,Line21‐22:Thereareplentyofotherreasonsfortheuncertaintyofmodelsimulations,forexample,thedifficultyofproperlyrepresentingcloudprocesses.

Weagreeandthepointhasbeenadded.13. Chapter2,Page31,Line11:Thetextdiscusses“BC”,whilethefiguresuse“EC”.It

appearsthatthesetwotermsarebeingusedassynonyms,whichisnotcorrect.Thissectionhasbeensignificantlymodified.Thereferredfigureandtexthavebeenremoved.14. Chapter2,Page41,Line21,26:Thestandarderrorusedhereisthestandarderror

ofthemean,andsoitisappropriatetoreportitalongwiththemean,notthemedian.

Fixed.15. Chapter2,Page46,Line4‐5:Iagreecompletelywiththisstatement.Itshouldbe

featuredprominentlyelsewhere,likeintheexecutivesummaryandasanintegralpartofthe“wayforward”chapter.ThisroleofuncertaintyanalysiswasacentralfeatureoftherecommendationsoftheSeinfeldNRC(1996)report.

Weagree.Executivesummary&WayForward????16. Chapter2,Page48,Line42:Howcanlidars“wellconstraintheaerosol‐induced

atmosphericheatingrate”whenmostarenotsensitivetoaerosollightabsorption?Wehaverewordedthesentences.17. Chapter2,Page50,Line34:change“inversed”to“retrieved”Done.

18. Chapter2,Page52,Line3:Suggestre‐wordingthebeginning“Toquantifyregional‐

scaletrends…”.Globalscaletrendsarenotparticularlyrelevant,andlocaltrendscanbemeasuredwithground‐basedinstruments.

Weagree.Done.19. Chapter2,Page53,Line38:Thissectionistoolong,andthereisalotofrepetition

ofmaterialalreadypresented.Rewordedtoavoidrepetition.20. Chapter2,Page71,Line22:Changeto“ESRLEarthSystemResearchLaboratory

(NOAA)”.Done.

SundarChristopher:

1. MuchofChapter2comesfromarecentreviewofYuetal(2006)anditmayrequiresomemodificationsandadditionstoincludepaperssince2006.Table2.1,‐includeDABEXtotable(Haywoodetal.,2008,JGR).Page23,Line18,ReferenceforOMI.NeweralgorithmsalsoexistforOMIotherthanTorresetal.Example–Veihelmannetal(2007‐ACP)andothers.

Yes,updated.Ageneralnote:thisteamhavetakenaseriouslookatthosepotentialreferencesandchosenmostimportantstudiestocite.Itisclearthatwehaveaddedalargeamountofmajorresultsinthisrevision.Asonecanunderstand,notallpublishedpapersshouldbeincludedinthisreport.2. Page25,Line8needsaccuracyestimatesforDeepBluesinceMODISoperational

algorithmestimatesareprovided.Added.

3. Page28,Lines39‐45andPage29,Lines1‐3areanabrupttransitionintheclosure

studiessection.Ratherthandiscussaspecificstudyitwillbeusefultogeneralizethemainpoints.

Done.

4. Section2.3.1doesnotappeartofitinthisreportforvariousreasons.AlthoughIagreethatthereneedstobeadiscussiononhowinsitumeasurementscanimprovemodelingestimatesofDRE,thisentiresectionistakenfromonepaper–Batesetal(2006).ThereportcansimplyprovidethenecessaryrelevantinformationandreferencetheBatesetal(2006)paperforfurtherreading.

This section has been changed substantially. The detailed description of the Bates et al. paper has been removed with only fundamental results described. Major results of aerosol characterization from NEAQS, MILAGRO, and ICARTT have been included. References to other work (e.g., Koch et al., 2007; Shindell et al., 2008) have been added.The section now reports on three ways in situ measurements can be used to improve models: validation of CTMs with measured in situ aerosol properties, use of measured optical properties in RTMs,

and development of simplifying parameterizations for use by RTMs based on field observations.

5. Page36,Lines21and22requiressomediscussion.Byhowmuchhavethesedifferencesreduced?Iagreethattherearedifferencesamongsatellite‐derivedproductsbutthisdoesnotnecessarilymeanthattheycanbeattributedtocloudcontaminationonlyandthereforethediscussiononPage26,Lines30‐33needstobeexpanded.

ThereductionofMODIS‐MISRAODdifferencehasbeenspecifiedbasedonLevyetal.(2007b)andKahnetal.(2005b).AnumberofreasonshavebeenidentifiedfortheMODIS‐MISRdifference,asdiscussedinKahnetal.(2007).Weneverbelievethatcloudcontaminationistheonlyfactor.

6. Page39,Line43‐44,referenceZhangetal(2005,JGR)whoaddressescloud

contaminationissuesandhowtocorrectforthese.Added.

7. Most,ifnotallofsection2.3.3.isfromYuetal(2006)anditisappropriateforthis

report.However,itisimportanttoeitherhaveaseparatesectionorsomewrite‐uponwhathasbeendonesincethattime.Forexample,Patadiaetal(2008)GRLnowshowthatwithimprovedangularmodelsforlandareasfromCERESalandDREispossiblefromMODIS/MISR/CERES.–Page44,Line11.Chandetal(2008)addressissuesrelatedtoverticaldistributionofaerosolsfromCALIPSO.Bellouinetal(2008)discusstheimportanceofusingthecorrectdefinitionsforforcing,etc.

Wehaveincludedthesereferencesinthisrevision.8. Attheendofthissectionandbeforethestartof2.3.5itwillbeusefultoprovidea

conciseitemizedsummaryofwhatthestateoftheartisandwhatarethenextstepsforward.Thisissprinkledthroughoutthetext(example,page45,Lines25‐29)anditwillbeusefultohavethisallinoneplace.

Wehavere‐organizedthissectiontoaddressthecomment.Butwehaveleft“thenextstepsforward”forthereport’schapter4(Thewayforward).Inchapter2,wefocusonthesummaryofcurrentunderstanding(i.e.,progressesandissues).

9. Section2.3.5onaerosol‐cloudinteractionisdefinitelyweakerwhencomparedto

theDREsection.Thissectioncouldbeseparatedintoiceandwatercloudeffects.Thenthewatercloudimpactsshouldbediscussedintermsofthevariousindirecteffects.TherehavebeenvariousreviewpapersregardingtheAIEandsynthesizingthiscouldbeextremelyuseful.

Thissectionhasbeensubstantiallyenhanced.Afterabriefintroduction,wenowhavetwosub‐sections:2.3.5.1:Remotesensingofaerosol‐cloudinteractionsandindirecteffects(bothsatelliteandground‐based);and2.3.5.2:In‐situstudiesofaerosol‐cloudinteractions.Thesetwosectionsgiveacomprehensivesummaryofwhatwehavelearnedfromdifferentapproaches,whataremajorreasonsforthedifferencesintheaerosol‐cloudinteractionsderivedfromdifferentapproaches/studies,andwhatare

currentmeasurementaccuracyforkeyvariables.Anumberofimportantstudieshavebeenreferenced.10. AfewitemsseemoutofplaceinSection2.4.Section2.4.1startswithadiscussionof

aerosolverticaldistributionsbutthenitquicklymovestodustthermaleffects.Theverticaldistributionsofcloudsandaerosolsareimportantforboththesolarandthermaleffects.Section2.4.2shouldincluderecentpapersforoverlandstudies.

Re‐organized.

11. Section2.4.alsorequirenewerreferences–example,Yuanetal,JonesandChristopher,Qaasetalandsomediscussiononhowmeteorologycouldconfoundthecause‐effectrelationshipsthatareusuallyexplored.

See our response to your comment #1.

12. Theconcludingremarksinsection5inpages54and55couldbebrokenintoaconciseformatthatwillallowthereadertogaugewhatthenextstepsmightbe.

Thanks for the suggestion and the section is streamlined.

CommentsonChapter3,modeling:

DanielJacob:

1. p.83:“required”‐1.2Wm‐2–explainACCEPTED

2. p.84‐:section3.2oncomparingaerosolobservationsandmodelsisjustanextendedsummaryoftheLiuetal.2006andGinouxetal.2006papersas“examples”.AlsoalotofrelianceonLacis,personalcommunication.Thatseemstooparochialforanassessmentpaper.ThebeginningofthesectionrepeatsmaterialfromtheIntro.

ACCEPTED;ORDERANDEMPHASISSOMEWHATCHANGED

3. p.105:notclearwhyscavengingdependsoncomposition.Evenhydrophobicparticleswouldbescavengedbywashout.Notclearwhythe“sulfateplusinsolubleparadigm”maybecomelessapplicable.

Differentaerosolcomponentshavedifferentsolubilitywhentheyarefreshlyemitted,sotheyhavescavengingefficiencies.Whentheyareaged,thedifferenceappearsverysmall.Wechangethewordinginthetext.Theothersentenceisalsomodifiedtomakeitmoreclear.

4. p.106:I’mnotconvincedthat‘extensiveobservations,underdifficultconditions’(whatarewetalkingabouthere?)arewhat’sneededtoimprovethetreatmentofupdraftsandassociateddropletactivationinmodels.Theoreticalworkleadingtoimprovedmodelparameterizationsseemslikeamoreproductivedirection.

ACCEPTED;CHANGEDEMPHASIS

5. p.118:Healdetal.2005referencemissingINAPPENDIXA2NOWREMOVED;ITWASUSEDINCHAPTER2NOW

6. p.119:Idon’tthinkthatthestateofknowledgeofSOAwarrantsastatementaboutisoprenebeingpossiblytheprincipalprecursor.

APPENDIXREMOVED.SOAISNOWDISCUSSEDINCHAPTER2

SteveGhan:

1. Page90,thirdparagraph.Excessiveblackcarbonabsorptionwoulddecrease,notincreasethesingle‐scatteringalbedo.

ACCEPTED2. Page96,line2.ReplacedonateswithconditionsACCEPTED3. Page98,figure3.16issameimageasfigure3.14.ACCEPTED;RIGHTFIGURESUBSTITUTED

JohnOgren:

1. Chapter3,Page83,Line27:whereisthe“followingfigure”?Pleasespecifythefigurenumber.

ACCEPTED,butfigureremoved

2. Chapter3,Page84,Line21:Aerosolscatteringisnever“primarilybackward”.Rayleighscatteringisequalinforwardandbackwardhemispheres,whileforwardscatteringisgreaterthanbackwardsscatteringforMieandgeometricscattering.

ACCEPTED

3. Chapter3,Page88,Line7:“Qualitativeagreement”isn’tparticularlyusefulhere,weneedtobedevelopingandusingquantitativemeasuresofagreement.Otherwise,wejustcontinuetowaveourhands,andareunabletodemonstratetheextenttowhichmodelormeasurementimprovementsarereducinguncertainties.

ACCEPTED

4. Chapter3,Page91,Line27‐29:Whybotherwiththiscomparison,then?ACCEPTED;PHRASEREMOVEDANDCOMPARISONEXPLAINEDBELOW

5. Chapter3,Page96,Line2:replace“donations”with“emissions”ACCEPTED,EXCEPTUSED“CONDITIONS”.

6. Chapter3,Page97,Line33‐34:Evenifthefullscopeisbeyondtheframeworkofthisdocument,itwouldbeusefultoprovideasummaryofthemostimportantissues.

SOMEDETAILSMENTIONEDINCHAPTER4

7. Chapter3,Page100,Line44‐45:Thedistinctionbetweenmeteorologicalfeedbackandaerosolforcingcanbecomequiteopaque.Iagree.Butdoyoureallywanttoleavethisstatementhangingoutthere,andforcetheintendedaudiencetofigureouthowtointerpretit?Thisreportcouldmakeavaluablecontributionifitweretoexplaintheimportanceofthedistinctionbetweenforcingandfeedbacktoapolicy‐makerwhohastomakepolicydecisionsbasedonthisscientificdiscussion.

ACCEPTED;DEFINITIONPROVIDED

8. Chapter3,Page101,Line10:“constantstuned”Thisisaverytroublingclause.Towhatextentareanyofthemodelstunedgogiveaparticularresponse?Whataretheconsequencesofsuchtuningforthecredibilityofmodel‐derivedforcings?

ACCEPTED;OBSERVATIONSUSEDFORCALLIBRATIONNOWREFERREDTO

9. Chapter3,Page106,Line18:Fromtheearlierdiscussions,Ithoughtthattherewasclearobservationalsupportforthefirstpartofthechain:increasedaerosol‐>increasedclouddropconcentration‐>smallerdrops

PHRASEREMOVEDANDPARAGRAPHPROVIDESEXPLANATION

10. Chapter3,Page106,Line27‐30:Didn'tBakerandCharlson(Nature,1990)makeasimilarpoint?Ifso,youshouldcitethemhere.

ACCEPTED;ADDITIONALPOINTISNOTMADE

11. Chapter3,Page108,Line10‐12:Thispointisworthemphasizing(“shoutitfromtherooftops”)intheexecutivesummary…weneedmuchbetterconstraintsontheaerosolforcinginordertoadequatelyconstraintheclimatesensitivityfactor.Thisreportwillbeafailureifthetargetaudiencedoesn’tgetthistake‐homemessage.

ACCEPTED

12. Chapter3,Page108,Line23:Theword“effect”canincludebothforcingsandfeedbacks.You’rejusttalkingaboutforcingshere,sosay“forcing”.

ACCEPTED

13. Chapter3,Page109,Line43‐45:There’snotmuchofaconstraintiftheverticalprofileofaerosolabsorptionisn’tknown.Thatreinforcestheneedformeasuringaerosollightabsorptionaloft.

ACCEPTED;PHRASEMODIFIED

14. Chapter3,Page110,Line21:Theobjectiveisn’ttoquantifyspeculation,itistoeliminatespeculationandreplaceitwithquantitativeresults.

ACCEPTED

15. Chapter3,Page115,Line18:Doesthetargetaudiencereallyneedthisappendix?

ACCEPTED;Appendixremoved

16. Chapter3,Page115,Line25:missingn(r)after“where”.Appendixremoved

17. Chapter3,Page116,Line23‐24:replace“scattering”with“extinction”Appendixremoved

18. Chapter3,Page119,Line18:Veryinterestingappendix,butwhat’sthebottomline?ThattargetaudienceneedstoknowtheuncertaintyofaerosolforcingduetouncertaintyofSOAformation.

ACCEPTED;Appendixremoved

19. Chapter3,Page120,Line1:Thisisaveryinterestingappendix.Itraisesthequestionofwhichdiscussionsshouldberelegatedtoappendicesandwhichbelonginthebodyofthereport.Asitstandsnow,thebodyofthereportissoweighteddownwithdetailsthatit’shardtoseethebigpicture.Reorganizationintoamorerationalstructureshouldhelp,butIsuggestconsideringtechniquesthatwillmakeiteasierforthetargetaudiencetoseetheforestinsteadofacollectionoftrees.Sidebarsandappendicesshouldbothbeconsidered.

ACCEPTEDINTERMSOFRORGANIZATIONBUTAPPENDIXREMOVED

SundarChristopher:

1. ChapterIIIdrawsheavilyupononepublishedstudy(Liuetal,2006)andmysuggestionistoreducethenumberoffiguresfromLiuetalbysimplyprovidinganoverviewandreferringthereaderformoredetailstothispaper.

ACCEPTED;FIGURESREMOVEDANDSECTIONSHORTENED2. Onpage91,thePenneretal(2002)referencecanbeupdatedorabetterdiscussion

canbeprovided.TheERBEprovidesclearskyfluxes–notreflectances(Page32)andtheCERESdatahasmuchimprovedcalibrationandneweralgorithmswithbetterfluxestimations.SeepapersbyLoebetal(2005).TheCEREShasmuchbettercloudclearingtechniqueswhencomparedtotheERBE.

TAKENINTOACCOUNT

SusanSolomon:

1. Page75,lines41‐46.Sameissueasmycommentabove.YoucannotrecastanIPCCconclusionthatclearlystated‘mostofthewarmingofthelast50yearsisverylikelyduetogreenhousegaseincreases’todropthe‘mostof’andtochange50yearsto100years.Thisisincorrectandmustbecorrected.Youcansaythatimprovedunderstandingofaerosolshasimprovedourunderstandingofradiativeforcing.Pleasethenadd:‘Aerosolconcentrationsarebelievedtohavechangedverylittlesinceaboutthemid‐1970s,theperiodofwhichmostofthewarmingofthepast50yearshasoccurred,andmostofthewarmingofthepast50yearsisthereforenotverydependentuponaerosolchangesbutratherupongreenhousegasincreases’

ACCEPTEDINPRINCIPLE;DIFFERENTIATIONBETWEENTHEREALITYOFGREENHOUSEWARMING,WHICHTHISDOCUMENTDOESNOTDISPUTE,ANDCALCULATIONOFCLIMATESENSITIVITYISNOWMADEMORECLEAR

2. Page76,lines1‐13.Theobviouslysarcasticuseofthephrase‘inconvenienttruth’isunprofessionalandinappropriateinascientificassessment.Further,therestoftheparagraphisbiasedandovergeneralizes,asnotedinmycommentsaboveregardingtheKiehletal.paper.Kiehletal.examined9models;thereare23intheIPCC(2007)reportsoitisnotappropriatetoovergeneralizethis,anditisalsoinappropriatetostatethatthereisa‘tuning’.Thishasnotbeenproventothelevelofthoroughnessacrossthefullsetofmodelsrequiredinanassessment.Also,Kiehletal.donotreachtheconclusionthatthisimpliesatuning.Finally,itisalsounbalancedandinappropriateinanassessmenttorefertotheSchwartetal.paper,andanewsarticlebyajournalist(Kerr),andomitthescientificrebuttalprovidedbyKnuttietal.,(2008).TheKnuttietal.paper,amongothermatters,madeclearthatthelast50yearsarewhatiskeyfortheattributiondebate,anditalsoshowedwhyovergeneralizationsaboutclimatesensitivityshouldbeavoidedgiventheimportantroleofoceanheatcontentandclimateinertia.Thisparagraphhasmanyproblemsandshouldbedeletedinitsentirety.

AGAIN,ACCEPTEDINPRINCIPLE;SEECOMMENTABOVE.‘OFFENDING’PHRASEREMOVED.

3. Page76,line40.Clarifywhatismeantbyrange.Pleaseensurethatallcomparisonsarequantitative,withappropriatediscussionsofuncertainties.Youcannotjusttalkabouta‘range’becausethathasmanydifferentmeanings.

ACCEPTED;RANGENOWGIVEN

4. Page103,lines33‐42.Needsabetterdiscussionofwhyindirecteffectsmaybedifficulttoconstrain.

ACCEPTED;morediscussionadded

5. Page104,line35.Pleaseadd:“Inthiscase,theeffectisafeedbackandnotaforcing.”

ACCEPTED

6. Page105,line42.Pleasedonotusecolloquialandunclearphraseslike‘achillesheel’inanassessment.Thenonspecialistwillnotknowifthismeanthemodelsaredead,dying,unfixable,orwhat.

ACCEPTED

7. Page107lines3‐23.Thisdiscussionneedserrorbars.Assertionsaboutinconsistenciescanonlybemadeifitisclearwhattheerrorbarsareonthingsbeingcompared.

ACCEPTED;ERRORBARSNOWPROVIDED

8. Page108,lines1‐12.Thisisoversimplified.Amongotherreasonsforparticularchoicesamongmodelstudiesareotherforcings–e.g.,amodelthatdoesnotincludetroposphericozonenorblackcarbonbecauseofcomputationallimitationsmightchoosetoreduceuncertainaerosoltermstocompensate.Thisdoesn’tmean‘tuning’isgoingonbutratherthatreasonablechoicesarebeingmadeforpracticalreasons.Pleaseaddthis.

ACCEPTEDINPRINCIPLE;PARAGRAPHHASBEENALTEREDAND‘TUNING’DROPPED

9. Page109,line34.Pleaseaddareferencetochapter9ofIPCC(2007),whichalsomadeclearthatadetailedanalysisofpatternsinspaceincludingSH/NHgradientsandheightgradientscanbeusedtoplaceimportantconstraintsonaerosols.

ACCEPTED

CommentsonChapter4,WayForward:

DanielJacob:

1. p.128:paragraphonaerosoltrendsisbeggingforafewreferences.ACCEPTED

2. p.129:intermsoffuturemodelimprovements,howaboutbeingabletosimulateTOAreflectancesforcomparisontosatellitedataasconstraintsonaerosolloadings?

ACCEPTEDANDPUTINTOTHETEXTINSEVERALPLACES

3. p.129‐130:againthedraftplaysupuncertaintiesinemissions,butIthinkuncertaintyinchemicalprocessesismoresevere.

ACCEPTEDANDTHEPROPERBALANCEBETWEENTHESETWOUNKNOWNSISRESTORED

4. p.130:itmightbestatedthatprogressinaerosolsimulationinGCMsistiedtoprogressinrepresentationofthehydrologicalcycle.

ACCEPTEDSteveGhan:

1. Page132,firstparagraph.Itistimetomovebeyondtheexternally‐mixedtreatmentofpastclimatemodels.TreatmentsofinternalmixingareavailableandhavebeenappliedtotheGISSandNCARmodels.TreatmentsofthedropletnucleatingandopticalpropertiesofinternalmixturesareavailableandhavebeenappliedtoatleasttheNCARmodel.WhetherthoseschemeswillbeusedinthenextIPCCassessmentwillbedecidedinthenextfewmonths.Butcertainlywithinthenextdecade(theapparenttimeframeoftherecommendations)suchtreatmentswillbeincorporatedinclimatemodels.

ACCEPTED–COMMENTADDED2. Page132,thirdparagraph.Thiscriticalparagraphreallyleavesthereaderhanging,

concludingthat“Thisapproach,however,wouldfailtoprovidemuchofthenecessaryinformation”.Isuggestawayout,whichmaybewhatwasintended,butisnotexplainedclearly.IncorporatealloftheadvancedaerosolprocessesintheGCMs,simulatethemonceforeachemissionsscenario,andthen,iftheaerosolprocessesaretooexpensivetoruninallotherrealizationsofthescenarios,inthesubsequentsimulationsreadinthemonthlymeanaerosolfromthehistoryoftheonlinesimulations.Itispossiblethataerosolvariationsonsubmonthlytimescalesareimportant,butpreliminaryexperimentsbymesuggestthatestimatesofaerosoldirectandindirecteffectsusingonlineandofflineaerosolsarecomparable.Isuggestthefollowingchanges.First,inthefirstlineofthesecondparagraphonthispage,insert“climate”before“models”.Thenreplacethethirdparagraphwiththefollowing:“Alltheseprocesseswillrequireobservationstounderstandthem,andextensivecomputertimetosimulatethem.Iftheaddedcomplexityistoo

computationallyexpensiveforalloftheensemblesofclimatesimulationsexpectedforIPCC,asolutionwouldbetotreattheaerosolprocessesonlineintheclimatemodelforonlythefirstmemberoftheensembleforeachaerosolemissionsscenario.Fortheothermembers,theprocessesthatchangetheaerosolcouldbereplacedwithtemporalinterpolationofthemonthlymeanaerosolsfromthefirstmemberoftheensemble.Preliminaryexperimentswithsuchaconfiguration(Ghan,personalcommunication2008)suggestthatestimatesofaerosoldirectandindirecteffectsusingofflineaerosolarecomparabletoestimatesusingonlineaerosols.Theinteractionbetweenclouds,relativehumidityandaerosolonsub‐monthlytimescalesisevidentlyunimportantprovidedthemonthlymeansareconstrainedtobethesame”.Thismightbebitlong.Thelastsentencecouldbeomitted.

ACCEPTEDINPRINCIPLE.THEPARAGRAPHHASBEENREWRITTEN,WITHAMOREEXPLICITDISCUSSIONOF‘THENEXTSTEPS’.

JohnOgren:

1. Chapter4,Page123,Line1:4.2readslikerequirementsfromthemeasurerspointofview.4.3readsliketherequirementsfromthemodellerspointofview.What'sneededisthesynthesisofthosetwosections,andeliminationofduplication,asthereadercaresaboutthewayforward,notthewriter'sperspective.

ACCEPTEDINPRINCIPLE.SOMEDEGREEOFSYNTHESISWASDONEBYINCORPORATINGTHEOBSERVATIONSNEEDEDTOIMPROVEMODELSTOGETHERWITHTHEMODELING.

2. Chapter4,Page123,Line43:Whatarethespatialandtemporalaveragingscalesthatcorrespondtothetargetuncertainty?

ACCEPTED;SCALESADDED

3. Chapter4,Page124,Line2:Needtospecifythatthemodelsbeingassessedareglobal,asmanymoreparameterscanbeevaluatedwithregional‐scalemodels.

ACCEPTED

4. Chapter4,Page124,Line5:It’sacurrentgoalaswellasafuturegoal.ACCEPTED

5. Chapter4,Page124,Line7:Shouldn’tthefuturegoalbeforthemodeltocalculatealltherequiredaerosolproperties,ratherthanspecifythem?

ACCEPTED

6. Chapter4,Page124,Line26‐28:Isn’tfiguringofftheoptimaltradeoffaresearchtopicinitself?Isthisresearchpartofthewayforward?

ACCEPTED

7. Chapter4,Page124,Line36:We’reallabittiredbythischapter,butisn’tthephrase“observationsareneededtoobserve”justaweebitmeaningless?

ACCEPTED

8. Chapter4,Page125,Line9‐11:Suggestaddingasentencethatwehavethefoundationforsuchanetwork(WMO/GAW,withNOAA/ESRLprovidingtheUScontribution),butthatadditionalmeasurementsandlocationsareneeded?

ACCEPTED

9. Chapter4,Page125,Line12:replace“do”with“to”,and“relation”with“relationships”

ACCEPTED

10. Chapter4,Page126,Line7:Change“GMD”to“ESRL”.ACCEPTED

11. Chapter4,Page126,Line7:TheNOAAnetworkbeganoperationoverthreedecadesago.

ACCEPTEDINTHESENSETHATWEHAVENOTLIMITEDTHEDISCUSSION(WHICHINCLUDESAERONET)TOONLYONEDECADE.

12. Chapter4,Page126,Line11:replace“systems”with“systematic”ACCEPTED

13. Chapter4,Page126,Line14:replace“remotesensing”with“observations”ACCEPTED

14. Chapter4,Page126,Line16:replace“rind”with“ring”ACCEPTED

15. Chapter4,Page126,Line35‐38:Toosatellite‐centric.Needtomakeitclearthatthesynergyincludessuborbitalplatforms.Thewayforwardforfuturesatellitemissionsneedstohaveintegratedsuborbitalmeasurementsaspartofthestrategy.Thatpointismadelaterintheparagraph,butbythenitseemslikeanafterthought.

ACCEPTED

16. Chapter4,Page128,Line3‐4:Thisiswheretheneedforin‐situmeasurementsofaerosolverticalprofilesisapparent,butnotexplicitlydiscussed.

ACCEPTED

17. Chapter4,Page128,Line22‐25:Theattributionalsorequiresdeterminationoftrendsincloudinessandcloudalbedo.

ACCEPTED

18. Chapter4,Page128,Line31:“Cross‐pollination”soundssocasualandrandom,itoccurswhenabeehappenstobuzzpast,whereaswhatisneededisaverysystematicanddeliberateintegrationofobservationsandmodels.

ACCEPTED

19. Chapter4,Page129,Line17‐19:Thisisanotheroneofthosekeytake‐homemessagesthatneedtobehighlightedintheexecutivesummary.

ACCEPTED

20. Chapter4,Page131,Line23:replace“wouldbevaluable”with“isrequired”TAKENINTOACCOUNT

21. Chapter4,Page132,Line28‐29:Notaveryoptimisticnotetoendthe“wayforward”chapteron.Itleavesthereaderthinkingthatwecan'tcurrentlydefineanadequatewayforward.

ACCEPTED–ENDINGHASBEENCHANGED.SundarChristopher:

1. Page124(lines1to4)arenotconsistentwithothersectionsandneedstobereworded.ThereareseveralsatellitesensorsthathaveimprovedalgorithmsforoverlandsuchasMODIS,MISRandOMIandtheseimprovementsareindeedreducinguncertaintiesandallowingfornewerresearchstudies.

ACCEPTED2. Whilesection4isusefulitmaybenefitthereaderiftherewasasummarytablethat

providedasynthesisofthewayforwardinallmeasurementandmodelingareas.ACCEPTED–THEWAYFORWARDISNOWWRITTENINACONCISEWAYWITHPOINTSCLEARLYLAIDOUT