Indian Ocean Experiment: An integrated analysis of the climate forcing and effects of the great Indo-Asian haze V. Ramanathan, P. Crutzen, J. Lelieveld, A. Mitra, D. Althausen, J. Anderson, M. Andreae, W. Cantrell, G. Cass, C. Chung, et al.

To cite this version:

V. Ramanathan, P. Crutzen, J. Lelieveld, A. Mitra, D. Althausen, et al.. Indian Ocean Experiment: An integrated analysis of the climate forcing and effects of the great Indo-Asian haze. Journal of Geophysical Research: Atmospheres, American Geophysical Union, 2001, 106 (D22), pp.28371-28398. ￿10.1029/2001JD900133￿. ￿hal-02903026￿

HAL Id: hal-02903026 https://hal.archives-ouvertes.fr/hal-02903026 Submitted on 28 Oct 2020

HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. D22, PAGES 28,371-28,398, NOVEMBER 27, 2001

Indian Ocean Experiment: An integrated analysis of the climate forcing and effects of the great Indo-Asian haze V. Ramanathan, • P. J. Crutzen, •,: J. Lelieveld,: A. P. Mitra, 3 D. Althausen,4 J. Anderson,s M. O. Andreae,: W. Cantrell,6 G. R. Cass,7 C. E. Chung,• A.D. Clarke,s J. A. Coakley,9 W. D. Collins,•ø W. C. Conant,• F. Dulac,TM J. Heintzenberg,4 A. J. Heymsfield,•ø B. Holben,•: S. Howell,s J. Hudson,•3 A. Jayaraman,TM J. T. Kiehl,•ø T. N. Krishnamurti,•s D. Lubin,• G. McFarquhar,•ø T. Novakov,•6 J. A. Ogren,•v I. A. Podgorny,• K. Prather,•s K. Priestley,•9 J. M. Prospero,:ø P. K. Quinn,:• K. Rajeev,:: P. Rasch,•ø S. Rupert,• R. Sadourny,:3 S. K. Satheesh,• G. E. Shaw,24 P. Sheridan,•7 and F. P. J. Valero •

Abstract. Every year, from December to April, anthropogenichaze spreadsover most of the North Indian Ocean, and South and SoutheastAsia. The Indian Ocean Experiment (INDOEX) documentedthis Indo-Asian haze at scalesranging from individualparticles to its contributionto the regionalclimate forcing.This studyintegrates the multiplatform observations(satellites, aircraft, ships,surface stations, and balloons)with one- and four- dimensionalmodels to derive the regional aerosolforcing resultingfrom the direct, the semidirectand the two indirect effects.The haze particlesconsisted of severalinorganic and carbonaceousspecies, including absorbing black carbon clusters,fly ash, and mineral dust. The most striking result was the large loading of aerosolsover most of the South Asian region and the North Indian Ocean. The January to March 1999 visible optical depthswere about 0.5 over most of the continent and reachedvalues as large as 0.2 over the equatorial Indian ocean due to long-rangetransport. The aerosollayer extended as high as 3 km. Black carboncontributed about 14% to the fine particle massand 11% to the visible optical depth. The single-scatteringalbedo estimatedby severalindependent methodswas consistentlyaround 0.9 both inland and over the open ocean.Anthropogenic sourcescontributed as much as 80% (_+10%) to the aerosolloading and the optical depth. The in situ data, which clearlysupport the existenceof the first indirect effect (increased aerosolconcentration producing more cloud dropswith smallereffective radii), are used to developa compositeindirect effect scheme.The Indo-Asian aerosolsimpact the radiativeforcing through a complexset of heating (positiveforcing) and cooling(negative forcing)processes. Clouds and black carbon emerge as the majorplayers. The dominant factor, however,is the large negativeforcing (-20 +_4 W m -t) at the surfaceand the comparablylarge atmosphericheating. Regionally,the absorbinghaze decreasedthe surfacesolar radiation by an amount comparableto 50% of the total ocean heat flux and nearly doubledthe lower troposphericsolar heating.We demonstratewith a general circulationmodel how this additional heating significantlyperturbs the tropical rainfall patternsand the hydrologicalcycle with implicationsto global climate.

•Centerfor AtmosphericSciences, Scripps Institution of Oceanog- •2NASAGoddard Space Flight Center, Greenbelt, Maryland. raphy, Universityof California, San Diego, California. •3DesertResearch Institute, Reno, Nevada. 2MaxPlanck Institute for Chemistry,Mainz, Germany. •4physicalResearch Laboratory, Ahmedabad, India. 3NationalPhysical Laboratory, New Delhi, India. •SDepartmentof Meteorology,Florida State University, Tallahas- 4Institutefor TroposphericResearch, Leipzig, Germany. see, Florida. SDepartmentof Geologyand Chemistry, Arizona State University, •6LawrenceBerkeley National Laboratory, Berkeley, California. Tempe, Arizona. •7ClimateMonitoring and Diagnostics Laboratory, NOAA, Boulder, 6Departmentof Chemistry,Indiana University, Bloomington, Indiana. Colorado. 7Schoolof Civiland Environmental Engineering, Georgia Institute •SDepartmentof Chemistry,University of California,Riverside, Cal- of Technology,Atlanta, Georgia. ifornia. SDepartmentof Oceanography, University of Hawaii,Honolulu, Hawaii. •9NASALangley Research Center, Hampton, Virginia. 9Collegeof Oceanicand Atmospheric Sciences, Oregon State Uni- 2øRosenstielSchool of Marineand Atmospheric Science, University versity, Corvallis, Oregon. of Miami, Miami, Florida. •øNationalCenter for AtmosphericResearch, Boulder, Colorado. :•PacificMarine Environmental Laboratory, NOAA, Seattle,Wash- •Laboratoire desSciences du Climat et de l'Environment,Gif sur ington. Yvette, France. :2SpacePhysics Laboratory, VSSC, Thiruvananthapuram,Kerala State, India. Copyright2001 by the American GeophysicalUnion. :3Laboratoirede MeteorologieDynamique, Paris, France. Paper number 2001JD900133. :4GeophysicalInstitute, University of AlaskaFairbanks, Fairbanks, 0148-0227/01/2001JD900133509.00 Alaska.

28,371 28,372 RAMANATHAN ET AL.: INDIAN OCEAN EXPERIMENT--AN INTEGRATED ANALYSIS

1. Introduction ing. If the availablewater vapor sourceis unaltered, the in- creasein the number of cloud dropswill lead to a decreasein It is now well documented[e.g., Langner and Rodhe, 1991; the drop size by inhibitingthe formation of larger drizzle size IntergovernmentalPanel on Climate Change (IPCC), 1995; drops.The suppressionof larger dropscan decreaseprecipi- Schwartz,1996] that anthropogenicactivities have increased tation (drizzlein the caseof low clouds)thus increasing cloud atmosphericaerosol concentrations. Both fossilfuel combus- lifetime and/or cloud water content, both of which can enhance tion and biomassburning [,4ndreaeand Crutzen,1997] contrib- cloud albedo. This second indirect effect has been observed in ute to aerosolproduction, by emittingprimary aerosol particles a limited set of aircraft studies[Albrecht, 1989] and with sat- (e.g., fly ash, dust, and black carbon)and aerosolprecursor ellite studies[Rosenfeld, 2000]. The global magnitudeof the gases(e.g., SO2,NO, c, and volatileorganic compounds) which indirect effect is highly uncertain,but can be large enoughto form secondaryaerosol particles through gas-to-particlecon- offset a large fraction of the entire greenhouseforcing [IPCC, version. Secondaryaerosols also have biogenicsources (di- 1995]. methyl sulfide from marine biota and nonmethanehydrocar- More recently,model studies[Hansen et al., 1997;Kiehl et bons from terrestrial vegetation). The secondaryparticles al., 1999;Ackerman et al., 2000] have indicatedthat the solar formed are categorizedas inorganic(e.g., sulfates,nitrates, heatingby absorbingaerosols (biomass burning or fossilfuel ammonia)or organic(e.g., condensedvolatile organics). The combustionrelated soot) can evaporate low clouds (e.g., combinationof organicsand black carbon aerosolsare also stratocumulusand trade cumulus);the resultingdecrease in referred to as carbonaceous aerosols. Aerosols exist in the cloud cover and albedo can lead to a net warming whose atmospherein a variety of hybridstructures: externally mixed, magnitudecan exceedthe coolingfrom the direct effect [Ack- internally mixed, coatedparticles or most likely a combination ermanet al., 2000]. of all of the above.Our primary interest in this paper is on the The climate forcing due to aerosolsis poorly characterized climate forcing of aerosols,which is definedfirst. A changein in climate models,in part due to the lack of a comprehensive radiative fluxes (be it at the surfaceor the top-of-the atmo- global databaseof aerosolconcentrations, chemical composi- sphere) due to aerosolsis referred to as aerosolradiative tion and optical properties.The uncertainrole of aerosolsin forcingand that due to anthropogenicaerosols is referred to as climateis one of the major sourcesof uncertaintiesin simula- anthropogenicaerosol forcing; and the differencebetween the tions of past and future climates.A primary goal of INDOEX two forcing terms is due to natural aerosols.Anthropogenic [Ramanathanet al., 1995,1996] is to quantifythe directand the aerosolscan modify the climateforcing directly by alteringthe indirect aerosol forcing from observations.This paper ad- radiativeheating of the planet [Coakleyand Cess,1985; Chad- dresses the climate effects of the haze over the Indian Ocean son et al., 1991], indirectly by altering cloud properties and South and SoutheastAsia (hereafter referred to as the [Twomey,1977; Albrecht, 1989; Rosenfeld, 2000], and semidi- Indo-Asianhaze). The followingaerosol field experimentspre- rectly by evaporatingthe clouds[Hansen et al., 1997;Kiehl et ceded INDOEX. al., 1999;Ackerman et al., 2000]: The overall goals of the Aerosol CharacterizationExperi- Aerosolsscatter solar radiation back to space,thus enhanc- ment (AGE 1) [Bateset al., 1998] and (AGE 2) [Raeset al., ing the planetary albedo and exerting a negative (cooling) 2000] are to reduce the uncertaintyin the calculationof cli- climate forcing. Models estimatethat the direct coolingeffect mate forcing by aerosolsand to understandthe multiphase of anthropogenicsulfate aerosols[e.g., Chadsonet al., 1992; atmosphericchemical system sufficiently to be able to provide Kiehl and Briegleb,1993] may have offset as much as 25% of a prognosticanalysis of future radiative forcing and climate the greenhouseforcing. Recently,the inclusionof other aero- response[Bates et al., 1998].ACE 1 took placein the minimally sols(e.g., organics, black carbon, and dust)have shown that the polluted SouthernHemisphere marine atmosphere,and ACE problem is cun•dcramy.... : ^ •"' more comp,ex' .... • more important 2 took placeover the subtropicalnortheast Atlantic [Raeset al., [e.g.,Penner et at., 1992;IPCC, 1995].Absorbing aerosols such 2000]. as black carbon can even changethe sign of the forcing from Smoke, Clouds, Aerosols, Radiation-Brazil (SCAR-B) negativeto positive[e.g., Haywood and Shine,1997; Heintzen- [Kaufmanet al., 1998], a comprehensivefield project to study berg et at., 1997; Liao and Seinfetd,1998; Haywood and Ra- biomassburning, smoke, aerosol and trace gasesand their maswamy,1998]. Observationalstudies have documentedthe climatic effects, took place in the Brazilian Amazon and cer- importance of absorbingaerosols even in remote oceanicre- rado regionsduring 1995. gions [e.g., Posfai et at., 1999]. There is now a substantial The TroposphericAerosol RadiativeForcing Observational amount of literature on absorbing aerosols [Grasst, 1975; Experiment(TARFOX) [Russellet al., 1999]was designedto Clarke and Chadson, 1985; Penner et at., 1992; Chytekand reduce the uncertainty in predicting climate change due to Wong,1995; Hobbs et at., 1997;Novakov et at., 1997;Heintzen- aerosoleffects and took place over the United Stateseastern berg et at., 1997; Kaufman and Fraser, 1997; Hobbs, 1999]. seaboardduring 1996 [Hobbs,1999; Russell et al., 1999]. Aerosolsalso influencethe longwaveradiation [e.g.,Lubin et INDOEX gathereddata on aerosolsand troposphericozone at., 1996], but the longwaveforcing is much smaller than the chemistryover the tropical Indian Ocean. The primary aero- solar forcing. The longwave forcing of INDOEX aerosolsis sol-relatedobjectives of INDOEX are to estimatethe direct includedin this study. and the indirect forcing of aerosolson climatologicallysignif- Aerosols influence the forcing indirectly by altering cloud icant timescalesand space scales;link the forcing with the opticalproperties, cloud water content,and lifetime. We begin chemicaland microphysicalcomposition of the aerosols;and with the first indirect effect, the so-called "Twomey effect" evaluate climate model estimatesof aerosolforcing. This pa- [Twomey,1977], which arisesbecause of the fundamentalrole per describesthe major findingsthat resultedfrom the aerosol of aerosolsin nucleatingcloud drops.An increasein aerosol objectives.The resultspresented here are for a heavily pol- number concentrationcan nucleate more cloud drops, make luted and a poorly understoodregion of the planet. It estab- the cloudsbrighter [Twomey,1977] and exert a negativeforc- lishesthe Indo-Asian haze as an important phenomenonwith RAMANATHAN ET AL.: INDIAN OCEAN EXPERIMENT--AN INTEGRATED ANALYSIS 28,373

globalclimate implications. The addedsignificance is that this for these5 yearsvary by lessthan 10% of the 1999value shown oceanicregion borders on rapidly developing,heavily popu- here. lated nationswith the potential for large increasesin the fu- ture. The INDOEX observationswere made over the tropical 2. Integration of Field Observations With Satellites and Models Indian Ocean during the Northern Hemisphere dry monsoon (December to April). Since 1996, an internationalgroup of A schematicof the integration schemeis shown in Plate 3. scientistshas been collectingaerosol, chemical and radiation The platforms(Plates la and lb) were designedto accomplish data from ships, satellitesand surface stations [e.g., Krish- two distinctivelydifferent objectives:process or "closurestud- namufti et al., 1998;Jayaraman et al., 1998], which culminated ies" and "gradient studies."Focusing on the C-130 flight tracks in a major field experimentwith five aircraft,two ships,several shownin Plate lb and 2b, flights for the processstudies were satellitesand numeroussurface observations conducted during directed from Male toward the northern Arabian Sea to un- Januarythrough March, 1999(Plates la and lb). In additionto derstandthe compositionof aerosolsand other gaseouspol- theseplatforms, the EuropeanMeteorological Satellite Orga- lutants close to their sources;process flights were also under- nization moved the geostationary satellite, METEOSAT-5, taken southward from Male to understand the aerosol over the Indian Ocean to support the INDOEX campaign. composition far away from the source regions. Next, long The first field phase (FFP) took place during February and transequatorialflights ("gradient flights") from Male to about March 1998, and the intensivefield phase (IFP) took place 10øSwere undertaken to estimate the horizontal gradient of from Januarythrough March 1999. aerosol concentration,aerosol composition,radiation fluxes, The tropical Indian Ocean provides an ideal and unique cloud condensationnuclei and drop concentration.In view of natural laboratoryto studythe role of anthropogenicaerosols the significantdaily and seasonalvariability in aerosolconcen- in climatechange. This is probablythe only regionin the world trations,we need continuousmeasurements through the year. where an intensesource of anthropogenicaerosols, trace gases For this purpose, during 1998, we establishedKCO on the and their reactionproducts (e.g., ozone) liesjuxtaposed to the island of Kaashidhoo,in the Republic of the Maidives, about 500 to 1000 km downwindof major cities in the subcontinent pristineair of the SouthernHemisphere [Ramanathan et al., (Plate lb). The islandis about3 km long and slightlymore than 1996;Krishnamufti et al., 1998]. The polluted and pristine air 1 km wide, isolatedfrom the nearbyislands (about 20 km from are connectedby a crossequatorial monsoonal flow into the the nearestMaldivian island) and about500 km awayfrom the Intertropical Convergence Zone (ITCZ), which, during nearest mainland, the Indian subcontinent.The population of INDOEX, was located between the equator and about 12øS. The monsoonal flow from the northeast to the southwest the island is about 1600 and is free from anthropogenicactiv- ities suchas industryor automobiletransport. The observatory (Plate2b) bringsdry continentalair overthe oceanwhich leads is located on the northeasttip of the island so that the local to a low level temperature inversion,mostly clear skieswith pollution, e.g., biomassburning on the island, is naturally re- scatteredcumuli (see the dark and light blue shadedregions in stricted at the observatorysince the synoptic-scalewinds are Plate 2a) and minimalrain, all of whichenable the haze laden mostlyfrom the northeastduring the period of interest. with pollutantsto accumulateand spreadon an oceanbasin In the remainder of this sectionwe summarizethe process scale. (closure)studies, we introduceMACR, which was developed The resultsand analysespresented in this paper are unique in part basedon the processstudy philosophy, we then review in many respects.It integratesobservations taken simulta- the gradient studies,and finally review how MACR will be neouslyfrom severalplatforms (Plates la and lb): top-of-the used to determine the direct and the first indirect radiative atmospheredata collectedby several satellites;in situ data forcing. from the Hercules C-130 and the Citation aircraft; surface and Near-continuous measurements were maintained at KCO column data from the R/V Ron Brown and the R/V Sagar for aerosol chemical composition,size distribution, spectral Kanya;surface and columndata from the KaashidhooClimate optical depths,vertical distributionof aerosolextinction coef- Observatory(KCO) at 4.97øN,73.5øE, in the Republicof the ficient (by multispectralLidar), cloud condensationnucleus Maidives; remotely measureddata from a multiwavelength- (CCN) spectraand radiation fluxes.These data were used to lidar station at Male (4.18øN,73.52øE) in the Republic of link aerosoloptical depth and direct aerosolforcing with aero- Maidives;and the INDOEX French stationsat Goa (15.45øN, sol chemical compositionand microphysicalproperties; link 78.3øE)and Dharwad(15.4øN, 75.98øE). These data are inte- aerosolconcentration with CCN for a wide variety of low and gratedwith a Monte Carlo Aerosol-CloudRadiation (MACR) heavily polluted conditions' and develop an aerosol optical model, satellite-derivedaerosol regional distributionover the model that agrees with surface measurementsof scattering, ocean and a four-dimensional aerosol assimilation model for absorbingcoefficients and aerosol humidity growth factors. the land regions[Collins et al., 2001].The directforcing under Aircraft data were then used to get a better representationof clear skieshas been documentedby Satheeshand Ramanathan the vertical distributionof aerosolcomposition, CCN and mi- [2000] for the KCO locationand by Rajeevand Ramanathan crophysicalproperties. These data were then incorporatedin [2001] for the entire tropicalIndian Ocean.This paper takes MACR to obtain the forcing on regional scales. on the more challengingtopics dealing with the direct forcing MACR usesmeasurements of physicaland chemicalprop- in cloudyskies and the indirectforcing. This paper focuseson erties of aerosolsas input and estimatesradiation fluxes for the IFP period of January-March1999. However, the conclu- three-dimensionalbroken cloud systemswith a realisticdistri- sionsof this INDOEX study shouldbe applicableto other bution of atmosphericgases and aerosol species.The cloud yearsas well. For example,analyses of satellitedata for 1996to parameters,such as cloud base and top heights,are obtained 2000[Tahnk, 2001] reveal that the seasonally(January-March) from the C-130, and cloud optical depth is calculatedas de-

crr[h•cl in c•ptic•n R nncl Annpncli,zt--i A The trnclp c'•rm•l•q clnuds 28,374 RAMANATHAN ET AL.: INDIAN OCEAN EXPERIMENT--AN INTEGRATED ANALYSIS

, METEOSAT5 INSAT

•,EOS •:.TRMM ScaRaB ., (RESURS) SeaW•FS NOAA14 15

Geophysica - 60 000 ft

/ Falcon C!tatton / / /

Const nt Level Balloon Sag r K nya

a, h•dhoo Ctimal Ronald H Brown Ohs rv tory

Plate 1. (a) The plate displaystwo separatebut related piecesof information.First is the INDOEX cube showingthe variousobserving platforms, which includedsatellites, aircraft, ships,balloons, dropsondes, and surfacestations at Maldives and India. The three photographsshow the picturesof the dense haze in the Arabian Sea, the trade cumuli imbedded in the haze, and the pristine southern Indian Ocean. (b) This illustrationof the INDOEX compositeobserving system shows the domainsof the variousplatforms. Geosta- tionary satellites(METEOSAT-5 and INSAT) and polar orbiters (NOAA 14, 15, and ScaRaB) provide synopticviews of the entire studyregion. The C-130, the Citation, and the Myst•re fly from Hululfi in the Maidives,while the Geophysicaand the Falcon depart from the Seychelles.These aircraft perform north- southflights from about 10øNto 17øSand east-westflights from about 75øEto 55øEmeasuring solar radiation fluxes, aerosol propertiesand distribution,cloud microphysics,chemical species,and water vapor vertical distribution.The ocean researchvessels Ronald H. Brown and Sagar Kanya travel between Goa, Malfi, La R•union, and Mauritius transectingthe intertropicalconvergence zone and samplingsurface solar radiation, aerosolabsorption and scattering,chemical species, ozone, winds, and water vapor vertical distribution.The SagarKanya also sails in the sourceregion along the westerncoast of India. Surfacestations in India (including Mount Abu, Pune, and Trivandrum), the Maldives (KaashidhooClimate Observatory),Mauritius, and La R•union measurechemical and physicalproperties of aerosols,solar radiation, ozone, and trace gases.Closely related to surfaceplatforms, constant level balloonsare flown from Goa to track low-levelairflow from the subcontinentand measureair pressure,temperature, and humidity. RAMANATHAN ET AL.: INDIAN OCEAN EXPERIMENT--AN INTEGRATED ANALYSIS 28,375 b

1¾i\1

Reunlor•

G•achentFlight Track

Plate 1. (continued) 28,376 RAMANATHAN ET AL.: INDIAN OCEAN EXPERIMENT--AN INTEGRATED ANALYSIS

Total Cloud Cover & Cruise Tracks 3O oo o

C 80.0

70.0

60.0

50.0 %

40.0

30.0

20.0 n ro nl-.1

Ron Brown I- ß3 10.0

___JORon, V BrownSagair Kanya i-.2 00

b Surface Wind, Low Clouds, & Flight Tracks

100.0 , C130 - Research 2O i% (• Flight, 17

66.7

0 . l • I ß

• • ' " ' ' ' 07 33.3

-10 , •--•..,,, ß , •-• 05

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40 50 60 70 • • 1 • 110

Plate 2. (a) Illustratesthe total cloudamounts for January-March1999 in percentcover over the tropical Indian Ocean.Cloud amountwas retrieved from METEOSAT-5 satelliteradiances provided by the European MeteorologicalSatellite Organization (EUMETSAT). The retrievalscheme allows the determinationof low clouds,cirrus, cumulus, and anvils.The band of near-overcastcloudiness represents the approximatelocation of the ITCZ. The orangeline representsthe trackof the R/V SagarKanya during February and March.The colorlines (dark blue, brown,and red) representthe cruisetrack of R/V Ron Brownduring late February throughthe end of March. The greenarrow illustrates the locationof the KaashidhooClimate Observatory. (b) Illustratesthe northeasterly to southwesterlymonsoonal flow during February 1999 and the low-level cloud amounts.Surface wind data for February were obtained from the National Center for Environmental Prediction/NationalCenter for AtmosphericResearch Reanalysis Project. The color lineson the map rep- resentthe flight pathsof the C-130. prevalentin thisregion have horizontal scales of few hundred fractionsbetween 0 (clear sky) and 1.0 (overcastsky). The meters to few kilometers,which we accountfor through the cloud fractions are retrieved once every 30 min from the Monte Carlo radiative transfer modeling [Podgornyet al., METEOSAT Infrared (IR) brightnesstemperature data. 2000]. We use fractal broken cloud scenesto simulatecloud MACR adoptsthree typesof skyconditions, specifically, clear RAMANATHAN ET AL.: INDIAN OCEAN EXPERIMENTmAN INTEGRATED ANALYSIS 28 377

KCO: - • • C-130

Male- "•sol C,Indii• ec ' KCO Lidar , ica•,!cro - Effect Sa•ar- Ron- Ch',,ical cheme Kanya Brown & Radiation CR c- 130& Model M ETEOSAT Citation R- ,_tonal AVIIRR Cloud 1 RM.M fraction & fox:canonly) eros O• '½. ep 'sl. ila i n , e •ant•)

KœO Clear Firs Indirect econ • lndirec

C-130 Direct Effect & •ud Effect & Fffect on Direct Set, i-Direct Effect CERES

nthropo•,eni 'rrade-Cu Fraction: D Clou, KCO; Rot Brown' m • el C-130 & C' tion

Ill! Oil o:

CIi ate Impact G 1 Studie

Plate 3. A schematicof the data integrationscheme. Central to the integrationscheme is the MACR model. The direct forcing in clear skiesis obtained directly from observationsat KCO, C-130, and the Clouds and Earth Radiation ExchangeSystem (CERES) radiation budget instrumenton board the Tropical Rainfall Mapping Mission(TRMM) satellite.The aerosol-chemical-microphysicalcomponents of MACR are devel- oped from observationsfrom KCO, the lidar in Male, the R/V Ron Brown, the C-130, and the Citation. The dependenceof cloudoptical properties on aerosolconcentration (the so-calledfirst indirecteffect) is incor- porated in MACR based on the C-130, the KCO, and the R/V Sagar Kanya observations.The regional distribution of the forcing is obtained by integrating satellite-retrievedaerosol optical depths and cloud fractionwith MACR. The resultingaerosol forcing is insertedin the NCAR-CCM3 to obtain climateimpacts.

skies,low clouds(including cirrus), and convectiveclouds in- MACR was used in two modes: First, it was used to retrieve cluding anvils. aerosoloptical depths from satellitedata, thusextending in situ MACR treats each aerosol speciesexplicitly by using the data to regionalscales. Second, it wasused in conjunctionwith correspondingphase functionsand single-scatteringalbedos satellite low-clouddata to obtain the regional direct and indi- for individual species[Satheesh et al., 1999]. The model also rect forcing. For the indirect effect the KCO data were com- accountsfor all multiple scatteringand absorptionby individ- bined with the C-130 data to derive a compositeindirect effect ual aerosol species,air molecules and reflections from the scheme(described in AppendixA) that relatesCCN, effective surface.The solar bandsfrom 0.2 to 4.0 •m are divided into 25 supersaturationand effective cloud drop radius with aerosol narrow bands.Hygroscopic aerosol species (all speciesexcept number density.In order to obtain regionalforcing values as a soot,dust and ash) are allowedto grow with relative humidity function of longitude and latitude from these point source [Hegg et al., 1993; Nemesureet al., 1995] using radiosonde estimates,we use the advancedvery high resolutionradiome- profilesof water vapor (launchedfrom KCO). The modeled ter (AVHRR) data collectedfrom the polar orbiting NOAA changein aerosol scatteringdue to growth with relative hu- satellite to retrieve aerosol optical depths (AODs), and the midity is comparedwith observedvalues in section6. MACR METEOSAT data for cloud fraction. also accountsfor the correspondingchanges in scattering The long transequatorialflights of the C-130 (Plate 2b) and phasefunction. A more detaileddescription of MACR is given the cruiseby the R/V Ron Brown (Plate 2a) establishedthe by Satheeshet al. [1999] and Podgornyet al. [2000]. sharp north-south gradients in aerosols, CCN and aerosol 28,378 RAMANATHAN ET AL.: INDIAN OCEAN EXPERIMENT--AN INTEGRATED ANALYSIS

1.0

© 1996 $agar Kanya Cruise

1997 0.8 [31998 ß 1999

0.•

.2o.o 45.o .•o.o .s.o o.o 5.o •o.o 15.o 2o.o Latitude, degree Plate 4. The latitudinalvariation of r•, (0.5/xm)measured by the Indianresearch vessel Sagar Kanya during 1996to 1999.The multiwavelengthSun photometer used for thisdata is describedby Jayaraman et al. [1998]. The precisionof the measurementis _+0.03 or 20%,whichever is smaller.Each color represents a different year. chemicalcomposition. These gradients were usedto evaluate [Satheeshet al., 1999,hereafter referred to asS99; Rajeev et al., the predictionof modelsfor the compositeindirect effect 2000; Conant, 2000; Collins et al., 2001; Rajeev and Ra- scheme.These transequatorial gradient data were alsoused to manathan,2001] and are brieflysummarized below. We adopt estimatethe anthropogeniccontribution to the aerosolforcing. the following conventionwith respect to aerosol optical The stepsfor estimatingclear-sky direct forcing by aerosols depths:% refersto aerosoloptical depths at 0.5 /xm;and r v have been documented extensivelyin the open literature refersto the aerosoloptical depth at 0.63/xm.

Jan-Mar 1999 25 • 0.6O

•.50

5 • , .20

,.15

,.10

IntegratedAerosol Optical Dc'pth (630 nm) Plate 5. The regionalmap of aerosolvisible optical depth %. The % valuesover ocean are retrievedfrom AVHRR data[Rajeev et al., 2000]and over land are estimated using a 4-D assimilationmodel [Collins et al., 2001].The figureillustrates the north-southgradient in % withvalues around 0.5 aroundthe coast and less thanabout 0.1 southof the equator.As describedby Rajeev and Ramanathan [2001], the standard error of the seasonalaverages shown in this plate is about_+0.02 or 15%,whichever is greater. RAMANATHAN ET AL.: INDIAN OCEAN EXPERIMENT--AN INTEGRATED ANALYSIS 28,379

1. The in situ data for size-resolved aerosol chemical com- indicativeof polluted air. The •-, frequentlyexceeded 0.2 north position;microphysics and vertical profile were used to de- of 5øN, in spite of being at least 500 to 1000 km away from velop an aerosol microphysicalmodel (S99). The single- major urban regions.The ship data clearly demonstratethe scatteringalbedo and the spectral variation in the column chronic recurrenceof the haze. The interannualvariability is aerosoloptical depth (AOD) simulatedby this model were alsolarge. At KCO, for example,•-, during 1999was more than verified with multiwavelengthSun photometer measurements a factor of 2 greater than in 1998. However, the interannual of AOD at KCO. variabilityof the spatiallyaveraged % is much smaller. 2. The aerosolmodel was employedin MACR to simulate Upon integratingthe in situ and satellitemeasurements with the solar radiation fluxes, which were validated with radiomet- the MACR model and the 4-D assimilationmodel [Collinset ric measurementsdeployed at the surface at KCO and the al., 2001], we discoveredthat during the winter months the TRMM satellite. thick haze layer spreadsover most of the northern Indian 3. The MACR aerosol model was used to retrieve aerosol Ocean(NIO) and over mostof the Asian continent(Plate 5). visibleoptical depth *v from satelliteradiances in the 0.63/am The % exceeds0.25 over the easternArabian Sea and most of region. We use 0.63/am region sincethis is the central wave- the Bay of Bengal; and exceeds0.4 over most of South and length of the satellite radianceused for retrievingthe optical SoutheastAsia. There is a sharp transition to background depth. It shouldbe noted that % • 1.30 values between 5øSto 8øSbecause of the ITCZ, which restricts 4. The retrieved •-• wasvalidated with •-• measuredby Sun interhemispherictransport. The fact that the haze can reach as photometersdeployed at the surfaceand aboardships [Rajeev far south as 5øSis also captured in the TransmissionElectron et al., 2000]. Steps1-4 yielded •-• only over the oceansince the Microscopeimage of particles(Figure 1) from the C-130. The surface reflectivities needed in the radiation retrieval models sootparticle is shownattached to the aerosolcollected at 6.1øS, are not well known for land surfaces. more than 1500 km from the source.The causeof the large- 5. For land a four-dimensional(4-D) globalaerosol assim- scalespread of the haze is demonstratedfrom the trajectories ilation model developedespecially for INDOEX [Collinset al., of the constant level balloons launched from Goa, India. Sev- 2001] was used.This model initializesdaily valuesof satellite- enteen overpressure (constant volume) balloons were retrieved •-• for the oceansand aerosolsurface emission data launched between January 16 and February 28, 1999. They for land and uses model winds to advect the aerosol horizon- followed quasi-isopycnictrajectories, except for eventual oc- tally and vertically. The model predictionswere extensively currence of diurnal oscillations in the vertical, due to occa- evaluatedagainst data collectedduring the field campaign. sionalwater loadingof the envelope.Their nominallevels were The cloud and aerosolmicrophysical data taken at KCO and comprisedbetween 960 and 810 hPa. The northeasterlyflow from the C-130 are usedto developa compositescheme for the was clearly evident in most of the trajectories.Most of the first indirect effect. This schemeestimates cloud optical depth balloonssurvived the flight to the ITCZ. The travel time across as a functionof aerosoloptical depth. The compositescheme the Arabian Sea to the equatorwas about 3 to 7 days.Because is insertedin MACR, in conjunctionwith the satellite data for of the scarcityof rain during the northeastmonsoon, the life- cloudcover and aerosoloptical depth, to estimatethe regional time of the aerosolsis of the order of 10 days[IPCC, 1995], and forcingfor the first indirect effect.The detailsare explainedin the haze spreadsover the whole NIO. Appendix A. However, INDOEX observationsdo not include The seasonalcycle of % is obtained from KCO. The dry data on the second indirect effect and the semidirect effect. monsoonbegins around November and initiatesthe buildup of We includethese two effectsin our forcingestimates using the the haze from the premonsoonaldaily mean valuesof 0.05 to model studyof Ackermanet al. [2000] and Kiehl et al. [2000]. 0.1 to values as high as 0.9 in late March. The same seasonal In summary,as depicted in Plate 3, the direct forcingand the cycle has been observedover western India by the stationsat first indirect effect are obtained directly from INDOEX ob- Goa and Dharwad [Leonet al., this issue].The transitionto the servationsand are describedfirst in sections7 and 8, respec- southwestmonsoon begins in April after which the pristine tively. The model-derived forcing values due to the second SouthernHemisphere air replacesthe haze. indirect effect and the semidirect effect are described in sec- tion 9. Section 10 uses these results to summarize the various contributionsto the aerosolforcing by anthropogenicsources. 4. Chemical, Microphysical, and Optical Properties: Anthropogenic Contribution 3. Space and Timescales of the Haze 4.1. Chemical Plate 4 shows4 yearsof aerosolvisible optical thickness data Aerosol optical and chemical measurements were per- collectedfrom a multispectralSun photometer [Jayaramanet formed on board the C-130, the R/V Ronald H. Brown, and al., 1998] on board the R/V SagarKanya during the winter from KCO at 5øN, 73.5øE. At KCO we measured the size monthsof 1996 to 1999. The optical depth at a wavelengthX, distributionand chemicalcomposition of fine particles, col- *x, is definedsuch that the direct solarbeam transmittedto the lected on filters and cascadeimpactors [Chowdhury et al., this surfacedecreases to exp (-,x/cos 0) due to aerosolscattering issue].The chemical data summarizedin Plate 6 are taken and absorption,where 0 is the solar zenith angle. Thus *x from Lelieveldet al. [2001].The filter analysisshows an average quantifiesthe optical effect of the aerosolcolumn between the massconcentration of nearly17/ag m -3 (Plate6). The aerosol surfaceand top-of-the atmosphere (TOA) at the wavelengthX. compositionwas stronglyaffected by both inorganic and or- Plate 4 showsthe optical depth at X = 0.5/am denotedby %. ganic pollutants,including black carbon (soot). Very similar For eachof the 4 yearsthere is a steeplatitudinal gradient with resultswere obtainedfrom boundarylayer flightsby the C-130 •-a increasingfrom about 0.05 near 20øSto 0.4 to 0.7 around aircraft and from the R/V Brown, indicatingthat the aerosol 15øN.Typically, % of 0.05 to 0.1 is consideredto be a back- compositionwas quite uniform over the northernIndian Ocean. groundvalue for pristineair (S99),while •-, in excessof 0.2 are The measurementsshow that the fine aerosol(dry massat 28,380 RAMANATHAN ET AL.: INDIAN OCEAN EXPERIMENT--AN INTEGRATED ANALYSIS

Figure 1. TransmissionElectron Microscope (TEM) imagesof aerosolssouth of the ITCZ (top left), within the ITCZ (top right), and north of the ITCZ (bottom). It can be seen that the soot is associatedwith sulfate/organicaerosols in the Northern Hemisphere and one is found as far as 6øS.The soot,while it seems to be attachedto the surfaceof the aerosol,may have been ejectedto the surfacefrom the core becauseof the location in the TEM. The location and altitude of the measurementis included in each panel. All the measurementsare betweenapproximately 690 and 820 m altitude. diameters<1/am) wastypically composed of 32% sulfate,26% 0.1-0.2 /am from fresh engine exhaust from Kleeman et al. organic compounds,14% black carbon (BC), 10% mineral [2000]; and 0.2-0.4 /am in ambient Los Angeles air from dust, 8% ammonium,5% fly ash,2% potassium,1% sea salt, Kleemanet al. [1997]). Massspectrometric single-particle anal- and trace amountsof methylsulfonic acid (MSA), nitrate, and ysisshows that the BC particleswere alwaysmixed with organic minor insolublespecies (Plate 6b). The coarseaerosol (dry compoundsand sulfate, indicatingsubstantial chemical aging massat diametersbetween 1 and 10/am), asobserved at KCO, and accumulationof gas-to-particleconversion products. This typically consistedof 25% sulfate, 11% mineral dust, 19% may partly arise from the processingby scatteredcumulus organiccompounds, 12% sea salt, 11% ammonium,10% BC, clouds (i.e., condensation,coalescence, and evaporation of 6% fly ash, and 1% potassium.Interestingly, at KCO the droplets; sulfate formation within BC-containing droplets). coarse aerosols also contained about 4% nitrate, while farther The prevalenceof suchclouds is shownin Plate 2b. north, as observedby the C-130 aircraft, this was even 7%, The BC aerosol and the fly ash, as observed during substantiallymore than the nitrate in the fine aerosols.This INDOEX, are unquestionablyhuman-produced since natural suggeststhat nitric acid from the gasphase preferentially par- sourcesare negligible [Cooke et al., 1999;Novakov et al., 2000]. titions into the coarseparticles. Likewise,non-sea-salt sulfate can be largely attributed to an- The fine particle (particlessmaller than 1 /am diameter) thropogenicsources. The filter samples,collected on board the concentrationsobserved over the Indian Ocean are compara- R/V Brown in the clean marine boundarylayer south of the ble to urban air pollution in North America and Europe that ITCZ, reveal a fine aerosol sulfate concentration of about 0.5 reducesair quality [Chtistoforouet al., 2000]. Moreover, the /agm-3, probablyfrom the oxidationof naturallyemitted di- high BC concentrationgives the aerosola strongsunlight ab- methyl sulfide.Considering that the sulfateconcentration over sorbingcharacter. The KCO impactor measurementsshow a the northernIndian Ocean was close to 7/ag m-3, we inferan dry particle size distributionthat peaksat about 0.5/am phys- anthropogenicfraction of about 90%. Similarly, the ammo- ical diameter.In this size range the sulfateaerosol (and asso- nium concentration south of the ITCZ, from natural ocean ciatedammonium) contribute most to the light scattering.The emissions,was 0.05/ag m -3, indicatingan anthropogeniccon- peak BC concentrationoccurred at 0.3-1.0/am physicaldiam- tributionof 97% to the nearly2 /agm-3 of ammoniumob- eter, largerthan typicallyfound in citieslike LosAngeles (e.g., served north of the ITCZ. RAMANATHAN ET AL.: INDIAN OCEAN EXPERIMENT--AN INTEGRATED ANALYSIS 28,381

30 In the aerosol south of the ITCZ, organic compoundswere negligible, whereas over the northern Indian Ocean it was

25 .• Residual almost6 tagm -3. We thusinfer that most (at least90%) of the I---1 Chloride particulateorganics north of the ITCZ were of anthropogenic I• Nitrate origin. INDOEX fine aerosol componentsof natural origin I Ammonium [::: 20 included a total mass fraction of 1% sea salt and 10% mineral I Sulfate r-• Calciumcarbonate dust. However, some of the mineral aerosol likely originated c3 15 I Organiccompounds I Blackcarbon from road dust and agriculturalemissions during the dry mon- soon, so that the 10% assumptionfor the natural dust repre- • 10 sents an upper limit. Taken together, the human-produced contribution to the aerosol over the northern Indian Ocean was about 80% (-+ 10%). Note that these aerosolmass frac- tions,in conjunctionwith Plate 6b, have been usedto perform the optical depth calculations. ø0.ø1.... •'' The followingadditional considerations also justify our con- Aerodynamicdiameter (ILtm) clusionthat anthropogenicsources contribute a major fraction (80%) to the aerosolloading. The observedBC/total carbon ratio of 0.5 and the sulfate/BC ratio of 2-2.5 are both more

5% typicalfor fossilfuel derived aerosolthan for biomassburning 10% 14% aerosol[Novakov et al., 2000]. The substantialmass fraction of 2% I Black carbon sulfatein the aerosolsupports this analysissince SO2 emissions 1% '" Organics are dominatedby fossilfuel use. Important informationabout 2% [] Ammonium aerosol sourcesis provided by the comparisonof potassium [] Sulfate (K +, mostlyfrom biomassburning) with sulfate(mostly from [] Potassium fossilfuel combustion).The K+/BC ratio in INDOEX aerosol 26% [] Sea-salt + nitrate [] MIS was about 5 timessmaller than that typicallyfound in biomass [] Mineral dust smoke [Novakovet al., 2000]. As concludedby Novakovet al.

32% [] Fly ash [2000], fossilfuel related emissionsof BC and sulfur are the major contributors(at least 70%) to the aerosolsover the

8% Indian Ocean. Fossil fuel related emissionsare not very effi- ciently removednear the sourcessince it takesseveral days to Plate 6. (a) Averagechemical composition of fine aerosolon convert SO2 into sulfate and BC into hygroscopicparticles. KCO (Maidives) as a function of particle size for February 1999(total mass 17 tagm-3). The residualincludes mineral Therefore the dry monsooncan carry thesecompounds across dust,fly ash, and unknowncompounds. For gravimetricmass the indiau Occa,• where they contribute to light extinction. determinationthe averageprecision of the impactormeasure- 4.2. Optical ments, calculatedfrom the nominally four replicate impactor samplestaken eachevent, was found to be _+5.3%for samples The scatteringand absorptionof solar radiation by aerosols greaterthan or equalto 2.0 tagm -3, and_+22.5% for samples are governedby the chemicalcomposition, the size distribu- lessthan 2.0 tag m-3 whosevalues were still significantly tion, and the number density.The total number of particles greaterthan zero. For sulfate,ammonium ion, organicmatter, variesfrom about 250 cm -3 in theunpolluted southern Indian and elemental carbon the averageprecision of the measure- Ocean(SIO) air to 1500cm -3 or morein pollutedNIO air ments based on repeated analysisof standardsolutions was (Plate 7a). Particlesof diametersfrom a few nanometers(nm) _+2.3%,_+5.4%, _+ 11.1%, and +_5.9%,respectively, for samples greaterthan or equalto 0.2 tagm-3; for samplessmaller than to a few micrometers(tam) contribute to the total number 0.2 tagm -3 the averageprecision was _+16.4%,_+12.4%, density,but thosebetween 0.05 and 2 tamcontribute more than _+18.4%,_+34.9%, and +_12.4%,respectively. These precision 90% to r,. The C-130 data (Plate 6b) accountfor only fine valuesrepresent +_ 1 standarderror of the determinationof a particles,i.e., diameterD < 1 tam.The coarseparticles (D > singleimpactor sample. Plate 6a is constructedfrom the aver- 1 tam) are primarily sea salt and dust,which contributemore ageof eightimpactor sampling events, so the standarderror of than 30% to the massbut lessthan 10% to r, sincethey are too the meanscalculated from thoseeight samplesis correspond- few. In MACR we use surface measurements at KCO to ac- ingly smaller.(b) Averagemass fractions of aerosol(D < 1 count for the coarseparticles. tam)as observedfrom the C-130 aircraftduring February and The chemicalcomposition determines the refractiveindex, March 1999(total mass22 tagm-3). Thirty-fourboundary which, in conjunctionwith solutionsof the Maxwell's equa- layer samplesare includedin the averages.MIS is minor in- tions,yields the scatteringand the absorbingcross sections for organicspecies (reproduced from Lelieveldet al. [2001] with permissionfrom the authorsand Science).When we account a singleparticle of diameterD. The calculatedlight extinction for spatialand temporalvariability, instrumental precision, and agreeswith direct opticalmeasurements both at KCO and on the numberof samples,our estimatefor the overall standard board the C-130 aircraft across the entire measurement do- error of the averagesshown in Plates6a and 6b is about +_20%. main down to the ITCZ. The results indicate a remarkably large absorptioncomponent of the aerosol,yielding a single- scatteringalbedo at ambient relative humidity of about 0.9 It is more difficultto attributethe organicaerosol fraction to (discussedlater in more detail). The contributionof various a particular source category.However, the BC/total carbon constituentsto the columnarr, is shownin Plate 8 (seeS99 for ratio of 0.5, as derivedfrom the filter samples,points to fossil details). The aerosolsingle-scattering albedo is wavelength- fuel combustionas the main contributor[Novakov et al., 2000]. dependent.MACR integratesthese solutions with the particle 28,382 RAMANATHAN ET AL.: INDIAN OCEAN EXPERIMENTmAN INTEGRATED ANALYSIS

INDOEX Flight03, 2/20/1999 20OO

1500 io.65 1 o00

500 CNhot (300øC) • 0.2 Ratio 0 •...... t 0 -6 -5 -4 -3 -2 -1 0 2 3 Latitude

1 O00 ...... - 10oo ? .A- SH ..._.Ambient 'l:emp. FJH _.,, Residualat 300øC

500 5oo

0 20 50 100 20 50 100 Diameter, nm Diameter, nm

Plate 7. (a) Latitudinalvariation of the condensationnuclei (CN) measuredduring the C-130 flight. Total CN (greencurve) and CNhot (blackcurve, samples collected with a preheaterat 300øC)are shownwith their ratio (pink curve).This ratio is the nonvolatilefraction of particlesremaining after volatile componentsare removed.The highgradient in total CN numberfrom north to southand the rapid drop in the CN nonvolatile fraction upon crossingthe ITCZ (approximatedby vertical line) are clearlyseen in the plate. Flow errors for the CN counterare < =3% Particleslarger than about 15 nm are countedwith high efficiency.The heated inlet wasmaintained at 300 +_2øC.(b) and (c) Size distributionsof dried aerosoland the residualcomponent after heating,as measuredwith a Differential Mobility Analyzer (DMA). The SouthernHemisphere (SH) distribution(Plate 7b) showsa typicalclean marine aerosol,with a dip at 100 nm due to cloudprocessing (a "Hoppel minimum") and very small amountsof refractorymaterial, while the Northern Hemisphere (NH) plot (Plate 7c) revealslarger particles with much greater refractoryparts typicalof the INDOEX haze. Errors in overall particle number and particle sizingare <10%. size distributionand vertical distribution(Plate 9, to be dis- and the forcing,consistent with earlier findings[S99; Pilinis et cussedlater) and sumsup the contributionsfrom all of the al., 1995]. Our confidencein the MACR treatment of the altitudes to obtain Plate 8, which enables us to determine the radiative effects of the INDOEX aerosols is derived from the anthropogeniccontribution to the radiative forcing. fact that the solar fluxesand the clear-skydirect forcing esti- The percent contributionshown in Plate 8 differs somewhat mated by MACR is in excellentagreement with the observed from the KCO valuesshown by S99 sincethe latter is basedon values (describedin section6). However, the aerosolmixing surfacedata for one location(that too for 1998), whereasthe state can exert a significantrole on the indirect effect. presentvalues integrate surface data with in situvalues (C-130 data) and for a much larger domain. The anthropogeniccon- tributionto the columnr,, (valuesare reportedhere only at 0.5 5. Role of Black Carbon in the Forcing /•m) is about 80% (_+10%). While the individualcontributions We highlightthe role of black carbon becauseof its major are broken down by species(using MACR), the aerosoltime- role in the surfaceand the atmosphericforcing [Podgorny et al., of-flight massspectrometer data [Coffeeet al., 1999] revealed 2000].The estimatesof aerosolforcing prior to 1990often used that these speciesdo not occur as separateparticles, i.e., as assumptionson the imaginarypart of the refractiveindex due pure sulfate or pure organic aerosols.Instead, substancesare to lack of adequatemeasurements. From the beginningof this mixed within individualparticles as imaged in Figure 1 which decade, more attention has been devoted to the absorbing shows the soot attached to a sulfate and organic aerosol. aerosols[Penner et al., 1992;Haywood and Shine,1997; Heint- MACR allowsfor this by treatingtwo extremecases: an exter- zenberget al., 1997;Hansen et al., 1997]. The aerosolforcing nally mixed particle in which each substanceoccurs as a sep- also depends on the altitude level at which the aerosol is arate particle; and an internally mixed aerosol in which each locatedand whetherthe aerosolis aboveor belowclouds [e.g., particle containsa mixture of all the substances.The difference Heintzenberget al., 1997;Haywood and Shine,1997]. While the between the two caseswas lessthan 10% in the calculatedr,, role of absorbingaerosols is enhanced at higher altitudes, RAMANATHAN ET AL.' INDIAN OCEAN EXPERIMENTmAN INTEGRATED ANALYSIS 28,383

Contribution to Aerosol Optical Depth

Black carbon

11% Dust 12% MISS Organics 2% .... 17øo

11Oo Sea-salt & NO3*

12% + NH4+

26%

SO4=

Plate 8. Relativecontributions of the variouschemical species to % (0.5/xm). When we accountfor spatial and temporalvariability, the standarderror of the mean valuesis about _+15%(relative error). The size distributionof aerosolsshows an effectiveradius of -0.34/xm. The value of Angstromcoefficient varies from 1.0 to 1.2.

particularlyif aboveclouds, the role of sulfatedecreases due to A key parameterin thisregard is the single-scatteringalbedo humidity effects [Heintzenberget al., 1997]. This is where the (SSA), w = C.•/(C.• + C.), where C,. is the scatteringcoef- importanceof simultaneousmeasurements of verticalprofiles ficient and C. is the absorptioncoefficient. Measured Cs.+ C. and absorptionproperties comes into play. are shown in Plate 9a for an aerosol layer imbedded in the

a , 3 / 25 / 1999 2 / 16/1999 (4.2øh, 73.5øE) (5.7øN, 73.3øE)

.... C-130 (5.7øN, 73.3øE) w Lidar (4.2øN,73.5øE)

.... , ,, •, , ,i, , ß ß ß i ...... 0 50 100 150 -10 10 30 50 70 90 110 ExtinctionCoefficient (Mm '1) Cs,Ca (Mm 4)

Plate 9. (a) Typicalprofiles of a "boundarylayer aerosol." The dataare for the aerosolextinction coefficient (scatteringplus absorption).The C-130 data includeonly submicronparticles, and the lidar includesall particles.This differencecontributes in part to the differencebetween the two profiles;the other causes includespatial gradients in the profilesand instrumentaluncertainties. (b) Typical"elevated aerosol layer." The data are for scattering(Cs, blue curve)and absorptioncoefficients (C,, red curve)measured from the C-130. The Cs is measuredwith an integratingnephelometer. C, is measuredwith a continuouslight absorptionphotometer. Since the C-130 inlet restrictslarger particles (diameter >1/xm), the valuesbelow 1 km maybe uncertainby asmuch as a factorof 2. Uncertaintyin the lidar data is 20% below2 km and is 30% above2 km. Uncertaintiesare greatlyreduced when simultaneously measured Sun photometer AODs canbe used to constrainthe columnar integral of Cs + C.. 28,384 RAMANATHAN ET AL.' INDIAN OCEAN EXPERIMENT--AN INTEGRATED ANALYSIS

Column

lto3km

Surface

0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00

Aerosol Single ScatteringAlbedo

Plate 10. The estimatesof aerosolsingle-scattering albedo •oat X = 0.53/am from variousplatforms which include the KCO, the C-130 aircraft, the R/V SagarKanya, and the R/V Ronald H. Brown (RHB). The horizontalbar representsuncertainties due to variabilityand instrumentalerrors. (bottom) Surfacemeasure- mentsfrom KCO and ships.KCO-AERONET standsfor AerosolRobotic Network measurementsat KCO; KCO-NP3 standsfor three wavelengthnephelometer and PSAP measurementsat KCO; NP standsfor single wavelengthnephelometer and PSAP;AS standsfor ArabianSea; and BOB standsfor Bayof Bengal.(middle) Measurementsbetween 1 and 3 km from the C-130 aircraft and lidar. (top) Column-integratedvalues estimatedusing MACR and retrievedfrom the sky radiancemeasurements using the Sun photometerand radiometer at KCO. The aerosolsingle-scattering albedo is wavelength-dependent.First set is the direct measurementsby a nephelometerand a photometer,in which the aerosol-ladenair is drawn into the instrument.The secondmethod employsa lidar [Althausenet al., 2000] at Male. The data measuredwith the multiwavelengthlidar are usedto determinethe backscattercoefficient at sixwavelengths and the extinction coefficientat two wavelengthsindependently and simultaneously.From these eight spectraldata, physical parametersof the aerosolsize distribution and the complexrefractive index are calculatedusing an inversion algorithm,which are then used for the calculationof •o. The third method employsthe measuredmass, composition,and size distribution of the aerosol(Plate 6) in conjunctionwith a Mie-scatteringmodel (solution of Maxwell's equation for a sphericalparticle) to determinethe aerosolradiative properties(Plate 8), averagedover the vertical column.

boundarylayer, and measuredCs and C a are shownseparately the near-surface values and between 0.8 to 0.9 above the sur- in Plate 9b for an elevated aerosollayer. The boundarylayer face layer, and about 0.86 to 0.9 for the column averages,in aerosolprofile occurredabout 1/3 of the time, and the elevated excellent agreementwith the column average MACR value profileoccurred about 2/3 of the time. The coalbedo,a = (1 - shownin Plate 10. Generally, these valuesrepresent a highly •o), is the basic measureof light absorption.In the visible absorbingaerosol. The surfacestations at Goa and Dharwad spectrum,a rangesfrom about lessthan 0.01 for pure sulfates also determined column average SSA to be about 0.9 from to 0.04 for fly ash, 0.2 for dust, and 0.77 for soot. Thus soot, optical measurements,the only data we have at the source even at 10% of the total fine particle mass,can enhancepar- [Leon et al., this issue]. ticle absorptionby an order of magnitude.Soot was found in In view of the diversesources and processesinfluencing the most samplesas far south as 6øS(Figure 1). The •o values, formation and evolution of continental aerosol, the narrow obtainedby severalindependent methods, are summarizedin range (0.85 to 0.9) of the mean •o in Plate 10 is somewhat Plate 10 and are generallyin the rangebetween 0.85 to 0.9 for surprising.There are two plausibleexplanations. First, aerosol RAMANATHAN ET AL.' INDIAN OCEAN EXPERIMENT--AN INTEGRATED ANALYSIS 28,385

KCO Growth Curves 400-700 nm Global and Diffuse Irradiance 2.8

2.6 ...... Measured at KCO KCO' MACR vs. Photodiode Radiometer MACR • 2.4 a o 2.2 -- 5OO G 2 E ßMACR with Soot o'* -r- 1.8 ßMACR without Soot ...15-ø' - 400 • '•e

ß ß o 1.4 ß

1.2 ß x• 300 ß•' Biaswith Soot: -1.8 40 50 60 70 80 90 100 'O ß o ß Bias without Soot: 9.2 RH(%) • 200• ! ! 2O0 300 400 500 Plate 11. A comparison of the model-estimated growth MeasuredGlobal (W m'=) curvewith that observedat KCO. The growthcurve is obtained by measuringscattering coefficient Cs as a functionof relative b humidity usingtwo 3-wavelengthnephelometers (one control at fixed relative humidity,the other at variable relative humid- E 160 'MACRwith Soot ' ' ity; see Ogren et al. [1992] for details). Some investigations showsthat the aerosolsare occasionallycoated with surface ß ß active materialsthat hinder their water uptake and thus influ- • 120 ence their growth with relative humidity [Ogrenet al., 1992; •:140 !' MACR without Sootß'ß Cantre#et al., 1997]. This effect is not includedin the present is 00[ , ;,."-'ß study.This could be one reasonfor the slightdifference in the x• t .:; A.-. growthcurves. This is alsosupported by the fact that measured e e•Pe e growth rate is lessthan the modeled. '•• 801-";. ;.,t ß Biaswith Soot: -0.2 O 60L: :" Biaswithout Soot:10.1 60 80 100 120 140 160 combustionsources are widespread(as opposedto being lo- MeasuredDiffuse (W m'2) calizedover few urban centers)and rather similar. Second,the persistenceof the monsoonflow often resultsin the mixingand Plate 12. MACR predictionsof clear-sky(a) global and (b) dispersionof this aerosolover large regionswith little modifi- diffusesurface irradiance in the 0.4-0.7 •m photosynthetically cation.A more direct piece of evidencefor the narrow range active spectral band versusKCO measurementsmade by the can be found in the C-130 data shown Plates 7a-7c. The data BSI GTR-511c photodiode flux radiometer from February- obtained from the cross-equatorialgradient flight of C-130 March 1999. Details of the model, the measurement, and the show the boundary layer concentrationof the total aerosol angular,spectral, and absolutecalibrations used to reduce the condensationnuclei concentration ("cold CN") alongwith the data are describedby Conant [2000]. Black points represent so-called"hot CN" (the aerosol CN left after heating it to the MACR model as describedby S99. Red points represent 300øC).The hot CN is indicativeof refractorymaterial (soot, MACR predictionswhen sootaerosol is removedfrom MACR fly ash,and seasalt). The ambientand heatedsize distribution (see Plate 8) and replacedwith proportionatecontributions from other aerosols so that % remains constrained to the of this aged NIO aerosol showedlittle change (other than spectralobservations. This plate demonstratesthe importance concentration)so that the variationsin CN (and hot CN) are of aerosolchemistry on the radiative fluxes. reflectedin the aerosolscattering (and the absorption).While CN and hot CN decreasewith latitude significantly,their ratio is nearly constant.The near constantratio suggeststhat the concluded that essentiallyall NH aerosolshave a primary, aerosolcharacteristics (e.g., w) remain the sameas the plume refractorycomponent consisting of soot or perhapsfly ash that dilutes in time. The reason for this near constancyin the amounts to 10-20% of the aerosol mass. The much smaller Northern Hemispherehot CN/cold CN ratio is most likely that size and negligibleassociated mass of the SH refractory com- the black carbon aerosol componentis first formed at high ponent found here is presumedto reflect incompletevolatil- temperaturesas primary particlesupon which organics,sul- ization of the aerosoldue possiblyto pyrolysisof surfacefilms, fates, and other compoundslater react or condense[Clarke et submicrometer sea salt, and/or other trace contaminants. al., 1998]. Evidence for this is shownin Plates 7b and 7c, in which the size distributions of the hot CN and the cold CN are shownfor a casein the SouthernHemisphere (Plate 7b) and 6. Successof the MACR Integration one in the Northern Hemisphere(Plate 7c). After the nonre- The ultimate test of MACR is its ability to simulate the fractorymaterial (e.g., sulfates,nitrates, and most organiccar- observations.It agreeswith the observedspectral dependence bon) is evaporatedfrom the aerosol,the remainingparticles of the aerosoloptical depth (S99), the growth of the aerosol are equal in number, but far smaller in size. The shift to with humidity (Plate 11), the observedscattering and absorb- smallersizes is far lessprominent for the NH aerosol,indicat- ing coefficientsat the surfacein KCO, and the column aver- ing a large refractory component.Because the total particle aged co(Plate 10). It simulatesthe broadband and spectral count is within 5% for the heated and the unheated scans,it is solar irradiance at the surfaceand the TOA under a variety of 28,386 RAMANATHAN ET AL.: INDIAN OCEAN EXPERIMENT--AN INTEGRATED ANALYSIS

aerosolconditions within a few percent [S99;Podgorny et al., low clouds[Kaufman et al., 1998] below 3 km for INDOEX 2000; Conant, 2000]. At KCO, 12 independentradiometers aerosolssince the bulk of the aerosolis belowthis level (Plate coveringthe entire solar spectrumwere deployedto validate 9). Becauseof this dependenceon the low cloudfraction, and MACR predictionsfor za rangingfrom 0.05 to 0.9. Plate 12 sincelow cloudshave significant spatial (Plate 2) and temporal showsa comparisonof the observedand the modeledglobal variations,the direct forcingfor cloudyskies has to be deter- (direct solar plus diffusesky radiation) radiation(Plate 12a) mined regionallyas a function of latitude and longitude.This and diffuseradiation (Plate 12b) for the 0.4 to 0.7/am region. would not havebeen possiblewithout the integratedINDOEX The agreementbetween the MACR and the observedvalues is data set. We use the observed forcing for clear skies and well within the instrumental standard error of about 5 to 10 W estimate the direct forcing for cloud-coveredregions with m-2. For the integrated0.3 to 4 /ambroadband irradiances MACR and weight the two with observedclear and cloud measured by a pyranometer and a thermistor bolometer fraction,respectively. For theseestimates, MACR employedin [Valeroet al., 1999], MACR agreedwithin 1-1.5% of the ob- situ data for cloudthickness (1 km) and verticallocation (0.5 servedvalues. In addition,the modeledclear-sky TOA albedo km for cloudbase and 1.5 km for cloud top), and the results valueshave been comparedwith the measuredalbedos by the describedin the next sectionfor cloud optical depth and ef- Cloudsand Earth's Radiant Energy System (CERES) [Weilicki fectivedrop diameter.Because of the strongdiurnal variation et al., 1996] radiationbudget instrument on board the Tropical of low clouds,we adopt the geostationaryMETEOSAT satel- Rainfall Mapping Mission satellite. Again, the model agrees lite data to obtain cloudcover. The averageNIO diurnal mean with the observedvalues to within 2% (S99). low cloudcover we retrieveis 29% (diurnalaverage cover), but Next, the radiancescomputed from MACR were used to the averagecloud cover can be as low as 15% (dependingon develop an algorithm for retrieving zv from the NOAA ad- the thresholdtemperature we use to distinguishclear from vancedvery high resolutionradiometer (AVHRR) in the 0.63 cloudypixels in the satellite radiances).Given the present /am spectralregion. As shownby Rajeevet al. [2000] andRajeev limitationsof the availablesatellite data (the 5 km resolution and Ramanathan [2001], the collocatedzv between the re- of the METEOSAT is one limitation; the AVHRR has 1 km trieved and the measured values from Sun photometersat resolution,but it doesnot have the diurnal samplingcapabil- KCO and R/V Sagar Kanya agreed within the instrumental ity), it is difficult to determinethe low cloud covermore ac- error of about _+0.03.Lastly, the MACR valuesfor the TOA curately.This twofold (15-29%) uncertaintyin the satellite- and surface aerosol forcing agree with the observedvalues retrievedlow cloudfraction is the largestsource of error in the [Satheeshand Ramanathan,2000, hereafterreferred to as SR] cloudy-skyforcing estimates. within 10%. We will beginwith the regionallyaveraged aerosol (natural plusanthropogenic) direct radiative forcing, shown in Plate 13. The gridvalues of % (Plate 5) and cloudfraction (Plate 2) are 7. Aerosol Direct Forcing in Cloudy Skies adopted to obtain the resultspresented here. The forcing is In this section we consider the direct forcing due to all firstestimated as a functionof latitudeand longitude (from the aerosols, including natural and anthropogenic. The term q'aand cloud fraction shownin Plates 5 and 2, respectively) "cloudyskies" refers to averageconditions with a mixture of from which the regional averagesshown in Plate 13 are ob- clearand overcastportions of the sky.The startingpoint for the tained. We account for the significantdiurnal variations in analysesis SR's clear-skyaerosol direct forcing efficiency, f (= cloud coverbut ignore its day-to-dayvariation. The regionally AF/A,a, the rate of changeof forcingper unit increasein averagednorthern Indian Ocean (0øN to 20øN) (NIO) values SR used simultaneousclear-sky solar irradiance observa- for the TOA forcing[F(T)] due to the directeffect is -7 _+1 tions at the surfaceand TOA and % for the KCO location,and W m-2 (Plate13a) for clearskies. For cloudyskies the TOA directforcing ranges from -0 to -4 W m-2, with a meanof demonstratedthat tthe TOA f isf(T) • -25 W m-2 andat the surface,f(S) • -75 W m-2. The atmosphericforcing -2 W m-2. Inclusionof cloudsintroduces a net heatingof efficiency,f(A) = f(T) - f(S), is positive,and thus aerosol about5 W m-2 (differencebetween clear- and cloudy-sky solarabsorption is the reasonfor f(S)/f(T) > 1. We checked forcing) largelybecause of the aerosolabsorption of the re- the validityof SR'sf valuesfor the rest of the INDOEX region flectedradiation from clouds. Roughly 50% of the _+2W m-2 by correlatingthe satellite CERES radiation budgetdata with estimatederror in the cloudy-skydirect forcingis due to the the observed% shownin Plate 5 for the entire tropicalIndian uncertaintyin cloud fraction; about 25% is due to the uncer- Ocean IRajeerand Ramanathan,2001]. For the entire tropical tainty in the verticalprofile of aerosols,and the balanceof 25% IndianOcean, f(T) • -22 W m-2 (witha 1-rrspatial varia- is due to the uncertaintyin clear-skyforcing. The plate also tionof 5 W m-2) comparedto SR's-25 W m-2. About50% showsthe forcingfor the surfaceIF(S)] and the atmosphere of the differenceis due to the larger domain of the satellite [F(A)]. In summary,the TOA, the atmosphere,and the sur- data, and the balance is due to revisions in CERES data since faceforcing values are F(T) - - 2 _+2.0 W m-2; F(A) - publicationof SR's paper. For f(S), C-130 radiometricdata 18 _+3 W m-2; andF(S) = -20 _+3 W m-2, respectively. [Valeroet al., 1999] confirmedSR's value of -70 to -75 W The cloudy-skysurface direct forcing is a factor of 10 larger m-2. Reassuredby this validation,we usedSR's values for than the TOA forcing, as opposedto a factor of 3 for clear f(S). We obtain clear-skyregional forcing as a function of skies(SR). About 60% of the largeatmospheric forcing F(A) latitude and longitudeby multiplyingthe observedf(T) [= is due to soot,and the balanceis due to fly ash,dust, and water -22 W m-2],f(S) [= -72 W m-2], andf(A) [= 50 W m-2] vapor absorptionof the radiation scatteredby aerosols. with the z• shownin Plate 5. Aerosols also absorb and emit infrared radiation. Their im- For absorbingaerosols, it is well known [Penneret al., 1992; portancefor the INDOEX region has been estimatedusing a Haywoodand Shine,1997; Kaufman et al., 1998]that f(T) can model similar to that presentedby Lubin et al. [1996]. Their changesign from cooling(negative) under clear skies to a large model was modified to include INDOEX aerosols and is used net heatingfor overcastskies. This effect is importantonly for in this study.The NIO averagedTOA IR aerosolforcing for RAMANATHAN ET AL.: INDIAN OCEAN EXPERIMENT--AN INTEGRATED ANALYSIS 28,387

AerosolRadiative Forcing(W m-2):NorthIndian Ocean (Jan - March, 1999; 0 - 20øN)

0.3

-7.0:t:l -2.0ñ2 -5ñ2.5

+ . +18.0:3 +l:C-o 5

-23-t-2 -203

Direct Direct First Indirect (Clear Sky) (Cloudy Sky) (2) (3) Plate 13. Aerosoldirect radiative forcing for theNorth Indian Ocean (0 ø to 20øN;40 ø to 100øE).The values includethe effectsof naturaland anthropogenicaerosols. The valueson top of eachpanel reflect TOA forcing;those within the box show the atmospheric forcing, and below the box is the surface forcing. cloudyskies is 1.3W m-2 at TOA and5.3 W m-2 at the servedvalues. The three INDOEX pointsshown in Plates14a surface,such that the atmosphereis subjectto a net coolingof and14b are averages for thepristine (N• -< 500 cm-3);the -4.3 W m-2. Thus they offset some of the solar forcing. so-calledtransition (500 -< N• -< 1500 cm-3),and the pol- Roughly90% of theTOA IR forcingand 75% of the surface luted(N• >- 1500 cm-3) cases.Each of thesepoints is an IR forcingare dueto naturalaerosols. The largersea-salt and averageover 10,000clouds sampled during 18 C-130flights dustparticles are moreeffective in their interactionwith IR from 15øNto about 8øS.The horizontalbar showsthe range of than the smalleranthropogenic particles. aerosolconcentrations over which the datawere averaged.The absoluteaccuracy of the clouddrop number density measure- ment is about 15%. The standarddeviations of Nc in each of 8. ForcingDue to the First Indirect Effect theseranges are large (as expected): 85 cm-3 forpristine; 135 The C-130 data clearlydemonstrated [Heymsfield and Mc- cm-3 for transition,and 197 cm -3 for the pollutedcase. How- Farquhar,this issue; McFarquhar and Heymsfield,this issue] ever,given the largenumber of cloudssampled, the standard that the pollutedclouds have N a (aerosolnumber density in erroris expectedto be muchsmaller than the abovestandard particlesizes larger than 50 nm diameter)exceeding 1500 deviation. We estimate that the overall standard error in the particlescm -3, aboutthe same average liquid water content, LWC•-- 0.15 g m-3, morecloud drops, N c -• 315 cm-3, and averagesshown in Plate14 is about25% or less[Heymsfield andMcFarquhar, this issue; McFarquhar and Heymsfield, this smallereffective drop radii,R e --<6 /•m, comparedwith the pristineclouds {Na -< 500;Nc = 90 cm-3;R e -->7.5 /•m). issue].The compositescheme is in agreementwith the ob- The mean cloud base vertical velocitywas about the same servedNc. With respectto Re, the observationsseem to sat- betweenthe polluted and the pristine clouds, thus enabling the uratearound Na of about1500, whereas the predicted ones do identificationof the anthropogeniceffect. The observedeffect not saturateuntil an N• of about2000. As a result,the pre- is capturedin MACR througha compositescheme (described dictedR e for the largerN• are smallerby about10% (or 0.5 in detailin AppendixA) whichrelates the effectiveradius of /•m in radius).Plates 14c and 14d show the computed relation- clouddrops and the numberdensity of clouddrops to N•. shipbetween ?• andNa (Plate14c) and between ?c, the cloud Plates14a and 14b comparethis composite scheme with ob- opticaldepth at 0.5/•m, and?• (Plate14d). We usethis plate 28,388 RAMANATHAN ET AL.: INDIAN OCEAN EXPERIMENT--AN INTEGRATED ANALYSIS

(a) CompositeINDOEX Scheme 0.3 8 3OO 7.5 0.25 250 0.2 • •E 200 z '-=6.5 0.15 '-- '-' 150 6 0.1 100

5.5 5O 0.05

0 500 1000 1500 2000 2500 0 500 1000 1500 2000 2500 (c) N,(cm N. (cm'3) 16 0.5 14 12 0.4

ß 10 0.3 8 0.2 6 4 0.1 2 0 0 500 1000 1500 2000 2500 0 0.2 0.4 0.6 N.(cm 'a) •a

Plate 14. Comparisonof the compositescheme with INDOEX in-cloudaircraft observations. (top left) Plot of R e againstNa. In this and the other panelsthe horizontalbars are not error bars.Data havebeen binned withinthe Na rangeshown by the horizontalbar, and averagedover several thousand points. (top right) Cloud drop numberdensity N c versusthe aerosolnumber density N,. (bottomleft) Aerosolvisible optical depth % versusN,. (bottom right) Cloud visibleoptical depth rc versus%. in conjunctionwith the regionaldistribution of % (shownin decreasethe negativedirect forcing by 5 W m-2 (fromthe Plate 5) to estimatethe cloudoptical depth for low cloudsas a clear-skyforcing of -7 to -2 W m-2 in cloudyskies), while on functionof longitud,eand latitude. These values are employed in the other hand they enhancethe negativeforcing by -5 W MACR to estimatethe forcingdue to the first indirecteffect. m-2 throughthe first indirect effect. This important new result The first step is to estimate the N, and % for the NIO is largelybecause the absorbingaerosol layer is locatedwithin without any anthropogenicinfluence, i.e., the backgroundnat- and abovethe tops of low clouds. ural aerosol over the tropical Indian ocean, subject to the The latitudinalgradient of the sumof the direct and the first continentaloutflow. If we adopt the resultgiven in section4 for indirect forcingis shownin Plate 15. The aerosolsintroduce a the anthropogeniccontribution of 80% to the columnaerosol large gradientin the solarheating, as shownin Plate 15. They opticaldepth, we obtainthe background% to be 0.06 (20% of exerta largepositive 25 W m-2 north-southgradient in the the 0.3 meanvalue for the NIO, shownin Plate 13). From Plate solarheating of the atmospherebetween 25øN and 25øS,and a 14cthis leads to a backgroundN, of about300 cm -3. Thisis correspondinglylarge negative gradient in the ocean solar slightlysmaller than the measured Na of about350 cm -3 south heating. of theITCZ (Plate7). We adopt% of 0.07and N• of 350cm -3 We will concludethe discussionon forcingwith a few com- for the backgroundunpolluted NIO value. Insertingthese in mentson its year-to-yearvariability. The forcingnumbers dis- MACR with the regional distribution of %, rc, and cloud cussedthus far are valid for the year 1999 and that too for fraction(including the diurnalvariation of clouds)on a 1ø x 1ø January,February, and March. Plate 4 suggestssignificant inter- (latitude, longitude)grid, we obtain for the regionalaverage annualvariability in the opticaldepths. We shouldcaution against forcingdue to the first indirecteffect: TOA equalto -5 + 2 W an overinterpretationof this plate, for the cruiseeach year was m-2; atmosphereequal to 1 ___0.5 W m-2; andsurface equal not identicalnor were the samplingdates. Tahnk [2001] uses to -6 _+3 W m-2. The largeerror bars are duemostly to the 5-yearAVHRR satellitedata to obtainthe followingaverage % uncertainties in the satellite-retrieved cloud fraction. We first (for January-February-Marchaverages) for the northernIndian note that the TOA negativeforcing due to the indirecteffect is Oceanfrom 0øNto 30øN:0.25 (1996);0.29 (1997);0.26 (1998); the samemagnitude but oppositein signto the positiveforcing 0.27 (1999);and 0.25 (2000). Clearly,the NIO averageforcing due to the direct effect of clouds; i.e., clouds on the one hand shownhere is representative of the averageforcing. However, the RAMANATHAN ET AL.: INDIAN OCEAN EXPERIMENT--AN INTEGRATED ANALYSIS 28,389

• , TOA Atmosphere ...... Surface variabilitydepends on the spatialscale. For example,at KCO the 1999values were higherthan the 1998and the 2000 %,values by 45 about40-50%. Thus the regionalvalues shown here (e.g., Plate Direct + Indirect 35 15) may havesignificant interannual variability. A 25 E 15 • 5 9. Second Indirect and Semidirect Effects l::n -5 For the second indirect effect and the semidirect effect we o -15 rely on a three-dimensionaleddy resolving trade cumulus o u. model describedby Ackermanet al. [2000]. Two independent -35 model studies[Ackerman et al., 2000;Kiehl et al., 1999] of the INDOEX soot heating showthat it leads to a reduction in the -45 -30' ,20..... "•'0 ' 0 10 20 30 trade cumuli prevalentover the tropical oceans.Ackerman et Latitude al. [2000] insert the INDOEX aerosolwith soot and estimate Plate 15. Latitudinal variation of the aerosol radiative forc- the forcingfor variouscloud drop concentrations(their Plate ing. Includesboth natural and anthropogenicaerosols. The 5). We adopthere their simulationsfor N,• (clouddrop number valueshave been averagedover oceanwith respectto longi- density)of 90 cm-3 (pristine)and 315 cm-3 (polluted),con- tude from 40øE to 100øE. sistentwith the compositeindirect effect schemeshown in

TOA

8

6

4

First Second 2 Direct Solar Indirect Indirect 0

-2 Direct IR Semi direct

-4

-6

-8

-10

Surf ce

Direct Sola First Second Direct IR Indirect Indirect

Semi direct -5

-10

-15

-0 Plate 16. Regionalaverage anthropogenic aerosol climate forcing for NIO (0ø to 20øN;40 ø to 100øE).The top panelshows the forcingat TOA, andthe bottompanel shows it at the surface.The differencebetween the TOA and the surfaceforcing yields the atmosphericforcing. The uncertaintyfor eachof the forcingvalues is the sameas in Plate 13. The mean for the individualforcing terms should lie within the gray shadedregion. 28,390 RAMANATHAN ET AL.: INDIAN OCEAN EXPERIMENT--AN INTEGRATED ANALYSIS

Plate 13. The inclusionof sootheating (semidirect effect) duringthe last 50 years.The surfaceforcing can also impact reducesthe low cloudfraction by about0.04 (day-night aver- the hydrologicalcycle. Roughly 80% of the net radiativeheat- age).The TOA forcingdue to thisincrease is about+4 W m-2 ingof thetropical oceans is balanced by evaporation [Oberhu- (day-nightaverage). Their modelalso shows that additionof bet,1988]. If 80%of the -20 W m-2 is balancedby reduced clouddrops (due to secondindirect effect) enhances cloud evaporation,it would constitutea 15% reduction in the win- lifetime,which in turn increasesthe low cloudfraction by tertime evaporationof about 100 W m-2 or a 5% reductionin about2% anda forcingof -2 W m-2. Givenour poor knowl- theannual mean evaporation. Ultimately, the decreased evap- edgeof howthe variousuncertainties accumulate, we prefer orationmust lead to decreasedtropical rainfall, thus perturb- notto attributean overall uncertainty range to thetotal forcing ingthe water budget. In addition,reduction in rainfallis equiv- (shownin the lastcolumn of Plate 16). alent to a reduction in the latent heat releasedto the middle andupper troposphere with implications for thelapse rate and 10. AnthropogenicAerosol Forcing the Hadley and Walker circulations. We describenext another potentially new effect of aerosols As describedearlier, the unpollutedra is takento be 0.07.In on climate.The atmosphericforcing of about15 W m-2 is additionto the direct and the first indirecteffect, we also equivalentto a heatingrate of about0.4 K/day for the0 to 3 km include the second indirect effect and the semidirect effect. layerand is about50% of theclimatological solar heating of Lastly,we alsoinclude the IR forcing.The individualforcing this region.Furthermore, the heatingrate increasesfrom valuesare shownin Plate 16. Furthermore,given the large uncertaintyand the opposingsigns of the variouseffects, we do aboutless than 0.1 to 0.8K/day over South Asia (Plate 18), formingadiabatic heating pattern asymmetric with respect to not givea meanvalue, but insteadshow a grayshaded region thatmust contain the mean value. The clear-skyTOA forcing theequator and concentrated north of ITCZ. Forcomparison, is -5 W m-2, butthe enhanced aerosol solar absorption in themidtropospheric heating anomaly during E1 Nifio events is cloudyregions cancels out as muchas 3 to 5 W m-2. Thus the nearlysymmetric around the equatorwith its localmaximum TOA directeffect (+0.5 to -2.5 W m-2) is a smalldifference reachingup to 2.0 K/daysouth of the ITCZ [Nigamet al., betweentwo large numbers. The surface forcing, however, is as 2000].What is the impactof theunique heating perturbation large as -14 to -18 W m-2. The other termsshown in Plate shownin Plate18 on the atmosphericgeneral circulation? 16have been described earlier. In viewof thelarge uncertainty We madea preliminaryassessment of such an impactwith in the satellite-retrievedlow cloudcover and the competing Version3 of theCommunity Climate Model (CCM3) [Kiehl et natureof the variousaerosol forcing terms, the total aerosol al., 1998].Eighty-four years of the CCM3 "controlrun" were forcingis not shownin Plate16. The regionaldistribution of contrastedto 30 yearsof the model"experiments," each with the directplus the first indirectforcing is shownin Plate 17. the sameprescribed climatological seasonal cycle of seasur- The solarheating of the easternArabian Sea and Bay of facetemperatures. In the experimentwe appliedthe radiative Bengalis reduced by as much as 20 to 30W m-2 (about15%). heatingincrease in thelevels below 700 mbar, which roughly The TOA forcingpeaks in the cloudyand polluted Bay of correspondto the first 3 km above the surface.The surface Bengal region,with valuesbetween -6 and -15 W m -2. solarflux is reduced with the ratio R, F (S)/F (A), to be - 1.5. Thisratio approximates the clear-sky direct forcing at thesur- face(see Plate 13). The imposedheating varied temporally 11. Discussion: Climate and startingfrom zero in October;increased linearly to thepeak Environmental Effects value in March; and was restrictedto NIO and the southAsian It is generallybelieved [IPCC, 1995] that the TOA forcing region with a smooth transition to zero outside this domain determinesglobal average surface temperature change. This (Plate19a) to prevent numerical "shocks." Plate 19 depicts the paradigmis appropriate for greenhouse gases and for primarily difference between the mean of control run and the mean of scatteringaerosols such as sulfates. For theabsorbing aerosols, experimentboth averaged for the monthsof January,Febru- however,since the surfaceand atmosphericforcing can be ary,March, and April. Plates 19b and 19c show the computed factorsof 3 to 10 largerand of oppositesigns, we needto changesintemperatures and precipitation asan example of the carefullyexamine the effects (regionally first and then globally) natureand magnitude of the calculatedchanges. withthree-dimensional climate models. As we discussnext, the Thelow-level air temperatureT increasesby about 0.5 to 1.5 mostimportant effect of theseaerosols may be on thehydro- K wherethe aerosolheating was imposed (Plate 19b). The logicalcycle and the atmosphericcirculation. warmingalso extended beyond the domainof the aerosolheat- The seasonaland NIO averagedsurface forcing of -20 W ing, in a southwestto northeast direction from the southern m-2 islarge compared with the downward heat fluxes into the Indian Oceanto east Asia. However,the surfaceover South ocean.It is about15% of the naturallyoccurring wintertime Asiacooled (not shown), but the magnitudewas strongly de- solarheating of NIO [Oberhuber,1988] and aboutthe same pendenton the ratioR. For a ratioof 1.5,as in thisplate magnitudeas the net downwardheat flux into the ocean of 30 (appropriatefor clearskies), the surface cooled by as much as to50 W m-2. Evenifwe make the extreme assumption that the 1 K,while for a ratioof about1.0 (similar to cloudy-sky values), aerosolforcing is zero duringthe other 9 months,the annual thecooling was a factorof 2 to3 smaller.The low-level heating mean forcingof the aerosolsis still about 4% of the total solar enhancesthe rising motion locally. The dynamical response to heatingof the ocean and about 20% of the net heat flux into this low-levelheating is to enhancemoist convectionand theNIO. In thiscontext, it is intriguingthat a recentstudy of strengthenthe rainfallalong the ITCZ (seethe red shaded worldocean surface temperatures [Levitus et al., 2000]has IndianOcean region in Plate 19c)by as muchas 15 to 30%. shownthat the NIO isone of thefew regions without a decadal Sincethe risingair has to sink elsewhere,the subsidencein- warmingtrend in the recentpast, while the SIO, the Atlantic, creasesaway from the regionof enhancedprecipitation and and the Pacificwere subject to a significantwarming trend leadsto decreasedrainfall north (in the ArabianSea) and RAMANATHAN ET AL.: INDIAN OCEAN EXPERIMENT--AN INTEGRATED ANALYSIS 28,391

AerosolForcing in CloudySkies (Wm'):Anthropogenic January- March 1999 TOA

25 _. -14.0

20 -lO.O

15 -6.0

10 -4.0

5 -2.0

2.0 -5

-I0 . 4.0

Surface 25 42.O

20 -36.0

15 30.0

10 + 24.0

q 18.0

0 • 12.0

-6.0 -5

-10 ...... 0.0 40 50 60 70 80 90 100

Plate 17. Regionaldistribution of the aerosolclimate forcing for January-March1999 at TOA (top panel) and at the surface(bottom panel).

south(over the westernIndian Ocean)of the regionof positive since absorbingaerosols are found in other regions of the rainfall. world as well [seePenner et al., 1992;Kaufman et al., 1998]. The enhancedITCZ precipitation over the Indian Ocean In addition to perturbingthe averagedforcing of the NIO, alsoperturbs the east-westWalker circulation,which resultsin the aerosolsperturb the interhemisphericheating gradientsin alternating patterns of suppressedrainfall over the western the oceansin major ways.The climatologicalwintertime Indian Pacific(see the negativerainfall regimeover the tropicalwest- Oceansolar heating varies from about175 W m-2 northof ern Pacific)and enhancedrainfall over the easternPacific (not 15øNto about225 W m-2 southof 15øS[Oberhuber, 1988]. The shown).This remote teleconnectionbetween the west Pacific, 50 W m-2 latitudinalheating gradient should be compared the east Pacific, and the Indian Ocean rainfall is well known with that of the surfaceaerosol forcing which varies from about and well studiedas part of the E1 Nifio problem [Rasmusson -25 W m -2 north of 15øN to -2 W m -2 south of 10øS. This and Carpenter,1983]. Becauseof a decreasein the low-level gradient,which (in additionto salinitygradients) is one of the wind, surfaceevaporation over the NIO decreasedby 10 to 20 driving forcesfor the thermohalinecirculation, is enhancedby W m-2, balancingmore than 50% of the negativeaerosol about 50%. surfaceforcing (Plate 16b). Theseresults have significantim- We shouldalso consider the ecosystemimpacts since 70% of plicationsfor globalclimate change and variability,particularly the surface forcing is concentratedin the photosynthetically 28,392 RAMANATHAN ET AL.' INDIAN OCEAN EXPERIMENT--AN INTEGRATED ANALYSIS

AnthropogenicAerosol Heating Rate (K/day) January-March, 1999

25

20

15 0.5

I0 ß ß ß . 0.4 5 - ' .... ß •. 0.3 0 ....- ,•' 0.2

0.1

0.0 -10 40 50 60 70 80 90 100 Plate 18. The 3-month (and diurnal) mean heating rate averagedbetween the surfaceand 3 km, due to anthropogenicaerosols. The ocean valueswere obtained from the integrated observations,while the land valueswere obtainedfrom the 4-D assimilationmodel of Collinset al. [2001].

activeUV and visiblepart of the solar spectrum[Meywerk and within a week to as far away as mid-Pacificto North America. Ramanathan,1999; Conant,2000]. Sincethe percentagereduc- Thus the absorbingaerosols may extendfar beyondthe source tion in the UV and the visibleregion is as large as 10% to 20%, regions. effectson terrestrialand marinebiological productivity need to The aerosoleffects on climate can be large and complex be examined.For example, Chameideset al. [1999] indicates involving many components of the global system. The that the haze-inducedreduction of sunlightby about 10% can INDOEX campaignhas successfullydemonstrated that an in- significantlyreduce wheat and rice productivityin China. In- tegratedobserving system that combinesin situ determination creasedaerosol concentration can also suppressprecipitation of the physico-chemicalproperties at the particlesscale up to from low clouds[Albrecht, 1989]. This effect togetherwith the the synopticscale with satellite measurementsof the entire soot effect on burninglow clouds[Ackornan et al., 2000] can ocean basin and models of the chemical-physical-dynamical decreasesignificantly the wet removalof aerosolsand poten- systemcan answerfundamental questions such as, What is the tially acceleratethe increasein the thicknessof the haze in the impact of developingnations in South Asia on regional and future. global radiation balance and climate? Advancesin observing The presentfindings can also have global implications.The systems,information technology, modeling, and collaboration large surfaceforcing and the atmosphericheating require ab- acrossdisciplinary and national boundarieswere the recipes sorbingaerosols with to < 0.95 and % > 0.2. Large surface that contributed to this success. forcing values similar to those shown here were found in the northern Atlantic off the East Coast of the United States [Hignettet al., 1999]. Second,absorbing aerosols with to of 0.9 Appendix A: The Composite First Indirect are typical of aerosolsresulting from biomassburning [Kauf- Effect Scheme Adopted in MACR man et al., 1998]. As describedby Andreae and Crutzen[1997], The aerosolindirect effect is difficult to quantify as it de- biomassburning was practiced in almost all the populated pends on the interactionsbetween aerosols,CCN, and cloud regionsand is still practicedin many parts of the tropicsand properties which are poorly understood [Joneset al., 1994; subtropics.Third, ice core data have revealed that soot con- Heintzenberget al., 1997].Many attemptshave been made since centrationshave tripled betweenthe 1850sto 1970sin western the 1990sto tackle this problem [Joneset al., 1994;Meehl et al., Europe [Lavanchyet al., 1999], thus suggestingthat absorbing 1996;Jones and Slingo,1996; Kogan et al., 1996; Chuanget al., aerosolssuch as thosefound in INDOEX were more prevalent 1997;Le Treut et al., 1998; Hah et al., 1998; Brenguieret al., in Europe and possiblyNorth America at leastuntil the 1970s. 2000]. For our compositescheme we adopt a more detailed Lastly, electronmicroscopic images and particle analysis(Fig- approachto estimatethe anthropogenicindirect aerosolforc- ure 1) show that the soot lodged in submicronsulfate and ing from combinedsurface, aircraft, and satelliteobservations. organicaerosols is transportedas far south as 6øS,providing Two fundamentalrelationships characterize the indirect ef- evidence that these particles survive in the atmospherefor fect: the dependenceof Nc on Na; and the dependenceof more than a week, the transport time from South Asia to the effectiveradii R e on Na. These parametersin turn dependon ITCZ. Furthermore, peak concentrationsof soot can extend the vertical velocity W, the cloud liquid water contentL, and up to 3 km (Plate 9; alsosee the soot layer extendingto cloud the chemicalcomposition of aerosolconcentration (which in- tops in Plate l). Forward trajectories(see a few samplesof fluencesthe CCN activationspectra). The verticalvelocity and 7-dayback trajectoriesin Plate 1) indicatethat the air masses the CCN supersaturationspectrum regulate the supersatura- between 500 mbar (shown in Plate 1) and 700 mbar in the tion S, which determines the number of activated CCN; in tropical Indian Ocean [Krishnamurtiet al., 1998] can travel addition, W plays some role in determiningL. Entrainment RAMANATHAN ET AL.: INDIAN OCEAN EXPERIMENTmAN INTEGRATED ANALYSIS 28,393

Simulationof RegionalClimate Change (NCAR/CCliI3) January-April ($0 years o• exp.)

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Plate 19. Difference (experiment - control run): (a) short-waveradiative heating at 850 mbar, (b) air temperature at 850 mbar, and (c) precipitation. The "control run" was generated from NCAR/CCM3 (atmosphericmodel) with the interactiveland surfacemodel and with the climatologicalseasonal cycle of observedSSTs. The January-Aprilaverage is taken here of the differencebetween 30 yearsof experimentand 84 yearsof control run. In the experiment,INDOEX aerosolclimate forcing of "best-guessed"ideal pattern was appliedto the periodbetween October and May. This additionalforcing consists of low-levelatmospheric heating and surfacesolar-flux reduction, in which the ratio R, F(S)/F(A), is about -1.5. In another experiment(not shown)where R - -1.0, air temperaturechange at 850 mbar is somewhatless, but the precipitationchange remains largely unchanged. 28,394 RAMANATHAN ET AL.: INDIAN OCEAN EXPERIMENT--AN INTEGRATED ANALYSIS and cloud thickness,two macroscopicparameters of cloud powerlaw assumedin moststudies [e.g., Twomey, 1977; Snider dynamics,also governL. As shownby Heyrnsfieldand Mc- and Brenguier,2000; HM] is Farquhar[this issue]and McFarquharand Heyrnsfield[this is- sue] (hereafterreferred to as HM and MH, respectively),the CCN = CSk k = 0.76. (A1) mean and median IV, L, and cloudwidth Ax, while undergo- The parameterS is the watervapor supersaturation in percent, ing significantcloud-to-cloud variability, was about the same and the parameter C, by definition,is the CCN for S - 1% betweenthe northern and southerntropical Indian Ocean and whichincreases with the aerosolconcentration as Na", with n - betweenpolluted and pristineconditions. HM and MH usethe 1.16 yieldingthe bestfit. The value of k = 0.76 is in excellent long transequatorialgradient flights to bin the aircraft data as agreementwith the averagevalue of 0.8 reportedfor ACE 2 by follows:pristine, N a --<500 (cm-3); polluted,N a -->1500 Sniderand Brenguier[2000]. Upon combiningthis dependence cm-3; andtransition region, 500 cm-3 < N a • 1500 cm-3. of C (CCN at S = 1%) on N a with (A1), we obtaina power For all these three regimesthe median valueswere W • 0.4 lawrelationship: CCN - bNa"Sk, whereb is a constant.How- m s-•; L • 0.15 g m-3; andAx • 300 m. Thisremarkable ever, when we fit this equationwith the KCO CCN data, we similarity enabled MH and HM to discernthe indirect effect find that n -- 1.25 givesthe best fit for S < 0.5 % and n = from the data and parameterizeit. 1.16 for S > 0.5 %. This empiricaldependence of n on S may Numerous attempts [Taylor and McHaffie, 1994; Martin et be simplydue to the measurementuncertainty in CCN (about al., 1994; Snider and Berenguier,2000] have been made to 40%), Na (about20%), and S (not quantifiedyet) or poorly characterize the indirect effect from aircraft data collected understoodmicrophysical processes. Since the effective in- over the Atlantic (mostly)and the easternPacific. Our scheme cloud S is lessthan or equal to 0.3%, we adopt the following is based on the conclusions of these earlier aircraft studies: equation: First is the Martin et al. [1994] studywhich includesa compre- hensiveset of data from the British meteorologicalservices CCN = 0.12Na•'25[S/0.3]ø'76 for S -< 0.3% C-130 data from the eastern Pacific, the South Atlantic, the subtropicalAtlantic, and the British Isles. These data reveal and300 - 1500). Entrainment,which plays an pendenceof the supersaturationon aerosolconcentration as importantrole in limitingNc, doesnot enter explicitlyin our measuredfrom aircraft;(3) the microphysicaldata reported by scheme,but is implicitly factored into the inferred value of HM for pristine and heavily and moderatelypolluted condi- effective supersaturation.The inversecorrelation between S tions;and (4) the dependenceof effectiveradii on clouddrop andN a is predictedby Twomey's[1977] theory (the equationis number concentrationas reported by MH. givenlater in thissection) and hasalso been observed by other Unless otherwise stated, all of the values and functional field studies(see summaryin the work of Finlayson-Pittsand Pitts [2000]). It arisesfrom the fact that as more drops are relationshipsdescribed below are valid only in the following nucleated,the vapor supplyis depleted,and as a result,S is range: reduced.In order to accountfor this,we let S varywith N a as 300 cm-3 --

? Compositescheme, this study • 25 k '/• 2000F © Cantrell etal., this issue; INDOEX KCO 0.24N, ' S /•

L- * Cantrell et al. (2000); INDOEX Sagar Kanya

1500 •

1000- ß ß ß Ha1'16 500 :• • • 0.24$k • E• S: Su•a•ration = 0.5% i.• & k • 0.76 O• ,• , • • , , • .... • , • ,•.... 0 500 1000 •500 2000 CN (cm Plate AI. Variation of cloudcondensation nuclei (CCN at 0.5% supersaturation)with condensationnuclei (CN) (Na in the text). Comparisonof observations(at KCO and over the tropicalIndian Ocean)with the compositescheme. The precisionof the CCN measurementsis about +20%.

Nc=fx CCN f= 0.8 + 0.2 shown in Plate 14. Typically, two types of aerosol vertical distributionswere observed(Plates 9a and 9b): one in which ß exp {(Na- 300) x 0.004} Na >_300. (A4) the aerosol layer is concentrated in the boundary layer The next parameter we need is the effective radius Re, (Plate 9a) and another in which an elevated layer is ob- defined as follows: served near 3 km (Plate 9b). The elevated aerosol vertical structure was also observedover India (from an INDOEX lidar in Goa) containing half or more of the total aerosol extinction. Because of uncertainties in the actual data, we do not use the actual values in Plate 9, but normalize the value •Re --' f R3n(R)dR. (AS) at each altitude with ,• (obtained from vertically integrating the profile) and then multiply it with the ,a from Plate 5. We f R2n(R)dR include both the profiles shownin Plates 9a and 9b, weight- We usethe parameterizationdeveloped by MH, who in turn ing our forcing estimateswith the frequency of occurrence, followMartin et al. [1994]for obtainingR e from aircraft data. 2/3 for the elevated layer aerosol profile and 1/3 for the Martin et al. expressRe in terms of L and No' boundary layer aerosol profile. The cloud fractions are retrieved from METEOSAT data (IR brightnesstemperature). In general,the atmosphericcon- (A6) Re= 4rtKNc 3L} 1/3 ' ditionsare classifiedinto eight as follows:clear sky;clear sky with cirrus; broken low clouds;broken low clouds with cirrus; where MH obtain the following K values from INDOEX low clouds;low cloudswith cirrus;anvils; and deep cumulus. data: K = 0.83 for pristine clouds(Na -< 500); and K = 0.73 The eightcategories are subgroupedinto three:clear sky; total for heavily polluted clouds (N• >- 1500). The factor K ac- low clouds;and high clouds.We usedthree typesof sky con- countsfor the deviation from monodisperse(all drops hav- ditions,specifically, clear skies,low clouds(including cirrus), ing the same radius) size distributions. It can easily be and convectiveclouds including anvils. shown from (A5) or (A6) that K = I for monodispersed distributions. With estimates for Nc, Re, and N• we can calculate,using MACR, the cloud optical depth and radia- tive forcing, provided we know L, cloud base altitude Zt,, Acknowledgments. INDOEX was funded by numerous national and cloud thicknessAZ. Following HM, we adopt L = 0.15 agenciesincluding the NationalScience Foundation (lead agencyfor g m-2; Zt, = 600m; andAZ = 300m. TheN• valuesat cloud the U.S. program),the Departmentof Energy,NASA, NOAA, Indian SpaceResearch Organization, European Union for Meteorological base are used for (A2) and (A4). Using the vertical distri- Satellites(EUMETSAT), Max Planck Institute, French Centre Na- bution of aerosols shown in Plate 9, the relationships be- tional d'EtudesSpatiales (CNES), and Centre National de la Recher- tween the various parameters as obtained by MACR are che Scientifique(CNRS). Seedfunds for INDOEX were providedby 28,396 RAMANATHAN ET AL.: INDIAN OCEAN EXPERIMENT--AN INTEGRATED ANALYSIS the Vetlesen foundation and the Alderson foundation. We thank EU- Collins,W. D., P. J. Rasch,B. E. Eaton, B. Khattatov,J.-F. Lamarque, METSAT for movingMETEOSAT to the Indian Ocean;the Govern- and C. S. Zender, Simulatingaerosols using a chemicaltransport ment of Maldivesfor hostingINDOEX; J. Fein for his keen interest modelwith assimilationof satelliteaerosol retrievals: Methodology and supportfor INDOEX; E. Frieman and C. Kennelfor their enthu- for INDOEX, J. Geophys.Res., 106, 7313-7336, 2001. siasticsupport; the UniversityCorporation for AtmosphericResearch Conant, W. 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A. Jayaraman,Physical Research Laboratory, Ahmedabad 380-009, D. Althausenand J. Heintzenberg,Institute for TroposphericRe- India. search,Leipzig D-04318, Germany. J. Anderson,Department of Geologyand Chemistry,Arizona State T. N. Krishnamurti,Department of Meteorology,Florida State Uni- University,Tempe, AR 85287. versity,Tallahassee, FL 32306. A. P. Mitra, National PhysicalLaboratory, New Delhi 110 012, M. O. Andreaeand J. Lelieveld,Max PlanckInstitute for Chemistry, India. Mainz 55020, Germany. T. Novakov,Lawrence Berkeley National Laboratory, Berkeley, CA W. Cantrell, Department of Chemistry,Indiana University,Bloom- 94720. ington, IN 47401. G. R. Cass,School of Civil andEnvironmental Engineering, Georgia J. A. Ogren and P. Sheridan,Climate Monitoringand Diagnostics Institute of Technology,Atlanta, GA 30332. Laboratory,NOAA, Boulder, CO 80303. C. E. Chung,W. C. Conant,P. J. Crutzen,D. Lubin, I. A. Podgorny, K. Prather,Department of Chemistry,University of California,Riv- erside, CA 92521. V. Ramanathan(corresponding author), S. Rupert, S. K. Satheesh, and F. P. J. Valero, Center for AtmosphericSciences, Scripps Institu- K. Priestley,NASA LangleyResearch Center, Hampton, VA 23665. tion of Oceanography,University of California,San Diego, CA 92037. J. M. Prospero,Rosenstiel School of Marine and AtmosphericSci- ([email protected]) ence,University of Miami, Miami, FL 33149. A.D. Clarke and S. Howell, Departmentof Oceanography,Univer- P. K. Quinn, PacificMarine EnvironmentalLaboratory, NOAA, Seattle, WA 98115. sity of Hawaii, Honolulu, HI 96822. J. A. Coakley,College of Oceanicand AtmosphericSciences, Ore- K. Rajeev,Space Physics Laboratory, VSSC, Thiruvananthapuram 695022, Kerala State, India. gon State University,Corvallis, OR 97331. R. Sadourny,Laboratoire de MeteorologieDynamique, Paris 75005, W. D. Collins,A. J. Heymsfield,J. T. Kiehl, G. McFarquhar,and P. France. Rasch, National Center for Atmospheric Research, Boulder, CO 80307. G. E. Shaw,Geophysical Institute, University of AlaskaFairbanks, Fairbanks, AK 99775. F. Dulac, Laboratoire des Sciencesdu Climat et de l'Environment, Gif sur Yvette 91191, France. B. Holben, NASA Goddard SpaceFlight Center, Greenbelt, MD 20771. (ReceivedOctober 3, 2000;revised February 27, 2001; J. Hudson, Desert Research Institute, Reno, NV 89506. acceptedFebruary 27, 2001.)