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,
TMJ. 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,24P. 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-Asianhaze at scalesrangingfrom individualparticlesto
its contributionto the regionalclimate forcing.This studyintegratesthe multiplatform
observations(satellites,aircraft,ships,surfacestations,and balloons)with one- and fourdimensionalmodelsto derive the regional aerosolforcing resultingfrom the direct, the
semidirectand the two indirect effects.The haze particlesconsistedof severalinorganic
and carbonaceousspecies,includingabsorbingblack carbon clusters,fly ash, and mineral
dust. The most strikingresult was the large loading of aerosolsover most of the South
Asian region and the North Indian Ocean. The Januaryto 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 extendedas
high as 3 km. Black carboncontributedabout 14% to the fine particle massand 11% to
the visibleoptical depth. The single-scattering
albedo estimatedby severalindependent
methodswas consistentlyaround0.9 both inland and over the open ocean.Anthropogenic
sourcescontributedas much as 80% (_+10%) to the aerosolloadingand the optical depth.
The in situ data, which clearlysupportthe existenceof the first indirect effect (increased
aerosolconcentrationproducingmore cloud dropswith smallereffectiveradii), are used
to developa compositeindirect effect scheme.The Indo-Asian aerosolsimpact the
radiativeforcingthrougha complexset of heating(positiveforcing) and cooling(negative
forcing)processes.
Cloudsandblackcarbonemergeasthemajorplayers.
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 significantlyperturbsthe tropical rainfall
patternsand the hydrologicalcyclewith implicationsto global climate.
•2NASAGoddardSpaceFlightCenter,Greenbelt,
Maryland.
•Centerfor Atmospheric
Sciences,
Scripps
Institutionof Oceanog-
•3DesertResearchInstitute,Reno, Nevada.
raphy,Universityof California, San Diego, California.
2MaxPlanckInstitutefor Chemistry,
Mainz,Germany.
•4physical
Research
Laboratory,
Ahmedabad,
India.
3NationalPhysical
Laboratory,
NewDelhi,India.
•SDepartment
of Meteorology,
FloridaStateUniversity,
Tallahas4Institute
for Tropospheric
Research,
Leipzig,Germany.
see, Florida.
SDepartment
of GeologyandChemistry,
ArizonaStateUniversity, •6Lawrence
BerkeleyNationalLaboratory,Berkeley,California.
Tempe, Arizona.
•7Climate
MonitoringandDiagnostics
Laboratory,
NOAA, Boulder,
6Department
of Chemistry,
IndianaUniversity,
Bloomington,
Indiana. Colorado.
7School
of CivilandEnvironmental
Engineering,
GeorgiaInstitute
•SDepartment
of Chemistry,
University
of California,
Riverside,
Calof Technology,Atlanta, Georgia.
ifornia.
SDepartment
ofOceanography,
University
ofHawaii,Honolulu,
Hawaii.
9College
of OceanicandAtmospheric
Sciences,
OregonStateUniversity,Corvallis,Oregon.
•øNational
Centerfor Atmospheric
Research,
Boulder,Colorado.
•LaboratoiredesSciences
du Climatet de l'Environment,Gif sur
Yvette, France.
Copyright2001 by the American GeophysicalUnion.
•9NASALangleyResearch
Center,Hampton,Virginia.
2øRosenstiel
School
of MarineandAtmospheric
Science,
University
of Miami, Miami, Florida.
:•PacificMarineEnvironmental
Laboratory,
NOAA, Seattle,Washington.
:2SpacePhysics
Laboratory,VSSC, Thiruvananthapuram,
Kerala
State, India.
:3Laboratoire
de Meteorologie
Dynamique,
Paris,France.
:4Geophysical
Institute,University
of AlaskaFairbanks,
Fairbanks,
Paper number 2001JD900133.
0148-0227/01/2001JD900133509.00
Alaska.
28,371
28,372
RAMANATHAN
ET AL.: INDIAN
OCEAN EXPERIMENT--AN
INTEGRATED
ANALYSIS
ing. If the availablewater vapor sourceis unaltered, the increasein the number of cloud dropswill lead to a decreasein
It is now well documented[e.g.,Langnerand Rodhe, 1991; the drop sizeby inhibitingthe formation of larger drizzle size
Intergovernmental
Panel on Climate Change (IPCC), 1995; drops.The suppression
of larger dropscan decreaseprecipiSchwartz,1996] that anthropogenicactivitieshave increased tation (drizzlein the caseof low clouds)thusincreasingcloud
atmosphericaerosolconcentrations.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 emittingprimaryaerosolparticles 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, andvolatileorganiccompounds)
which indirect effect is highlyuncertain,but can be large enoughto
form secondaryaerosolparticlesthrough gas-to-particlecon- offseta large fraction of the entire greenhouseforcing[IPCC,
version.Secondaryaerosolsalso have biogenicsources(di- 1995].
methyl sulfidefrom marine biota and nonmethanehydrocarMore recently,model studies[Hansenet al., 1997;Kiehl et
bons from terrestrial vegetation). The secondaryparticles al., 1999;Ackermanet al., 2000] have indicatedthat the solar
formed are categorizedas inorganic(e.g., sulfates,nitrates, heatingby absorbingaerosols(biomassburningor fossilfuel
ammonia)or organic(e.g., condensed
volatileorganics).The combustionrelated soot) can evaporate low clouds (e.g.,
combinationof organicsand black carbon aerosolsare also stratocumulusand trade cumulus);the resultingdecreasein
referred
to as carbonaceous
aerosols. Aerosols exist in the
cloud cover and albedo can lead to a net warming whose
atmospherein a varietyof hybridstructures:externallymixed, magnitudecan exceedthe coolingfrom the direct effect [Ackinternally mixed, coatedparticlesor most likely a combination ermanet al., 2000].
of all of the above.Our primary interestin this paper is on the
The climate forcing due to aerosolsis poorly characterized
climate forcingof 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,chemicalcomposisphere) due to aerosolsis referred to as aerosolradiative tion and optical properties.The uncertainrole of aerosolsin
forcingand that due to anthropogenicaerosolsis referred to as climateis one of the major sourcesof uncertaintiesin simulaanthropogenicaerosolforcing;and the differencebetweenthe tionsof past and future climates.A primarygoal of INDOEX
two forcing terms is due to natural aerosols.Anthropogenic [Ramanathanet al., 1995,1996]is to quantifythe directandthe
aerosolscan modify the climateforcingdirectlyby alteringthe indirect aerosol forcing from observations.This paper adradiativeheatingof 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 followingaerosolfield experiments
prerectly by evaporatingthe clouds[Hansenet al., 1997;Kiehl et ceded INDOEX.
al., 1999;Ackermanet al., 2000]:
The overall goals of the Aerosol CharacterizationExperiAerosolsscattersolar radiationback 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 cliclimate forcing. Models estimatethat the direct coolingeffect mate forcing by aerosolsand to understandthe multiphase
of anthropogenicsulfate aerosols[e.g., Chadsonet al., 1992; atmosphericchemicalsystemsufficientlyto be able to provide
Kiehl and Briegleb,1993] may have offset as much as 25% of a prognosticanalysisof future radiative forcing and climate
the greenhouseforcing. Recently,the inclusionof other aero- response[Bateset al., 1998].ACE 1 took placein the minimally
sols(e.g.,organics,blackcarbon,anddust)haveshownthat the polluted SouthernHemispheremarine atmosphere,and ACE
problem is cun•dcramy
.... : ^ •"' more comp,ex
' .... • more important 2 tookplaceoverthe subtropicalnortheastAtlantic [Raeset al.,
[e.g.,Penneret at., 1992;IPCC, 1995].Absorbingaerosolssuch 2000].
as black carbon can even changethe signof the forcing from
Smoke, Clouds, Aerosols, Radiation-Brazil (SCAR-B)
negativeto positive[e.g.,Haywoodand Shine,1997;Heintzen- [Kaufmanet al., 1998], a comprehensive
field projectto study
berg et at., 1997; Liao and Seinfetd,1998; Haywood and Ra- biomassburning, smoke, aerosol and trace gasesand their
maswamy,1998]. Observationalstudieshave documentedthe climatic effects, took place in the Brazilian Amazon and cerimportance of absorbingaerosolseven in remote oceanicre- rado regionsduring 1995.
gions [e.g., Posfai et at., 1999]. There is now a substantial
The TroposphericAerosol RadiativeForcingObservational
amount of literature on absorbingaerosols[Grasst, 1975; Experiment(TARFOX) [Russell
et al., 1999]wasdesignedto
Clarke and Chadson, 1985; Penner et at., 1992; Chytekand reduce the uncertaintyin predicting climate change due to
Wong,1995;Hobbset at., 1997;Novakovet at., 1997;Heintzen- aerosoleffectsand took place over the United Stateseastern
berg et at., 1997; Kaufman and Fraser, 1997; Hobbs, 1999]. seaboardduring 1996 [Hobbs,1999;Russellet al., 1999].
Aerosolsalsoinfluencethe longwaveradiation [e.g.,Lubin et
INDOEX gathereddata on aerosolsand troposphericozone
at., 1996], but the longwaveforcing is much smallerthan the chemistryover the tropical Indian Ocean. The primary aerosolar forcing. The longwaveforcing of INDOEX aerosolsis sol-relatedobjectivesof INDOEX are to estimatethe direct
includedin this study.
and the indirect forcingof aerosolson climatologicallysignifAerosols influence the forcing indirectly by altering cloud icant timescalesand space scales;link the forcing with the
opticalproperties,cloudwater content,and lifetime. We begin chemicaland microphysicalcompositionof the aerosols;and
with the first indirect effect, the so-called "Twomey effect" evaluateclimate model estimatesof aerosolforcing. This pa[Twomey,1977],which arisesbecauseof the fundamentalrole per describes
the major findingsthat resultedfrom the aerosol
of aerosolsin nucleatingcloud drops.An increasein aerosol objectives.The resultspresentedhere are for a heavily polnumber concentrationcan nucleate more cloud drops, make luted and a poorly understoodregion of the planet. It estabthe cloudsbrighter [Twomey,1977] and exert a negativeforc- lishesthe Indo-Asian haze as an importantphenomenonwith
1.
Introduction
RAMANATHAN
ET AL.: INDIAN
OCEAN EXPERIMENT--AN
globalclimateimplications.The addedsignificance
is that this
oceanicregion borderson rapidly developing,heavily populated nationswith the potential for large increasesin the future.
The INDOEX observationswere made over the tropical
Indian Ocean during the Northern Hemisphere dry monsoon
(Decemberto April). Since 1996, an internationalgroup of
scientistshas been collectingaerosol,chemicaland radiation
data from ships,satellitesand surface stations [e.g., Krishnamuftiet al., 1998;Jayaramanet al., 1998],which culminated
in a majorfield experimentwith five aircraft,two ships,several
satellitesand numeroussurfaceobservationsconductedduring
JanuarythroughMarch, 1999(Platesla and lb). In additionto
theseplatforms,the EuropeanMeteorologicalSatellite Organization moved the geostationarysatellite, METEOSAT-5,
over the Indian Ocean to support the INDOEX campaign.
The first field phase (FFP) took place during February and
March 1998, and the intensivefield phase (IFP) took place
from Januarythrough March 1999.
The tropical Indian Ocean provides an ideal and unique
natural laboratoryto studythe role of anthropogenicaerosols
in climatechange.This is probablythe onlyregionin the world
where an intensesourceof anthropogenicaerosols,trace gases
and their reactionproducts(e.g.,ozone)liesjuxtaposedto the
pristineair of the SouthernHemisphere[Ramanathanet al.,
1996;Krishnamuftiet al., 1998]. The polluted and pristine air
are connectedby a crossequatorialmonsoonalflow into the
Intertropical Convergence Zone (ITCZ), which, during
INDOEX, was located between the equator and about 12øS.
INTEGRATED
ANALYSIS
28,373
for these5 yearsvaryby lessthan 10% of the 1999value shown
here.
2.
Integration of Field Observations
With
Satellites
and Models
A schematicof the integration schemeis shownin Plate 3.
The platforms(Platesla and lb) were designedto accomplish
two distinctivelydifferent objectives:processor "closurestudies" and "gradientstudies."Focusingon the C-130 flight tracks
shownin Plate lb and 2b, flights for the processstudieswere
directed
from
Male
toward
the northern
Arabian
Sea to un-
derstandthe compositionof aerosolsand other gaseouspollutants closeto their sources;processflightswere also undertaken
southward
from
Male
to
understand
the
aerosol
composition far away from the source regions. Next, long
transequatorialflights("gradientflights")from Male to about
10øSwere undertaken to estimate the horizontal gradient of
aerosol concentration,aerosol composition,radiation fluxes,
cloud condensationnuclei and drop concentration.In view of
the significantdaily and seasonalvariability in aerosolconcentrations,we need continuousmeasurementsthroughthe year.
For this purpose, during 1998, we establishedKCO on the
island of Kaashidhoo,in the Republic of the Maidives, about
500 to 1000 km downwindof major cities in the subcontinent
(Plate lb). The islandis about3 km longand slightlymore than
1 km wide, isolatedfrom the nearbyislands(about 20 km from
the nearestMaldivianisland)and about500 km awayfrom the
nearest mainland, the Indian subcontinent.The population of
the island is about 1600 and is free from anthropogenicactivThe monsoonal flow from the northeast to the southwest
ities suchasindustryor automobiletransport.The observatory
(Plate2b) bringsdrycontinentalair overthe oceanwhichleads
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 rescatteredcumuli(seethe dark andlight blue shadedregionsin
stricted at the observatorysincethe synoptic-scale
winds are
Plate 2a) and minimalrain, all of whichenablethe hazeladen
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 analysespresentedin this paper are unique in part basedon the processstudyphilosophy,we then review
in many respects.It integratesobservationstaken simulta- the gradient studies,and finally review how MACR will be
neouslyfrom severalplatforms(Platesla and lb): top-of-the used to determine the direct and the first indirect radiative
atmospheredata collectedby severalsatellites;in situ data forcing.
from the Hercules C-130 and the Citation aircraft; surface and
column data from the R/V Ron Brown and the R/V Sagar
Kanya;surfaceand columndata from the KaashidhooClimate
Observatory(KCO) at 4.97øN,73.5øE,in the Republicof the
Maidives; remotely measureddata from a multiwavelengthlidar station at Male (4.18øN,73.52øE)in the Republic of
Maidives;and the INDOEX Frenchstationsat Goa (15.45øN,
78.3øE)and Dharwad(15.4øN,75.98øE).Thesedata are integratedwith a Monte Carlo Aerosol-CloudRadiation(MACR)
model, satellite-derivedaerosolregional distributionover the
ocean and a four-dimensional
aerosol assimilation
model for
the land regions[Collinset al., 2001].The directforcingunder
clear skieshasbeen documentedby Satheeshand Ramanathan
[2000] for the KCO locationand by Rajeevand Ramanathan
[2001]for the entire tropicalIndian Ocean.This paper takes
on the more challengingtopicsdealingwith the direct forcing
in cloudyskiesand the indirectforcing.This paper focuseson
the IFP period of January-March1999. However,the conclusionsof this INDOEX study shouldbe applicableto other
yearsaswell. For example,analyses
of satellitedatafor 1996to
2000[Tahnk,2001]revealthat the seasonally
(January-March)
Near-continuous
measurements
were
maintained
at KCO
for aerosol chemical composition,size distribution, spectral
optical depths,vertical distributionof aerosolextinctioncoefficient (by multispectralLidar), cloud condensationnucleus
(CCN) spectraand radiation fluxes.These data were used to
link aerosolopticaldepth and direct aerosolforcingwith aerosol chemical compositionand microphysicalproperties;link
aerosolconcentrationwith CCN for a wide variety of low and
heavily polluted conditions' and develop an aerosol optical
model that agreeswith surface measurementsof scattering,
absorbingcoefficientsand aerosol humidity growth factors.
Aircraft data were then used to get a better representationof
the vertical distributionof aerosolcomposition,CCN and microphysicalproperties.These data were then incorporatedin
MACR to obtain the forcingon regional scales.
MACR usesmeasurementsof physicaland chemicalproperties of aerosolsas input and estimatesradiation fluxes for
three-dimensionalbroken cloud systemswith a realisticdistribution of atmosphericgasesand aerosol species.The cloud
parameters,suchas cloud base and top heights,are obtained
from the C-130, and cloud optical depth is calculatedas decrr[h•cl in c•ptic•n R nncl Annpncli,z
A The trnclp c'•rm•l•q
t--i
clnuds
28,374
RAMANATHAN
ET AL.: INDIAN
,
OCEAN EXPERIMENT--AN
METEOSAT5
•,EOS •:.TRMM
INTEGRATED
ANALYSIS
INSAT
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 variousobservingplatforms,which includedsatellites,aircraft,ships,balloons,dropsondes,and
surfacestationsat Maldivesand India. The three photographsshowthe picturesof the densehaze in the
Arabian Sea, the trade cumuli imbeddedin the haze, and the pristine southernIndian Ocean. (b) This
illustrationof the INDOEX compositeobservingsystemshowsthe domainsof the variousplatforms.Geostationary satellites(METEOSAT-5 and INSAT) and polar orbiters (NOAA 14, 15, and ScaRaB) provide
synopticviewsof 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 northsouthflightsfrom about 10øNto 17øSand east-westflightsfrom about 75øEto 55øEmeasuringsolarradiation
fluxes,aerosolpropertiesand distribution,cloud microphysics,chemicalspecies,and water vapor vertical
distribution.The ocean researchvesselsRonald H. Brown and SagarKanya travel between Goa, Malfi, La
R•union, and Mauritius transectingthe intertropicalconvergencezone and samplingsurfacesolarradiation,
aerosolabsorptionand scattering,chemicalspecies,ozone,winds,and water vapor vertical distribution.The
SagarKanyaalsosailsin the sourceregionalongthe westerncoastof India. Surfacestationsin India (including
Mount Abu, Pune, and Trivandrum), the Maldives (KaashidhooClimate Observatory),Mauritius, and La
R•union measurechemicaland physicalpropertiesof aerosols,solarradiation,ozone,and tracegases.Closely
related to surfaceplatforms,constantlevel 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•achentFlightTrack
Plate 1. (continued)
28,376
RAMANATHAN
ET AL.: INDIAN OCEAN EXPERIMENT--AN
Total Cloud
Cover
& Cruise
INTEGRATED ANALYSIS
Tracks
3O
oo o
C
80.0
70.0
60.0
50.0
%
40.0
30.0
n
ro
20.0
nl-.1
___J
Ron
i-.2
O
V Brown
SagairKanya
Ron Brown
10.0
I- ß3
,
00
b
Surface Wind, Low Clouds, & Flight Tracks
100.0
,
C130
Research
-
i%
2O
(• Flight,
17
66.7
0
. l• I
• •
-10
'
ß
"
,
'
'
•--•..,,,
ß
'
,
07
•-•
33.3
05
0.0
40
50
60
70
•
•
1•
110
Plate 2. (a) Illustratesthe total cloudamountsfor January-March1999in percentcoveroverthe tropical
Indian Ocean.Cloud amountwasretrievedfrom METEOSAT-5 satelliteradiancesprovidedby the European
MeteorologicalSatelliteOrganization(EUMETSAT). The retrievalschemeallowsthe determinationof low
clouds,cirrus,cumulus,and anvils.The bandof near-overcastcloudiness
representsthe approximatelocation
of the ITCZ. The orangeline represents
the trackof the R/V SagarKanyaduringFebruaryand March.The
colorlines(dark blue, brown,and red) representthe cruisetrackof R/V Ron Brownduringlate February
throughthe end of March.The greenarrowillustratesthe locationof the KaashidhooClimateObservatory.
(b) Illustrates
thenortheasterly
to southwesterly
monsoonal
flowduringFebruary1999andthelow-levelcloud
amounts.Surfacewind data for Februarywere obtained from the National Center for Environmental
Prediction/National
Centerfor AtmosphericResearchReanalysisProject.The colorlineson the map representthe flight pathsof the C-130.
prevalentin thisregionhavehorizontalscalesof few hundred fractionsbetween0 (clear sky) and 1.0 (overcastsky). The
meters to few kilometers,which we accountfor through the
Monte Carlo radiative transfer modeling [Podgornyet al.,
2000]. We use fractal broken cloud scenesto simulatecloud
cloud fractions are retrieved once every 30 min from the
METEOSAT Infrared (IR) brightnesstemperature data.
MACR adoptsthreetypesof skyconditions,specifically,
clear
RAMANATHAN
KCO:
ET AL.:
INDIAN
OCEAN
EXPERIMENTmAN
INTEGRATED
- ••
Male-
"•sol
, !cro
Ron
-
Ch',,ical
ica•,
C,
Indii•ec'
-
Effect
cheme
&
Radiation
Model
c- 130&
28 377
C-130
Lidar
Brown
ANALYSIS
KCO
Sa•arKanya
CR
Citation
R- ,_tonal
Cloud
fraction
&
eros
M ETEOSAT
AVIIRR
1 RM.M
fox:canonly)
O• '½.
ep
'sl. ila i n
, e •ant•)
KœO
Clear
Direct
C-130
Firs Indirect
Effect &
•ud
Fffect on Direct
econ • lndirec
Effect &
Set, i-Direct Effect
CERES
'rrade-Cu
D Clou,
nthropo•,eni
Fraction:
m • el
KCO; Rot Brown'
C-130 & C'
tion
Ill!
Oil o:
CIi ate Impact
G
1 Studie
Plate 3. A schematicof the data integrationscheme.Central to the integrationschemeis 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-microphysical
componentsof MACR are developed from observations
from KCO, the lidar in Male, the R/V Ron Brown,the C-130, and the Citation. The
dependenceof cloudopticalpropertieson aerosolconcentration(the so-calledfirst indirecteffect) is incorporated in MACR based on the C-130, the KCO, and the R/V Sagar Kanya observations.The regional
distributionof the forcing is obtained by integrating satellite-retrievedaerosol optical depths and cloud
fractionwith MACR. The resultingaerosolforcingis insertedin the NCAR-CCM3 to obtainclimateimpacts.
skies,low clouds(includingcirrus),and convectivecloudsincludinganvils.
MACR treats each aerosol speciesexplicitlyby using the
correspondingphase functionsand single-scatteringalbedos
for individual species[Satheeshet al., 1999]. The model also
accountsfor all multiple scatteringand absorptionby individual aerosol species,air moleculesand reflections from the
surface.The solar bandsfrom 0.2 to 4.0 •m are divided into 25
narrow bands.Hygroscopicaerosolspecies(all speciesexcept
soot,dustand ash) are allowedto growwith relativehumidity
[Hegg et al., 1993; Nemesureet al., 1995] using radiosonde
profilesof water vapor (launchedfrom KCO). The modeled
changein aerosolscatteringdue to growth with relative humidity is comparedwith observedvaluesin section6. MACR
also accountsfor the correspondingchangesin scattering
phasefunction.A more detaileddescriptionof MACR is given
by Satheeshet al. [1999] and Podgornyet al. [2000].
MACR was used in two modes: First, it was used to retrieve
aerosolopticaldepthsfrom satellitedata, thusextendingin situ
data to regionalscales.Second,it wasusedin conjunctionwith
satellitelow-clouddata to obtain the regionaldirect and indirect forcing. For the indirect effect the KCO data were combined with the C-130 data to derivea compositeindirect effect
scheme(describedin AppendixA) that relatesCCN, effective
supersaturationand effective cloud drop radius with aerosol
numberdensity.In order to obtain regionalforcingvaluesas a
function of longitude and latitude from these point source
estimates,we use the advancedvery high resolutionradiometer (AVHRR) data collectedfrom the polar orbiting NOAA
satellite to retrieve aerosoloptical depths (AODs), and the
METEOSAT
data for cloud fraction.
The long transequatorialflightsof the C-130 (Plate 2b) and
the cruiseby the R/V Ron Brown (Plate 2a) establishedthe
sharp north-south gradients in aerosols,CCN and aerosol
28,378
RAMANATHAN ET AL.: INDIAN OCEAN EXPERIMENT--AN INTEGRATED ANALYSIS
1.0
$agar Kanya Cruise
© 1996
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 latitudinalvariationof r•, (0.5/xm)measured
bytheIndianresearch
vessel
SagarKanyaduring
1996to 1999.The multiwavelength
Sunphotometer
usedfor thisdatais described
byJayaraman
etal. [1998].
The precision
of themeasurement
is _+0.03
or 20%,whichever
is smaller.
Eachcolorrepresents
a different
year.
chemicalcomposition.
Thesegradientswere usedto evaluate
the predictionof modelsfor the compositeindirecteffect
scheme.Thesetransequatorial
gradientdatawerealsousedto
estimatethe anthropogenic
contribution
to theaerosolforcing.
The stepsfor estimatingclear-skydirectforcingby aerosols
have been documented extensivelyin the open literature
[Satheesh
et al., 1999,hereafterreferredto asS99;Rajeevet al.,
2000; Conant, 2000; Collins et al., 2001; Rajeev and Ra-
manathan,2001]and are brieflysummarizedbelow.We adopt
the following conventionwith respectto aerosol optical
depths:% refersto aerosolopticaldepthsat 0.5 /xm;and r v
refersto the aerosolopticaldepthat 0.63/xm.
Jan-Mar 1999
25
•
0.6O
•.50
5
•
, .20
,.15
,.10
Integrated
Aerosol
Optical
Dc'pth
(630nm)
Plate 5. The regionalmapof aerosolvisibleopticaldepth%. The % valuesoveroceanare retrievedfrom
AVHRR data[Rajeev
etal., 2000]andoverlandareestimated
usinga 4-D assimilation
model[Collins
etal.,
2001].The figureillustrates
thenorth-south
gradientin % withvaluesaround0.5aroundthecoastandless
thanabout0.1 southof theequator.As described
byRajeevandRamanathan
[2001],thestandarderrorof the
seasonal
averages
shownin thisplateis about_+0.02or 15%,whicheveris greater.
RAMANATHAN
1.
ET AL.: INDIAN
The in situ data for size-resolved
OCEAN EXPERIMENT--AN
aerosol chemical
INTEGRATED
ANALYSIS
28,379
com-
indicativeof polluted air. The •-, frequentlyexceeded0.2 north
of 5øN, in spite of being at least 500 to 1000 km away from
major urban regions.The ship data clearly demonstratethe
chronicrecurrenceof the haze. The interannualvariability is
alsolarge.At KCO, for example,•-, during1999wasmore than
a factor of 2 greater than in 1998. However, the interannual
of AOD at KCO.
variabilityof the spatiallyaveraged% is much smaller.
2. The aerosolmodelwas employedin MACR to simulate
Upon integratingthe in situ and satellitemeasurementswith
the solar radiation fluxes, which were validated with radiometthe 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 overmostof the Asiancontinent(Plate 5).
visibleopticaldepth *v from satelliteradiancesin 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 satelliteradianceusedfor retrievingthe optical SoutheastAsia. There is a sharp transition to background
values between 5øSto 8øSbecause of the ITCZ, which restricts
depth. It shouldbe noted that % • 1.30
4. The retrieved•-• wasvalidatedwith •-• measuredby Sun interhemispherictransport.The fact that the haze can reach as
photometersdeployedat the surfaceand aboardships[Rajeev far south as 5øSis also captured in the TransmissionElectron
et al., 2000]. Steps1-4 yielded•-• only over the oceansincethe Microscopeimageof particles(Figure 1) from the C-130. The
surface reflectivities needed in the radiation retrieval models
sootparticleis shownattachedto the aerosolcollectedat 6.1øS,
are not well known for land surfaces.
more than 1500 km from the source.The causeof the large5. For land a four-dimensional
(4-D) globalaerosolassim- scalespreadof the haze is demonstratedfrom the trajectories
ilationmodeldevelopedespeciallyfor INDOEX [Collinset al., of the constant level balloons launched from Goa, India. Sev2001] was used.This model initializesdaily valuesof satellite- enteen overpressure (constant volume) balloons were
retrieved •-• for the oceansand aerosolsurfaceemissiondata launched between January 16 and February 28, 1999. They
for land and uses model winds to advect the aerosol horizonfollowed quasi-isopycnic
trajectories,exceptfor eventual octally and vertically. The model predictionswere extensively currence of diurnal oscillations in the vertical, due to occaevaluatedagainstdata collectedduring the field campaign.
sionalwater loadingof the envelope.Their nominallevelswere
The cloudand aerosolmicrophysicaldata taken at KCO and comprisedbetween 960 and 810 hPa. The northeasterlyflow
from the C-130 are usedto developa compositeschemefor the was clearly evident in most of the trajectories.Most of the
first indirecteffect.This schemeestimatescloud optical depth balloonssurvivedthe 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 satellitedata for of the scarcityof rain during the northeastmonsoon,the lifecloudcoverand aerosolopticaldepth,to estimatethe regional time of the aerosolsis of the order of 10 days[IPCC, 1995],and
forcingfor the first indirecteffect.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.
monsoonbeginsaroundNovemberand initiatesthe buildupof
We includethesetwo effectsin our forcingestimatesusingthe the haze from the premonsoonaldaily mean valuesof 0.05 to
modelstudyof Ackermanet al. [2000] and Kiehl et al. [2000]. 0.1 to valuesas high as 0.9 in late March. The same seasonal
In summary,asdepictedin Plate 3, the directforcingand the cyclehas 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- southwestmonsoonbegins in April after which the pristine
tively. The model-derived forcing values due to the second SouthernHemisphere air replacesthe haze.
position;microphysicsand vertical profile were used to develop an aerosol microphysicalmodel (S99). The singlescatteringalbedo and the spectralvariation in the column
aerosoloptical depth (AOD) simulatedby this model were
verified with multiwavelengthSun photometer measurements
indirect
effect and the semidirect
tion 9. Section
effect are described
10 uses these results to summarize
in sec-
the various
contributionsto the aerosolforcingby anthropogenicsources. 4.
Chemical, Microphysical, and Optical
Properties: Anthropogenic Contribution
3.
Space and Timescales of the Haze
Plate4 shows4 yearsof aerosolvisibleopticalthicknessdata
collectedfrom a multispectralSun photometer [Jayaramanet
al., 1998] on board the R/V SagarKanya during the winter
monthsof 1996 to 1999. The optical depth at a wavelengthX,
*x, is definedsuchthat the direct solarbeam transmittedto the
surfacedecreases
to exp (-,x/cos 0) due to aerosolscattering
and absorption,where 0 is the solar zenith angle. Thus *x
quantifiesthe opticaleffectof the aerosolcolumnbetweenthe
surfaceandtop-of-theatmosphere(TOA) at the wavelengthX.
Plate 4 showsthe optical depth at X = 0.5/am denotedby %.
For eachof the 4 yearsthere is a steeplatitudinalgradientwith
•-a increasingfrom about 0.05 near 20øSto 0.4 to 0.7 around
15øN.Typically,% of 0.05 to 0.1 is consideredto be a backgroundvaluefor pristineair (S99),while •-, in excessof 0.2 are
4.1.
Chemical
Aerosol optical and chemical measurements were performed on board the C-130, the R/V Ronald H. Brown, and
from KCO at 5øN, 73.5øE. At KCO we measured the size
distributionand chemicalcompositionof fine particles,collectedon filters and cascadeimpactors[Chowdhuryet al., this
issue].The chemical data summarizedin Plate 6 are taken
from Lelieveldet al. [2001].The filter analysisshowsan average
massconcentration
of nearly17/agm-3 (Plate6). The aerosol
compositionwas stronglyaffected by both inorganic and organic pollutants,includingblack carbon (soot). Very similar
resultswere obtainedfrom boundarylayer flightsby the C-130
aircraft and from the R/V Brown, indicatingthat the aerosol
compositionwas quite uniform over the northernIndian Ocean.
The measurementsshowthat the fine aerosol(dry massat
28,380
RAMANATHAN
ET AL.: INDIAN
OCEAN EXPERIMENT--AN
INTEGRATED
ANALYSIS
Figure 1. Transmission
ElectronMicroscope(TEM) imagesof aerosolssouthof 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/organicaerosolsin 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 includedin each panel. All the
measurementsare betweenapproximately690 and 820 m altitude.
diameters<1/am) wastypicallycomposed
of 32% sulfate,26% 0.1-0.2 /am from fresh engine exhaust from Kleeman et al.
organiccompounds,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]). Massspectrometricsingle-particleanaland trace amountsof methylsulfonicacid (MSA), nitrate, and ysisshowsthat the BC particleswere alwaysmixedwith organic
minor insolublespecies(Plate 6b). The coarseaerosol(dry compoundsand sulfate,indicatingsubstantialchemicalaging
massat diametersbetween1 and 10/am), asobservedat KCO, and accumulationof gas-to-particleconversionproducts.This
typicallyconsistedof 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 evaporationof
6% fly ash, and 1% potassium.Interestingly,at KCO the droplets; sulfate formation within BC-containingdroplets).
coarse aerosolsalso contained about 4% nitrate, while farther
The prevalenceof suchcloudsis 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-producedsincenatural
suggests
that nitric acidfrom the gasphasepreferentiallypar- sources
arenegligible
[Cooke
etal., 1999;Novakov
etal.,2000].
titions into the coarseparticles.
Likewise,non-sea-saltsulfate can be largely attributed to anThe fine particle (particlessmaller than 1 /am diameter) thropogenicsources.The filter samples,collectedon boardthe
concentrations
observedover 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, probably
fromthe oxidation
of naturallyemitteddihigh BC concentrationgivesthe aerosola strongsunlightab- methylsulfide.Consideringthat the sulfateconcentrationover
sorbingcharacter.The KCO impactor measurementsshow a thenorthernIndianOceanwascloseto 7/agm-3, we inferan
dry particle size distributionthat peaksat about 0.5/am phys- anthropogenicfraction of about 90%. Similarly, the ammoical diameter.In this sizerangethe sulfateaerosol(and asso- nium concentration south of the ITCZ, from natural ocean
ciatedammonium)contributemostto the light scattering.The emissions,
was0.05/agm-3, indicating
an anthropogenic
conpeak BC concentrationoccurredat 0.3-1.0/am physicaldiam- tributionof 97% to the nearly2 /agm-3 of ammonium
obeter, largerthan typicallyfoundin citieslike LosAngeles(e.g., served north of the ITCZ.
RAMANATHAN
ET AL.: INDIAN
OCEAN EXPERIMENT--AN
.•
c3
•
15
28,381
almost6 tagm-3. We thusinferthatmost(at least90%)of the
Residual
particulateorganicsnorth of the ITCZ were of anthropogenic
origin. INDOEX fine aerosol componentsof natural origin
I---1 Chloride
I•
Nitrate
[::: 20
ANALYSIS
In the aerosol south of the ITCZ, organic compoundswere
negligible,whereas over the northern Indian Ocean it was
30
25
INTEGRATED
I
I
r-•
Ammonium
Sulfate
Calciumcarbonate
I
Organic
compounds
I
Blackcarbon
included
a total mass fraction
of 1% sea salt and 10% mineral
dust. However, some of the mineral aerosol likely originated
from road dust and agriculturalemissionsduring the dry monsoon, so that the 10% assumptionfor the natural dust represents an upper limit. Taken together, the human-produced
10
contribution
to the aerosol
over the northern
Indian
Ocean
was about 80% (-+10%). Note that these aerosolmassfractions,in conjunctionwith Plate 6b, havebeen usedto perform
the optical depth calculations.
The followingadditionalconsiderationsalsojustify our conclusionthat anthropogenicsourcescontributea major fraction
(80%) to the aerosolloading.The observedBC/total carbon
ø0.ø1
.... •''
Aerodynamic
diameter
(ILtm)
ratio of 0.5 and the sulfate/BC ratio of 2-2.5 are both more
5%
10%
14%
2%
I
Black carbon
1%
'" Organics
2%
[]
Ammonium
[]
Sulfate
[]
Potassium
[]
[]
Sea-salt + nitrate
MIS
[]
Mineral dust
26%
typicalfor fossilfuel derivedaerosolthan for biomassburning
aerosol[Novakovet al., 2000]. The substantialmassfraction of
sulfatein the aerosolsupportsthis analysissinceSO2emissions
are dominatedby fossilfuel use.Important informationabout
aerosol sourcesis provided by the comparisonof potassium
(K +, mostlyfrom biomass
burning)with sulfate(mostlyfrom
fossilfuel combustion).
The K+/BC ratio in INDOEX aerosol
was about 5 timessmallerthan that typicallyfound in biomass
smoke [Novakovet al., 2000]. As concludedby Novakovet al.
[2000], fossilfuel related emissionsof BC and sulfur are the
[]
Fly
ash
32%
major contributors(at least 70%) to the aerosolsover the
Indian Ocean. Fossilfuel related emissionsare not very effi8%
cientlyremovednear the sourcessinceit takesseveraldaysto
Plate 6. (a) Averagechemicalcomposition
of fine aerosolon convert SO2 into sulfate and BC into hygroscopicparticles.
KCO (Maidives) as a functionof particle size for February
Therefore the dry monsooncan carry thesecompoundsacross
1999(totalmass17 tagm-3). The residualincludes
mineral
dust,fly ash, and unknowncompounds.For gravimetricmass the indiau Occa,• where they contribute to light extinction.
determinationthe averageprecisionof the impactormeasure4.2. Optical
ments, calculatedfrom the nominally four replicate impactor
The scatteringand absorptionof solar radiationby aerosols
samplestaken eachevent,wasfoundto be _+5.3%for samples
greaterthanor equalto 2.0tagm-3, and_+22.5%
for samples are governedby the chemicalcomposition,the size distribulessthan 2.0 tag m-3 whosevalueswere still significantly tion, and the number density.The total number of particles
greaterthan zero. For sulfate,ammoniumion, organicmatter, variesfromabout250cm-3 in theunpolluted
southern
Indian
and elemental carbon the averageprecisionof the measure- Ocean(SIO) air to 1500cm-3 or morein pollutedNIO air
ments based on repeated analysisof standardsolutionswas (Plate 7a). Particlesof diametersfrom a few nanometers(nm)
_+2.3%,_+5.4%,_+11.1%, and +_5.9%,respectively,for samples
to a few micrometers(tam) contribute to the total number
greaterthanor equalto 0.2tagm-3; for samples
smallerthan
0.2 tagm-3 the averageprecisionwas _+16.4%,_+12.4%, density,but thosebetween0.05 and 2 tamcontributemore 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 >
singleimpactorsample.Plate 6a is constructedfrom the aver- 1 tam) are primarilyseasalt and dust,which contributemore
ageof eightimpactorsamplingevents,sothe standarderror of than 30% to the massbut lessthan 10% to r, sincethey are too
the meanscalculatedfrom thoseeight samplesis correspond- few. In MACR we use surface measurements at KCO to acingly smaller.(b) Averagemassfractionsof aerosol(D < 1 count for the coarseparticles.
tam)asobservedfrom the C-130 aircraftduringFebruaryand
The chemicalcompositiondeterminesthe refractiveindex,
March1999(total mass22 tagm-3). Thirty-fourboundary which, in conjunctionwith solutionsof the Maxwell's equalayer samplesare includedin the averages.MIS is minor intions,yieldsthe scatteringand the absorbingcrosssectionsfor
organicspecies(reproducedfrom Lelieveldet al. [2001]with
permissionfrom the authorsand Science).When we account a singleparticleof diameterD. The calculatedlight extinction
both at KCO and on
for spatialandtemporalvariability,instrumentalprecision,and agreeswith directopticalmeasurements
board the C-130 aircraft across the entire measurement
dothe numberof samples,our estimatefor the overallstandard
error of the averagesshownin Plates6a and6b is about+_20%. main down to the ITCZ. The results indicate a remarkably
large absorptioncomponentof the aerosol,yieldinga singlescatteringalbedo at ambient relative humidity of about 0.9
later in more detail). The contributionof various
It is more difficultto attributethe organicaerosolfractionto (discussed
to the columnarr, is shownin Plate 8 (seeS99for
a particular sourcecategory.However, the BC/total carbon constituents
albedo is wavelengthratio of 0.5, as derivedfrom the filter samples,pointsto fossil details). The aerosolsingle-scattering
fuel combustionasthe main contributor[Novakovet al., 2000]. dependent.MACR integratesthesesolutionswith the particle
28,382
RAMANATHAN
ET AL.: INDIAN
OCEAN EXPERIMENTmAN
INTEGRATED
ANALYSIS
INDOEX Flight03, 2/20/1999
20OO
1500
io.6
5
1 o00
500
CN
hot
(300øC)
• 0.2
Ratio
•............ t0
0
-6
-5
-4
-3
-2
-1
2
0
3
Latitude
?
1 O00 .................
-
.A- SH ..._.
Ambient
'l:emp.
10oo
FJH
_.,, Residualat 300øC
5oo
500
0
20
50
100
20
Diameter, nm
50
100
Diameter, nm
Plate 7. (a) Latitudinalvariationof the condensationnuclei (CN) measuredduringthe C-130 flight. Total
CN (greencurve)and CNhot (blackcurve,samplescollectedwith a preheaterat 300øC)are shownwith their
ratio (pink curve).This ratio is the nonvolatilefractionof particlesremainingafter volatilecomponentsare
removed.The highgradientin total CN numberfrom north to southand the rapid drop in the CN nonvolatile
fraction uponcrossingthe ITCZ (approximatedby vertical line) are clearlyseenin the plate. Flow errorsfor
the CN counterare < =3% Particleslarger than about 15 nm are countedwith high efficiency.The heated
inlet wasmaintainedat 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 typicalcleanmarine 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) revealslargerparticleswith muchgreaterrefractorypartstypicalof the INDOEX haze. Errors
in overall particle number and particle sizingare <10%.
size distributionand vertical distribution(Plate 9, to be discussedlater) and sumsup the contributionsfrom all of the
and the forcing,consistentwith earlier findings[S99;Piliniset
al., 1995]. Our confidencein the MACR treatment of the
altitudes to obtain Plate 8, which enables us to determine the
radiative
anthropogeniccontributionto the radiative forcing.
The percentcontributionshownin Plate 8 differs somewhat
from the KCO valuesshownby S99 sincethe latter is basedon
surfacedata for one location(that too for 1998), whereasthe
presentvaluesintegratesurfacedatawith in situvalues(C-130
data) and for a much larger domain.The anthropogeniccontributionto the columnr,, (valuesare reportedhere only at 0.5
/•m) is about80% (_+10%). While the individualcontributions
are broken down by species(usingMACR), the aerosoltimeof-flight massspectrometerdata [Coffeeet al., 1999] revealed
that these speciesdo not occur as separateparticles,i.e., as
pure sulfate or pure organicaerosols.Instead, substancesare
mixedwithin individualparticlesas imagedin Figure 1 which
shows the soot attached to a sulfate and organic aerosol.
MACR allowsfor thisby treatingtwo extremecases:an externally mixed particle in which each substanceoccursas a separate particle; and an internally mixed aerosol in which each
particlecontainsa mixtureof all the substances.
The difference
betweenthe two caseswas lessthan 10% in the calculatedr,,
fact that the solar fluxesand the clear-skydirect forcing estimated by MACR is in excellentagreementwith the observed
values(describedin section6). However, the aerosolmixing
state can exert a significantrole on the indirect effect.
5.
effects of the INDOEX
aerosols is derived from the
Role of Black Carbon in the Forcing
We highlightthe role of black carbonbecauseof its major
role in the surfaceand the atmosphericforcing[Podgorny
et al.,
2000].The estimatesof aerosolforcingprior to 1990oftenused
assumptions
on the imaginarypart of the refractiveindex due
to lack of adequatemeasurements.From the beginningof this
decade, more attention has been devoted to the absorbing
aerosols[Penneret al., 1992;Haywoodand Shine,1997;Heintzenberget al., 1997;Hansenet al., 1997]. The aerosolforcing
also depends on the altitude level at which the aerosol is
locatedandwhetherthe aerosolis aboveor belowclouds[e.g.,
Heintzenberg
et al., 1997;Haywoodand Shine,1997].While the
role of absorbingaerosolsis 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
2%
....
Organics
17øo
11Oo
Sea-salt & NO3*
12%
+
NH4+
26%
SO4=
Plate 8. Relativecontributions
of the variouschemicalspeciesto % (0.5/xm). When we accountfor spatial
and temporalvariability,the standarderror of the mean valuesis about _+15%(relativeerror). The size
distributionof aerosolsshowsan effectiveradiusof -0.34/xm. The value of Angstromcoefficientvariesfrom
1.0 to 1.2.
A keyparameterin thisregardis the single-scattering
albedo
particularlyif aboveclouds,the role of sulfatedecreases
dueto
humidityeffects[Heintzenberg
et al., 1997]. This is where the (SSA), w = C.•/(C.• + C.), where C,. is the scatteringcoefimportanceof simultaneous
measurements
of verticalprofiles ficientand C. is the absorptioncoefficient.MeasuredCs.+ C.
are shown in Plate 9a for an aerosol layer imbedded in the
and absorptionpropertiescomesinto play.
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)
....
0
, ,, •, , ,i, ,
50
ß ß ß i ..............
100
150
ExtinctionCoefficient(Mm'1)
-10
10
30
50
70
90
110
Cs,Ca(Mm4)
Plate 9. (a) Typicalprofilesof a "boundary
layeraerosol."The dataarefor the aerosolextinction
coefficient
(scatteringplus absorption).
The C-130 data includeonly submicronparticles,and the lidar includesall
particles.This differencecontributesin part to the differencebetweenthe two profiles;the other causes
includespatialgradientsin the profilesand instrumental
uncertainties.
(b) Typical"elevatedaerosollayer."
The data are for scattering(Cs, blue curve)and absorptioncoefficients
(C,, red curve)measuredfrom the
C-130. The Cs is measuredwith an integratingnephelometer.C, is measuredwith a continuouslight
absorption
photometer.Sincethe C-130inlet restrictslargerparticles(diameter>1/xm), the valuesbelow1
km maybe uncertainby asmuchasa factorof 2. Uncertaintyin the lidar datais 20% below2 km andis 30%
above2 km. Uncertaintiesare greatlyreducedwhensimultaneously
measuredSunphotometerAODs 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 SingleScatteringAlbedo
Plate 10. The estimatesof aerosolsingle-scattering
albedo•oat X = 0.53/am from variousplatformswhich
include the KCO, the C-130 aircraft, the R/V SagarKanya, and the R/V Ronald H. Brown (RHB). The
horizontalbar representsuncertaintiesdue to variabilityand instrumentalerrors.(bottom)Surfacemeasurementsfrom KCO and ships.KCO-AERONET standsfor AerosolRoboticNetworkmeasurements
at KCO;
KCO-NP3 standsfor three wavelengthnephelometerand PSAP measurements
at KCO; NP standsfor single
wavelengthnephelometerandPSAP;AS standsfor ArabianSea;andBOB standsfor Bayof Bengal.(middle)
Measurementsbetween 1 and 3 km from the C-130 aircraft and lidar. (top) Column-integratedvalues
estimatedusingMACR and retrievedfrom the skyradiancemeasurements
usingthe 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 secondmethodemploysa lidar [Althausenet al., 2000] at Male. The data measuredwith the
multiwavelengthlidar are usedto determinethe backscattercoefficientat sixwavelengthsand the extinction
coefficientat two wavelengthsindependentlyand simultaneously.
From these eight spectraldata, physical
parametersof the aerosolsizedistributionand the complexrefractiveindexare calculatedusingan inversion
algorithm,which are then used for the calculationof •o. The third method employsthe measuredmass,
composition,
andsizedistributionof the aerosol(Plate6) in conjunction
with a Mie-scattering
model(solution
of Maxwell'sequationfor a sphericalparticle) to determinethe aerosolradiativeproperties(Plate 8),
averagedover the vertical column.
boundarylayer, and measuredCs and Ca are shownseparately
in Plate 9b for an elevatedaerosollayer. The boundarylayer
aerosolprofileoccurredabout1/3 of the time, and the elevated
profileoccurredabout2/3 of the time. The coalbedo,a = (1 •o), is the basic measureof light absorption.In the visible
spectrum,a rangesfrom aboutlessthan 0.01 for pure sulfates
to 0.04 for fly ash, 0.2 for dust, and 0.77 for soot.Thus soot,
even at 10% of the total fine particle mass,can enhanceparticle absorptionby an order of magnitude.Sootwas found in
most samplesas far south as 6øS(Figure 1). The •o values,
obtainedby severalindependentmethods,are summarizedin
Plate 10 and are generallyin the rangebetween0.85 to 0.9 for
the near-surface
values and between
0.8 to 0.9 above the sur-
face layer, and about 0.86 to 0.9 for the column averages,in
excellent agreementwith the column average MACR value
shownin Plate 10. Generally,thesevaluesrepresenta highly
absorbingaerosol.The surfacestationsat Goa and Dharwad
also determined column averageSSA to be about 0.9 from
optical measurements,the only data we have at the source
[Leonet al., this issue].
In view of the diversesourcesand processes
influencingthe
formation and evolution of continental aerosol, the narrow
range (0.85 to 0.9) of the mean •o in Plate 10 is somewhat
surprising.There are two plausibleexplanations.First, aerosol
RAMANATHAN
ET AL.' INDIAN
KCO Growth
OCEAN EXPERIMENT--AN
Curves
400-700
2.8
2.6
......
Measured
nm Global
KCO'
at KCO
INTEGRATED
MACR
ANALYSIS
28,385
and Diffuse
vs. Photodiode
Irradiance
Radiometer
MACR
•o 2.2
2.4
a
-E
G 2
5OO
ßMACR with Soot
o'*
ßMACR
without
Soot
-r- 1.8
-
400
• '•e
ß
o
1.4
...15
-ø'
ß
ß
ß
1.2
x•
40
50
60
70
80
90
100
300
'O
ß
o
ß•'
BiaswithSoot:-1.8
ß
Bias without
• 200• !
RH(%)
2O0
300
can be found in the C-130 data shown Plates 7a-7c.
The data
which the size distributions
400
500
b
E 160 'MACR
with
Soot
' '
•:140
!'MACR
without
Soot
ß'ß
ß
•
ß
120
is 00[
x•
, ;,."-'ß
t
.:;e eA.-.
e e•P
• 801-";.;.,t ß BiaswithSoot:-0.2
O 60L::"
Bias
without
Soot:
10.1
'•
60
obtained from the cross-equatorialgradient flight of C-130
show the boundary layer concentrationof the total aerosol
condensation
nucleiconcentration("cold CN") alongwith the
so-called"hot CN" (the aerosol CN left after heating it to
300øC).The hot CN is indicativeof refractorymaterial (soot,
fly ash,and seasalt). The ambientand heatedsizedistribution
of this aged NIO aerosol showedlittle change (other than
concentration)so that the variationsin CN (and hot CN) are
reflectedin the aerosolscattering(and the absorption).While
CN and hot CN decreasewith latitude significantly,their ratio
is nearly constant.The near constantratio suggeststhat the
aerosolcharacteristics
(e.g., w) remain the sameas the plume
dilutes in time. The reason for this near constancyin the
Northern Hemispherehot CN/cold CN ratio is mostlikely that
the black carbon aerosolcomponentis first formed at high
temperaturesas primary particlesupon which organics,sulfates,and other compoundslater react or condense[Clarkeet
al., 1998]. Evidencefor this is shownin Plates 7b and 7c, in
9.2
MeasuredGlobal(W m'=)
Plate 11. A comparison of the model-estimated growth
curvewith that observedat KCO. The growthcurveis obtained
by measuringscatteringcoefficientCs as a functionof relative
humidityusingtwo 3-wavelengthnephelometers(one control
at fixed relative humidity,the other at variable relative humidity; see Ogrenet al. [1992] for details). Some investigations
showsthat the aerosolsare occasionallycoated with surface
activematerialsthat hinder their water uptake and thus influence their growth with relative humidity [Ogrenet al., 1992;
Cantre#et al., 1997].This effect is not includedin the present
study.This could be one reasonfor the slightdifferencein the
growthcurves.This is alsosupportedby the fact that measured
growth rate is lessthan the modeled.
combustionsourcesare widespread(as opposedto being localizedover few urban centers)and rather similar. Second,the
persistenceof the monsoonflowoften resultsin the mixingand
dispersionof this aerosolover large regionswith little modification.A more direct piece of evidencefor the narrow range
Soot:
!
80
100
120
140
160
MeasuredDiffuse(W m'2)
Plate 12. MACR predictionsof clear-sky(a) globaland (b)
diffusesurfaceirradiancein the 0.4-0.7 •m photosynthetically
active spectralband versusKCO measurementsmade by the
BSI GTR-511c photodiode flux radiometer from FebruaryMarch 1999. Details of the model, the measurement, and the
angular,spectral,and absolutecalibrationsusedto reduce the
data are describedby Conant [2000]. Black points represent
the MACR model as describedby S99. Red points represent
MACR predictionswhen sootaerosolis removedfrom MACR
(see Plate 8) and replacedwith proportionatecontributions
from other aerosolsso that % remains constrained to the
spectralobservations.This plate demonstratesthe importance
of aerosolchemistryon the radiative fluxes.
concluded that essentiallyall NH aerosolshave a primary,
refractorycomponentconsistingof sootor perhapsfly ashthat
amounts
to 10-20%
of the aerosol
mass. The much smaller
size and negligibleassociatedmassof the SH refractory component found here is presumedto reflect incompletevolatilization of the aerosoldue possiblyto pyrolysisof surfacefilms,
submicrometer sea salt, and/or other trace contaminants.
of the hot CN and the cold CN are
shownfor a casein the SouthernHemisphere(Plate 7b) and
one in the Northern Hemisphere(Plate 7c). After the nonrefractorymaterial (e.g., sulfates,nitrates,and mostorganiccarbon) is evaporatedfrom the aerosol,the remainingparticles
are equal in number, but far smaller in size. The shift to
smallersizesis far lessprominentfor the NH aerosol,indicating a large refractory component.Becausethe total particle
count is within 5% for the heated and the unheated scans,it is
6.
Successof the MACR Integration
The ultimate test of MACR is its ability to simulate the
observations.It agreeswith the observedspectraldependence
of the aerosoloptical depth (S99), the growthof the aerosol
with humidity (Plate 11), the observedscatteringand absorbing coefficientsat the surfacein KCO, and the column averaged co(Plate 10). It simulatesthe broadbandand spectral
solar irradianceat the surfaceand the TOA under a variety of
28,386
RAMANATHAN
ET AL.: INDIAN
OCEAN EXPERIMENT--AN
INTEGRATED
ANALYSIS
low clouds[Kaufmanet al., 1998] below 3 km for INDOEX
aerosolssincethe bulk of the aerosolis belowthislevel (Plate
9). Becauseof this dependenceon the low cloudfraction,and
sincelow cloudshavesignificantspatial(Plate2) andtemporal
variations,the direct forcingfor cloudyskieshas to be determined regionallyas a functionof latitude and longitude.This
wouldnot havebeen possiblewithout the integratedINDOEX
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-coveredregionswith
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-skyTOA albedo km for cloudbaseand 1.5 km for cloudtop), and the results
valueshavebeen comparedwith the measuredalbedosby the describedin the next sectionfor cloud optical depth and efCloudsandEarth'sRadiantEnergySystem(CERES) [Weilicki fectivedrop diameter.Becauseof the strongdiurnalvariation
et al., 1996]radiationbudgetinstrumenton board the Tropical of low clouds,we adopt the geostationaryMETEOSAT satelRainfall Mapping Mission satellite. Again, the model agrees lite data to obtaincloudcover.The averageNIO diurnalmean
with the observedvaluesto within 2% (S99).
low cloudcoverwe retrieveis29% (diurnalaveragecover),but
Next, the radiancescomputed from MACR were used to the averagecloudcovercanbe as low as 15% (dependingon
develop an algorithm for retrieving zv from the NOAA ad- the thresholdtemperaturewe use to distinguishclear from
vancedveryhighresolutionradiometer(AVHRR) in the 0.63 cloudypixelsin the satelliteradiances).Given the present
/amspectralregion.As shownbyRajeevet al. [2000]andRajeev limitationsof the availablesatellitedata (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 measuredvalues from Sun photometersat resolution,but it doesnot have the diurnal samplingcapabilKCO and R/V Sagar Kanya agreed within the instrumental ity), it is difficultto determinethe low cloud covermore acerror of about _+0.03.Lastly, the MACR valuesfor the TOA
curately.This twofold (15-29%) uncertaintyin the satelliteand surface aerosol forcing agree with the observedvalues retrievedlow cloudfractionis the largestsourceof error in the
[Satheesh
and Ramanathan,2000, hereafterreferredto as SR] cloudy-skyforcingestimates.
within 10%.
We will beginwith the regionallyaveragedaerosol(natural
plusanthropogenic)
directradiativeforcing,shownin Plate 13.
The gridvaluesof % (Plate5) andcloudfraction(Plate2) are
7. Aerosol Direct Forcing in Cloudy Skies
adopted to obtain the resultspresentedhere. The forcing is
In this sectionwe considerthe direct forcing due to all firstestimatedasa functionof latitudeandlongitude(fromthe
aerosols, including natural and anthropogenic.The term q'aand cloud fraction shownin Plates5 and 2, respectively)
"cloudyskies"refers to averageconditionswith a mixture of from which the regional averagesshownin Plate 13 are obclearand overcastportionsof the sky.The startingpoint for the tained. We account for the significantdiurnal variationsin
cloudcoverbut ignore its day-to-dayvariation.The regionally
analyses
is SR's clear-skyaerosoldirectforcingefficiency,
f (=
AF/A,a, the rate of changeof forcingper unit increasein
averagednorthernIndian Ocean(0øNto 20øN)(NIO) values
SR used simultaneousclear-skysolar irradiance observa- for the TOA forcing[F(T)] due to the directeffectis -7 _+1
tions at the surfaceand TOA and % for the KCO location,and W m-2 (Plate13a)for clearskies.For cloudyskiestheTOA
aerosolconditionswithin a few percent [S99;Podgornyet al.,
2000; Conant, 2000]. At KCO, 12 independentradiometers
coveringthe entire solar spectrumwere deployedto validate
MACR predictionsfor za rangingfrom 0.05 to 0.9. Plate 12
showsa comparisonof the observedand the modeledglobal
(direct solarplus diffusesky radiation)radiation(Plate 12a)
and diffuseradiation(Plate 12b) for the 0.4 to 0.7/am region.
The agreementbetweenthe MACR andthe observedvaluesis
demonstrated
thattheTOA f isf(T) • -25 W m-2 andat directforcingrangesfrom-0 to -4 W m-2, witha meanof
a net heatingof
the surface,
f(S) • -75 W m-2. The atmospheric
forcing -2 W m-2. Inclusionof cloudsintroduces
efficiency,
f(A) = f(T) - f(S), is positive,and thusaerosol about5 W m-2 (difference
betweenclear-and cloudy-sky
t
solarabsorptionis the reasonforf(S)/f(T) > 1. We checked
the validityof SR'sf valuesfor the rest of the INDOEX region
by correlatingthe satelliteCERES radiationbudgetdata with
the observed% shownin Plate 5 for the entire tropicalIndian
Ocean IRajeerand Ramanathan,2001]. For the entire tropical
forcing)largelybecauseof the aerosolabsorptionof the re-
flectedradiation
fromclouds.
Roughly
50%of the _+2W m-2
estimatederror in the cloudy-skydirect forcingis due to the
uncertaintyin cloud fraction;about 25% is due to the uncertaintyin the verticalprofile of aerosols,and the balanceof 25%
IndianOcean,
f(T) • -22 W m-2 (witha 1-rrspatialvaria- is due to the uncertaintyin clear-skyforcing. The plate also
tionof 5 W m-2) compared
to 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 surdata, and the balance is due to revisions in CERES data since
publicationof SR's paper. For f(S), C-130 radiometricdata
[Valeroet al., 1999] confirmedSR's value of -70 to -75 W
m-2. Reassured
by thisvalidation,we usedSR'svaluesfor
f(S). We obtain clear-skyregional forcing as a function of
latitude and longitudeby multiplyingthe observedf(T) [=
-22 W m-2],f(S) [= -72 W m-2], andf(A) [= 50W m-2]
with the z• shownin Plate 5.
For absorbingaerosols,it is well known [Penneret al., 1992;
Haywoodand Shine,1997;Kaufmanet al., 1998]thatf(T) can
changesignfrom cooling(negative)underclearskiesto a large
net heatingfor overcastskies.This effect is importantonly for
faceforcingvaluesareF(T) - - 2 _+2.0 W m-2; F(A) 18 _+3 W m-2; andF(S) = -20 _+3 W m-2, respectively.
The cloudy-skysurfacedirect forcing is a factor of 10 larger
than the TOA forcing, as opposedto a factor of 3 for clear
skies(SR). About 60% of the largeatmospheric
forcingF(A)
is due to soot,and the balanceis due to fly ash,dust,andwater
vapor absorptionof the radiation scatteredby aerosols.
Aerosols
also absorb and emit infrared
radiation.
Their im-
portancefor the INDOEX region has been estimatedusinga
model similarto that presentedby Lubin et al. [1996]. Their
model was modified
to include INDOEX
aerosols and is used
in this study.The NIO averagedTOA IR aerosolforcingfor
RAMANATHAN ET AL.: INDIAN OCEAN EXPERIMENT--AN
INTEGRATED ANALYSIS
28,387
AerosolRadiativeForcing(W m-2):NorthIndian Ocean
(Jan - March, 1999; 0 - 20øN)
0.3
-7.0:t:l
+
-2.0ñ2
.
+18.0:3
-23-t-2
-203
Direct
Direct
(Clear Sky)
(Cloudy Sky)
(2)
-5ñ2.5
+l:C-o 5
First
Indirect
(3)
Plate 13. Aerosoldirectradiative
forcingfor theNorthIndianOcean(0øto 20øN;40øto 100øE).
Thevalues
includethe effectsof naturalandanthropogenic
aerosols.
The valueson top of eachpanelreflectTOA
forcing;
those
withintheboxshow
theatmospheric
forcing,
andbelowtheboxisthesurface
forcing.
cloudy
skiesis 1.3W m-2 at TOA and5.3W m-2 at the servedvalues.The three INDOEX pointsshownin Plates14a
forthepristine
(N• -< 500 cm-3);the
surface,suchthatthe atmosphere
is subjectto a net coolingof and14bareaverages
transition
(500 -< N• -< 1500cm-3),andthepol-4.3 W m-2. Thus they offset someof the solar forcing. so-called
Roughly
90% of theTOA IR forcingand75% of the surface luted(N• >- 1500 cm-3) cases.
Eachof thesepointsis an
IR forcingaredueto naturalaerosols.
The largersea-salt
and averageover 10,000cloudssampledduring18 C-130flights
dustparticles
are moreeffectivein theirinteraction
with IR from 15øNto about 8øS.The horizontalbar showsthe rangeof
than the smalleranthropogenic
particles.
aerosolconcentrations
overwhichthe datawere averaged.The
absoluteaccuracy
of the clouddropnumberdensitymeasure-
8. ForcingDue to the First Indirect Effect
The C-130 data clearlydemonstrated
[Heymsfield
and Mc-
ment is about 15%. The standarddeviationsof Nc in each of
these
ranges
arelarge(asexpected):
85cm-3 forpristine;
135
cm-3 for transition,
and197cm-3 for thepollutedcase.HowFarquhar,
thisissue;McFarquhar
andHeymsfield,
thisissue] ever,giventhelargenumberof cloudssampled,
the standard
that the pollutedcloudshaveN a (aerosolnumberdensityin erroris expected
to be muchsmallerthanthe abovestandard
particlesizeslargerthan50 nm diameter)exceeding
1500 deviation. We estimate that the overall standard error in the
particles
cm-3, aboutthesameaverage
liquidwatercontent,
shownin Plate14 is about25% or less[Heymsfield
LWC•--0.15g m-3, moreclouddrops,
Nc -• 315 cm-3, and averages
andMcFarquhar,
thisissue;McFarquhar
andHeymsfield,
this
smallereffectivedropradii,Re --<6 /•m, comparedwith the
issue].
The
composite
scheme
is
in
agreement
with
the
obpristine
clouds
{Na -< 500;Nc = 90 cm-3;Re -->7.5 /•m).
seemto satThe mean cloud base vertical velocitywas about the same servedNc. With respectto Re, the observations
thepredicted
onesdo
between
thepollutedandthepristineclouds,
thusenabling
the uratearoundNa of about1500,whereas
not
saturate
until
an
N•
of
about
2000.
As
a
result,
the
preidentification
of the anthropogenic
effect.The observed
effect
dicted
R
e
for
the
larger
N•
are
smaller
by
about
10%
(or
0.5
iscaptured
in MACR through
a composite
scheme
(described
relationin detailin AppendixA) whichrelatesthe effectiveradiusof /•min radius).Plates14cand14dshowthecomputed
?• andNa (Plate14c)andbetween
?c,thecloud
clouddropsand the numberdensityof clouddropsto N•. shipbetween
Plates14a and 14b comparethiscompositeschemewith ob- opticaldepthat 0.5/•m,and?• (Plate14d).We usethisplate
28,388
RAMANATHAN
(a)
ET AL.: INDIAN OCEAN EXPERIMENT--AN
INTEGRATED
ANALYSIS
Composite
INDOEX
Scheme
0.3
8
3OO
0.25
7.5
250
0.2 •
•E 200
'-=6.5
'-'
6
z
0.15
150
0.1
100
5.5
0.05
5O
0
500
(c)
1000
1500
2000
2500
N,(cm
'--
0
500
1000
1500
2000
2500
N. (cm'3)
16
0.5
14
12
0.4
10
ß
0.3
8
6
0.2
4
0.1
2
0
0
500
1000
1500
2000
2500
N.(cm'a)
0
0.2
0.4
0.6
•a
Plate 14. Comparisonof the compositeschemewith INDOEX in-cloudaircraftobservations.
(top left) Plot
of R e againstNa. In this and the other panelsthe horizontalbarsare not error bars.Data havebeenbinned
withintheNa rangeshownby the horizontalbar, andaveragedoverseveralthousandpoints.(top right)Cloud
drop numberdensityN c versusthe aerosolnumberdensityN,. (bottomleft) Aerosolvisibleopticaldepth%
versusN,. (bottomright) Cloudvisibleopticaldepth rc versus%.
in conjunctionwith the regionaldistributionof % (shownin
Plate5) to estimatethe cloudopticaldepthfor low cloudsasa
decrease
the negative
directforcingby 5 W m-2 (fromthe
clear-sky
forcing
of -7 to -2 W m-2 in cloudy
skies),
whileon
function
oflongitud,e
andlatitude.
These
values
areemployed
in the other hand they enhancethe negativeforcing by -5 W
MACR to estimatethe forcingdue to the first indirecteffect.
The first step is to estimate the N, and % for the NIO
without any anthropogenicinfluence,i.e., the backgroundnatural aerosol over the tropical Indian ocean, subject to the
continentaloutflow.If we adoptthe resultgivenin section4 for
the anthropogeniccontributionof 80% to the columnaerosol
opticaldepth,we obtainthe background% to be 0.06 (20% of
the 0.3 meanvaluefor the NIO, shownin Plate 13). From Plate
m-2 throughthefirstindirecteffect.Thisimportant
newresult
is largelybecausethe absorbingaerosollayer is locatedwithin
and abovethe topsof low clouds.
The latitudinalgradientof the sumof the directand the first
indirectforcingis shownin Plate 15. The aerosolsintroducea
large gradientin the solarheating,as shownin Plate 15. They
exerta largepositive25 W m-2 north-south
gradientin the
solarheatingof the atmospherebetween25øNand 25øS,and a
14cthisleadsto a background
N, of about300cm-3. Thisis correspondinglylarge negative gradient in the ocean solar
slightly
smallerthanthemeasured
Na of about350cm-3 south heating.
We will concludethe discussion
on forcingwith a few comof theITCZ (Plate7). We adopt% of 0.07andN• of 350cm-3
for the backgroundunpollutedNIO value. Insertingthesein mentson its year-to-yearvariability.The forcingnumbersdisMACR with the regional distribution of %, rc, and cloud cussedthus far are valid for the year 1999 and that too for
significant
interfraction(includingthe diurnalvariationof clouds)on a 1ø x 1ø January,February,and March. Plate4 suggests
(latitude,longitude)grid, we obtain for the regionalaverage annualvariabilityin the opticaldepths.We shouldcautionagainst
of thisplate,for the cruiseeachyear was
forcingdue to the first indirecteffect:TOA equalto -5 + 2 W an overinterpretation
m-2; atmosphere
equalto 1 ___
0.5W m-2; andsurface
equal not identicalnor were the samplingdates.Tahnk [2001] uses
to -6 _+3 W m-2. The largeerrorbarsare duemostlyto the 5-yearAVHRR satellitedatato obtainthe followingaverage%
uncertainties in the satellite-retrieved
cloud fraction. We first
(for January-February-March
averages)for the northernIndian
note that the TOA negativeforcingdue to the indirecteffectis Oceanfrom 0øNto 30øN:0.25 (1996);0.29 (1997);0.26 (1998);
the samemagnitudebut 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 shownhereisrepresentative
of the averageforcing.However,the
RAMANATHAN
•
,TOA
ET AL.: INDIAN
Atmosphere
OCEAN EXPERIMENT--AN
Direct
A
25
E
15
•
+ Indirect
9.
5
l::n -5
o
o
u.
-35
-30'
,20..... "•'0 '
0
10
20
30
Latitude
Plate
Second Indirect
For the second indirect
-15
-45
15.
Latitudinal
variation
of the aerosol radiative
forc-
ing. Includesboth natural and anthropogenicaerosols.The
valueshave been averagedover oceanwith respectto longitude from 40øE to 100øE.
ANALYSIS
28,389
variabilitydependson the spatialscale.For example,at KCO the
1999valueswere higherthan the 1998and the 2000 %,valuesby
about40-50%. Thus the regionalvaluesshownhere (e.g.,Plate
15) may havesignificantinterannualvariability.
............
Surface
45
35
INTEGRATED
and Semidirect
Effects
effect and the semidirect
effect we
rely on a three-dimensionaleddy resolving trade cumulus
model describedby Ackermanet al. [2000]. Two independent
model studies[Ackermanet al., 2000;Kiehl et al., 1999] of the
INDOEX soot heating showthat it leadsto a reductionin the
trade cumuli prevalentover the tropical oceans.Ackermanet
al. [2000] insert the INDOEX aerosolwith soot and estimate
the forcingfor variouscloud drop concentrations(their Plate
5). We adoptheretheir simulations
for N,• (clouddropnumber
density)of 90 cm-3 (pristine)and315 cm-3 (polluted),consistentwith the compositeindirect effect schemeshown in
TOA
8
6
4
2
First
Indirect
Direct Solar
Second
Indirect
0
-2
Direct
Semi direct
IR
-4
-6
-8
-10
Surf
ce
Direct Sola
Direct
IR
First
Second
Indirect
Indirect
Semi direct
-5
-10
-15
-0
Plate 16. Regionalaverageanthropogenic
aerosolclimateforcingfor NIO (0øto 20øN;40ø to 100øE).The
top panelshowstheforcingat TOA, andthe bottompanelshowsit at the surface.The differencebetweenthe
TOA and the surfaceforcingyieldsthe atmospheric
forcing.The uncertaintyfor eachof the forcingvaluesis
the sameas in Plate 13. The mean for the individualforcingtermsshouldlie within the grayshadedregion.
28,390
RAMANATHAN ET AL.: INDIAN OCEAN EXPERIMENT--AN INTEGRATED ANALYSIS
Plate 13. The inclusion
of sootheating(semidirect
effect) duringthelast50years.Thesurface
forcingcanalsoimpact
reducesthe low cloudfractionby about0.04(day-night
aver- the hydrological
cycle.Roughly80% of the net radiativeheatage).The TOA forcingdueto thisincreaseisabout+4 W m-2 ingof thetropical
oceans
isbalanced
byevaporation
[Oberhu(day-nightaverage).Their modelalsoshowsthat additionof bet,1988].If 80%of the -20 W m-2 isbalanced
byreduced
clouddrops(due to secondindirecteffect)enhances
cloud evaporation,it would constitutea 15% reduction in the winlifetime,whichin turn increases
the low cloudfractionby tertimeevaporationof about100W m-2 or a 5% reductionin
about2% anda forcing
of -2 W m-2. Givenourpoorknowl- theannual
meanevaporation.
Ultimately,
thedecreased
evapedgeof howthevariousuncertainties
accumulate,
we prefer orationmustleadto decreased
tropicalrainfall,thusperturbnotto attributeanoveralluncertainty
rangeto thetotalforcing ingthewaterbudget.
In addition,
reduction
inrainfallisequiv(shownin the lastcolumnof Plate16).
alent to a reduction in the latent heat releasedto the middle
anduppertroposphere
withimplications
forthelapserateand
10. AnthropogenicAerosolForcing
the Hadley and Walker circulations.
We describe
nextanotherpotentiallyneweffectof aerosols
As described
earlier,theunpollutedra istakento be 0.07.In on climate.The atmospheric
forcingof about15 W m-2 is
additionto the direct and the first indirecteffect,we also
equivalent
to
a
heating
rate
of
about
0.4K/dayforthe0 to 3 km
include the second indirect effect and the semidirect effect.
layerandis about50%of theclimatological
solarheating
of
Lastly,we alsoincludetheIR forcing.Theindividual
forcing this region.Furthermore,the heatingrate increases
from
valuesare shownin Plate16. Furthermore,
giventhe large
uncertaintyandthe opposing
signsof thevariouseffects,we do aboutlessthan0.1to 0.8K/dayoverSouthAsia(Plate18),
forming
adiabaticheating
patternasymmetric
withrespect
to
notgivea meanvalue,butinsteadshowa grayshaded
region
the
equator
and
concentrated
north
of
ITCZ.
For
comparison,
thatmustcontain
themeanvalue.Theclear-sky
TOA forcing
themidtropospheric
heatinganomaly
duringE1Nifioevents
is
is -5 W m-2, buttheenhanced
aerosol
solarabsorption
in
aroundthe equatorwith its localmaximum
cloudyregionscancelsout as muchas 3 to 5 W m-2. Thusthe nearlysymmetric
TOA directeffect(+0.5 to -2.5 W m-2) is a smalldifference reachingup to 2.0 K/daysouthof the ITCZ [Nigamet al.,
perturbation
between
twolargenumbers.
Thesurface
forcing,
however,
isas 2000].Whatistheimpactof theuniqueheating
shown
in
Plate
18
on
the
atmospheric
general
circulation?
largeas -14 to -18 W m-2. The othertermsshownin Plate
assessment
of suchan impactwith
16havebeendescribed
earlier.In viewof thelargeuncertainty We madea preliminary
ClimateModel(CCM3)[Kiehlet
in the satellite-retrieved
lowcloudcoverandthe competingVersion3 of theCommunity
natureof the variousaerosolforcingterms,the total aerosol al., 1998].Eighty-fouryearsof the CCM3 "controlrun"were
to 30 yearsof the model"experiments,"
eachwith
forcingis not shownin Plate16.The regionaldistribution
of contrasted
the
same
prescribed
climatological
seasonal
cycle
of
seasurthe directplusthe firstindirectforcingis shownin Plate 17.
In theexperiment
we appliedtheradiative
The solarheatingof the easternArabianSeaand Bay of facetemperatures.
increase
in thelevelsbelow700mbar,whichroughly
Bengal
isreduced
byasmuchas20to 30W m-2 (about15%). heating
correspond
to
the
first 3 km above the surface.The surface
The TOA forcingpeaksin the cloudyandpollutedBayof
solarfluxisreduced
withtheratioR, F (S)/F(A), to be - 1.5.
Bengal region,with valuesbetween -6 and -15 W m-2.
Thisratioapproximates
theclear-sky
directforcingat thesur11. Discussion: Climate
Environmental
Effects
and
face(seePlate13).The imposed
heatingvariedtemporally
starting
fromzeroin October;
increased
linearlyto thepeak
value in March; and was restrictedto NIO and the southAsian
region with a smooth transition to zero outside this domain
It is generally
believed
[IPCC,1995]thattheTOA forcing
numerical
"shocks."
Plate19depicts
the
determines
globalaveragesurfacetemperature
change.
This (Plate19a)toprevent
paradigm
isappropriate
forgreenhouse
gases
andforprimarily difference between the mean of control run and the mean of
bothaveraged
for the monthsof January,
Februscattering
aerosols
suchassulfates.
Fortheabsorbing
aerosols, experiment
ary,
March,
and
April.
Plates
19b
and
19c
show
the
computed
however,sincethe surfaceand atmospheric
forcingcanbe
intemperatures
andprecipitation
asanexample
ofthe
factorsof 3 to 10 largerand of oppositesigns,we needto changes
of the calculated
changes.
carefully
examine
theeffects
(regionally
firstandthenglobally) natureandmagnitude
Thelow-level
airtemperature
T increases
byabout0.5to 1.5
withthree-dimensional
climatemodels.Aswediscuss
next,the
heatingwasimposed
(Plate19b).The
mostimportant
effectof theseaerosols
maybeonthehydro- K wherethe aerosol
warmingalsoextendedbeyondthe domainof the aerosolheatlogicalcycleandthe atmospheric
circulation.
The seasonal
andNIO averaged
surfaceforcingof -20 W ing, in a southwestto northeastdirection from the southern
m-2 islargecompared
withthedownward
heatfluxes
intothe Indian Oceanto eastAsia. However,the surfaceover South
ocean.It is about15% of the naturallyoccurring
wintertime Asiacooled(notshown),
butthemagnitude
wasstrongly
desolarheatingof NIO [Oberhuber,
1988]andaboutthe same pendenton the ratioR. For a ratioof 1.5,asin thisplate
magnitudeas the net downwardheat flux into the oceanof 30 (appropriate
for clearskies),thesurface
cooledbyasmuchas
to50W m-2.Evenifwemaketheextreme
assumption
thatthe 1K,whilefora ratioofabout1.0(similar
tocloudy-sky
values),
aerosolforcingis zero duringthe other 9 months,the annual thecooling
wasa factorof2 to3 smaller.
Thelow-level
heating
mean forcingof the aerosolsis still about 4% of the total solar enhances
therisingmotionlocally.
Thedynamical
response
to
heatingof the oceanand about 20% of the net heat flux into
this low-levelheating is to enhancemoist convectionand
theNIO. In thiscontext,
it isintriguing
thata recentstudyof strengthen
the rainfallalongthe ITCZ (seethe red shaded
worldoceansurfacetemperatures
[Levitus
et al., 2000]has IndianOceanregionin Plate 19c)by asmuchas 15 to 30%.
shownthattheNIO isoneof thefewregions
withouta decadal Sincethe risingair has to sinkelsewhere,the subsidence
inwarmingtrendin the recentpast,whilethe SIO, the Atlantic, creases
awayfrom the regionof enhanced
precipitation
and
andthe Pacificweresubjectto a significant
warmingtrend leadsto decreased
rainfallnorth(in the ArabianSea)and
RAMANATHAN
ET AL.: INDIAN
OCEAN EXPERIMENT--AN
INTEGRATED
ANALYSIS
28,391
AerosolForcingin 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
40
......
50
0.0
. .....
60
70
80
90
100
Plate 17. Regionaldistributionof the aerosolclimateforcingfor January-March1999 at TOA (top panel)
and at the surface(bottom panel).
south(overthe westernIndian Ocean)of the regionof positive since absorbingaerosolsare found in other regions of the
world as well [seePenneret al., 1992;Kaufman et al., 1998].
The enhancedITCZ precipitationover the Indian Ocean
In addition to perturbingthe averagedforcingof the NIO,
alsoperturbsthe east-westWalker circulation,which resultsin the aerosolsperturb the interhemisphericheating gradientsin
alternating patterns of suppressedrainfall over the western the oceansin majorways.The climatologicalwintertimeIndian
Pacific(seethe negativerainfall regimeover the tropicalwest- Oceansolarheatingvariesfrom about175W m-2 northof
[Oberhuber,
1988].The
ern Pacific)and enhancedrainfall over the easternPacific(not 15øNto about225W m-2 southof 15øS
shown).This remote teleconnectionbetweenthe west Pacific, 50 W m-2 latitudinalheatinggradientshouldbe compared
the east Pacific, and the Indian Ocean rainfall is well known with that of the surfaceaerosolforcingwhichvariesfrom 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, surfaceevaporationover the NIO decreasedby 10 to 20 drivingforcesfor the thermohalinecirculation,is enhancedby
rainfall.
W m-2, balancingmore than 50% of the negativeaerosol
about 50%.
We shouldalsoconsiderthe ecosystemimpactssince70% of
surfaceforcing(Plate 16b). Theseresultshave significantimplicationsfor globalclimatechangeandvariability,particularly the surfaceforcing is concentratedin the photosynthetically
28,392
RAMANATHAN
ET AL.' INDIAN
OCEAN EXPERIMENT--AN
INTEGRATED
ANALYSIS
AnthropogenicAerosolHeating Rate (K/day)
January-March, 1999
25
20
15
0.5
I0
ß
5
ß
-
ß
' ....
0
ß
....-
0.4
.
•.
0.3
,•'
0.2
0.1
0.0
-10
40
50
60
70
80
90
100
Plate 18. The 3-month (and diurnal) mean heatingrate averagedbetweenthe surfaceand 3 km, due to
anthropogenicaerosols.The ocean valueswere obtained from the integratedobservations,while the land
valueswere obtainedfrom the 4-D assimilationmodel of Collinset al. [2001].
activeUV and visiblepart of the solarspectrum[Meywerkand
Ramanathan,1999;Conant,2000]. Sincethe percentagereduction in the UV and the visibleregionis aslargeas 10% to 20%,
effectson terrestrialand marinebiologicalproductivityneedto
be examined.For example,Chameideset al. [1999] indicates
that the haze-inducedreductionof sunlightby about 10% can
significantlyreducewheat and rice productivityin China. Increasedaerosolconcentrationcan also suppressprecipitation
from low clouds[Albrecht,1989].This effect togetherwith the
soot effect on burninglow clouds[Ackornanet al., 2000] can
decreasesignificantlythe wet removalof aerosolsand potentially acceleratethe increasein the thicknessof the haze in the
future.
The presentfindingscan alsohaveglobalimplications.The
large surfaceforcing and the atmosphericheatingrequire absorbingaerosolswith to < 0.95 and % > 0.2. Large surface
forcing valuessimilar to those shownhere were found in the
northern
Atlantic
off
the
East
Coast
of the
United
within a week to as far awayas mid-Pacificto North America.
Thus the absorbingaerosolsmay extendfar beyondthe source
regions.
The aerosoleffectson climate can be large and complex
involving many components of the global system. The
INDOEX campaignhassuccessfully
demonstratedthat an integratedobservingsystemthat combinesin situ determination
of the physico-chemical
propertiesat the particlesscaleup to
the synopticscalewith satellite measurementsof the entire
ocean basin and models of the chemical-physical-dynamical
systemcan answerfundamentalquestionssuchas,What is the
impact of developingnationsin South Asia on regional and
global radiation balance and climate? Advancesin observing
systems,informationtechnology,modeling,and collaboration
acrossdisciplinaryand national boundarieswere the recipes
that contributed
to this success.
States
[Hignettet al., 1999]. Second,absorbingaerosolswith toof 0.9
are typical of aerosolsresultingfrom biomassburning [Kaufman et al., 1998].As describedbyAndreaeand Crutzen[1997],
biomassburning was practiced in almost all the populated
regionsand is still practicedin many parts of the tropicsand
subtropics.Third, ice core data have revealedthat soot concentrationshave tripled betweenthe 1850sto 1970sin western
Europe [Lavanchyet al., 1999],thus suggesting
that absorbing
aerosolssuchas thosefound in INDOEX were more prevalent
in Europe and possiblyNorth America at leastuntil the 1970s.
Lastly,electronmicroscopicimagesand particle analysis(Figure 1) show that the soot lodged in submicronsulfate and
organicaerosolsis transportedas far southas 6øS,providing
evidence that these particles survive in the atmospherefor
more than a week, the transporttime from South Asia to the
ITCZ. Furthermore, peak concentrationsof soot can extend
up to 3 km (Plate 9; alsoseethe sootlayer extendingto cloud
tops in Plate l). Forward trajectories(see a few samplesof
7-dayback trajectoriesin Plate 1) indicatethat the air masses
between 500 mbar (shown in Plate 1) and 700 mbar in the
tropical Indian Ocean [Krishnamurtiet al., 1998] can travel
Appendix A: The Composite First Indirect
Effect SchemeAdopted in MACR
The aerosolindirect effect is difficult to quantify as it dependson the interactionsbetween aerosols,CCN, and cloud
propertieswhich are poorly understood[Joneset al., 1994;
Heintzenberg
et al., 1997].Many attemptshavebeenmadesince
the 1990sto tacklethisproblem[Joneset al., 1994;Meehlet al.,
1996;Jonesand Slingo,1996;Koganet al., 1996; Chuanget al.,
1997;Le Treut et al., 1998; Hah et al., 1998;Brenguieret al.,
2000]. For our compositeschemewe adopt a more detailed
approachto estimatethe anthropogenicindirect aerosolforcing from combinedsurface,aircraft, and satelliteobservations.
Two fundamentalrelationshipscharacterizethe indirecteffect: the dependenceof Nc on Na; and the dependenceof
effectiveradii R e on Na. Theseparametersin turn dependon
the vertical velocity W, the cloud liquid water contentL, and
the chemicalcompositionof aerosolconcentration(which influencesthe CCN activationspectra).The verticalvelocityand
the CCN supersaturationspectrumregulate the supersaturation S, which determines the number of activated CCN; in
addition, W plays some role in determiningL. Entrainment
RAMANATHAN
ET AL.: INDIAN
OCEAN EXPERIMENTmAN
INTEGRATED
ANALYSIS
Simulationof RegionalClimateChange(NCAR/CCliI3)
January-April ($0 years o• exp.)
a) Solar heating rate at 8,5,0,rob
.........
--,--'-X
08
unit=K/day
....
.
0.2
10N
'
-0.2
-0.4
EO
105
b) Temperature at 850mb
:SON
"<---"::
.....
........
/
-
unit=K
'•
1.2
,o.
0.6
,;
10N
d-"•-"":-'•-.N•-•.•
-•'•--'2• k"•ff,;-•-••
- •.6•.
0.5
-[•'•
-0.5
-0.6
1.2
4•
60E
80E
1•E
120E
-1
-4
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 climatologicalseasonalcycleof
observedSSTs.The January-Aprilaverageis taken here of the differencebetween30 yearsof experimentand
84 yearsof controlrun. In the experiment,INDOEX aerosolclimate forcingof "best-guessed"
ideal pattern
wasappliedto the periodbetweenOctoberand May. This additionalforcingconsistsof low-levelatmospheric
heating and surfacesolar-fluxreduction,in which the ratio R, F(S)/F(A), is about -1.5. In another
experiment(not shown)where R - -1.0, air temperaturechangeat 850 mbar is somewhatless,but the
precipitationchangeremainslargelyunchanged.
28,393
28,394
RAMANATHAN
ET AL.: INDIAN
OCEAN EXPERIMENT--AN
and cloud thickness,two macroscopicparameters of cloud
dynamics,also governL. As shownby Heyrnsfieldand McFarquhar[thisissue]and McFarquharand Heyrnsfield[thisissue] (hereafterreferred to as HM and MH, respectively),the
mean and median IV, L, and cloudwidth Ax, while undergoing significantcloud-to-cloudvariability,was about the same
betweenthe northern and southerntropicalIndian Ocean and
betweenpollutedand pristineconditions.HM and MH usethe
long transequatorialgradientflightsto bin the aircraftdata as
INTEGRATED
ANALYSIS
powerlaw assumedin moststudies[e.g.,Twomey,1977;Snider
and Brenguier,2000; HM] is
CCN = CSk
k = 0.76.
(A1)
The parameterS is thewatervaporsupersaturation
in percent,
and the parameterC, by definition,is the CCN for S - 1%
whichincreases
with the aerosolconcentrationasNa",with n 1.16 yieldingthe bestfit. The valueof k = 0.76 is in excellent
agreementwith the averagevalueof 0.8 reportedfor ACE 2 by
follows:pristine,N a --<500 (cm-3);polluted,N a -->1500 SniderandBrenguier[2000].Upon combiningthisdependence
cm-3;andtransition
region,500cm-3 < Na • 1500 cm-3. of C (CCN at S = 1%) on N a with (A1), we obtaina power
For all thesethree regimesthe medianvalueswere W • 0.4
lawrelationship:
CCN - bNa"S
k, whereb is a constant.
Howm s-•; L • 0.15 g m-3; andAx • 300 m. Thisremarkable ever, when we fit this equationwith the KCO CCN data, we
similarityenabled MH and HM to discernthe indirect effect
from the data and parameterizeit.
Numerous attempts [Taylorand McHaffie, 1994;Martin et
al., 1994; Snider and Berenguier,2000] have been made to
find that n -- 1.25 givesthe best fit for S < 0.5 % and n =
1.16 for S > 0.5 %. This empiricaldependence
of n on S may
be simplydue to the measurement
uncertaintyin CCN (about
40%), Na (about20%), and S (not quantifiedyet) or poorly
characterize the indirect effect from aircraft data collected
understoodmicrophysicalprocesses.Since the effective inover the Atlantic (mostly)and the easternPacific.Our scheme
cloudS 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] studywhichincludesa comprehensiveset 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
that Nc andN a are correlatedin a statisticallysignificantmanner. Secondis the Sniderand Berenguier[2000]ACE 2 study,
which concludesthat Twomey's[1977] theoreticalexpressions
relating droplet concentrationsbasedon updraft velocityand
the CCN activationspectrumare consistentwith direct observations.Next is the findingof severalstudies[e.g.,Martin et al.,
1994;TaylorandMcHaffie, 1994;MH] that the effectivecloud
drop radius is related to L and Na in a predictablemanner.
We follow the approachproposedin these studies,which is
to relate Re in terms of L and Nc [e.g., seeMartin et al., 1994,
equation (11)] and to relate Nc in terms of Na, the supersaturation S, and the vertical velocity W. The composite
INDOEX schemedescribedhere adoptsthe MH parameterization for Re and a combinationof CCN spectrameasuredat
KCO and aircraft data for Na, L, and AZ (cloudthickness).
The resultingschemeis tested againstthe binned data presentedby HM and then comparedwith data from other field
studies.
The schemeaccountsfor the followingmeasurements:
(1)
CCN spectrameasuredat KCO from 0.1 to 1%; (2) the dependenceof the supersaturationon aerosolconcentrationas
measuredfrom aircraft;(3) the microphysical
datareportedby
HM for pristine and heavily and moderatelypolluted conditions;and (4) the dependenceof effectiveradii on clouddrop
number concentrationas reported by MH.
Unless otherwise stated, all of the values and functional
relationshipsdescribedbelow are valid only in the following
range:
300 cm-3 --<Na--<2000 cm-3.
and300 -<Na -< 2000 cm-3.
(A2)
PlateA1 comparesthe CCN predictedby (A2) with Canttell
et al.'s [2000, this issue]observedCCN for S = 0.5% from
R/V RonBrown(for the entire cruiseshownin Plate 2a) along
with KCO data.Equation(A2) is shownfor bothn -- 1.16 and
1.25, and the two curvescapturethe observedCCN. We show
both values of n, sincen • 1.16 for S = 1% and is about 1.25
for S -< 0.3%, with a value somewhere in between for S =
0.5 %. Sincewe usein situdata,the equationaccountsfor the
possibilityof somefraction of the aerosolbeing hydrophobic
dependingon the fractionof anthropogenicand natural aerosols.Modelingof the cloudactivationprocessusingthe supersaturationspectrafrom KCO or (A2) indicatesthat cloud
droplet concentrationsshould be significantlyelevated over
"cleanregion"clouds,asindeedwasfound to be the casefrom
measurements
made aboard the NCAR
C-130.
The effectivesupersaturation
that explainsthe observed(averaged)Nc [Hudsonand Yum, 1997,1999]is 0.1-0.2% for the
pristine case (N a --< 500) and about 0.05% for the most
polluted case (N a > 1500). Entrainment,which plays an
importantrole in limitingNc, doesnot enter explicitlyin our
scheme,but is implicitly factored into the inferred value of
effective supersaturation.The inversecorrelationbetweenS
andN a ispredictedby Twomey's
[1977]theory(the equationis
givenlater in thissection)andhasalsobeenobservedby other
field studies(see summaryin the work of Finlayson-Pitts
and
Pitts [2000]). It arisesfrom the fact that as more drops are
nucleated,the vapor supplyis depleted,and as a result,S is
reduced.In order to accountfor this,we let S varywith N a as
follows:
At KCO, cloudcondensation
nuclei(CCN) at differentap- S = 0.18exp(-6.5 x 10-4ma) 300 --<ma-< 2000. (A3)
pliedwater vaporsupersaturations
(0.1 to 1%) were measured
along with simultaneousdeterminationsof the aerosol size
Thus, givenNa, we can estimatethe CCN from (A2) and
distributionsin the particlediameterrangefrom 10 to 500 nm (A3). Only a fractionf of the CCN becomeclouddrops.We
[Cantrellet al., 2000,thisissue].The cloudcondensation
nuclei adoptthe aircraftdatareportedbyMcFarquharandHeymsfield
measurements
were made with a cloud condensation nucleus
[thisissue]and by Hudsonand Yum [1997, 1999],whichindicounter,while the aerosolsize spectrumwas derivedwith a cate that f = 1 for pristineconditions(N a --<300) and for
scanningmobilityparticlesizeras describedbyJi et al. [1998]. largervaluesof N a the observedNc wasabout0.8 of CCN. The
The CCN spectra[Canttellet al., 2000,thisissue]whichfits the followingequationaccountsfor this behavior:
RAMANATHAN
ET AL.:
?
INDIAN
OCEAN
EXPERIMENT--AN
INTEGRATED
ANALYSIS
Composite
scheme,
thisstudy
28 395
• 25 k '/•
2000
F©Cantrell
etal.,
this
issue;
INDOEX
KCO 0.24
N,' S /•
- * Cantrell et al. (2000); INDOEX Sagar Kanya
L
1500 •
1000-
ß
ß
ß
:
500 •
•
i.•
O•
Ha1'16
•
•
0.24
$k
E•
&
,• ,
•
0
•
,
,
500
•
....
1000
S: Su•a•ration = 0.5%
k • 0.76
•
,
•
•500
,•....
2000
CN (cm
Plate AI. Variation of cloudcondensationnuclei(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 measurements
is about +20%.
Nc=fx
CCN
f=
0.8 + 0.2
ß exp {(Na- 300) x 0.004}
Na >_300.
(A4)
The next parameter we need is the effective radius Re,
defined as follows:
•Re --'
f R3n
(R)
dR
f R2n(R)
dR
.
(AS)
We usethe parameterizationdevelopedby MH, who in turn
followMartin et al. [1994]for obtainingR e from aircraftdata.
Martin et al. expressRe in termsof L and No'
3L}1/3
Re= 4rtKNc'
(A6)
where MH obtain the following K values from INDOEX
data:K = 0.83 for pristine clouds(Na -< 500); and K = 0.73
for heavily polluted clouds (N• >- 1500). The factor K accountsfor the deviation from monodisperse(all drops having the same radius) size distributions. It can easily be
shownfrom (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 radiative forcing, provided we know L, cloud base altitude Zt,,
and cloud thicknessAZ. Following HM, we adopt L = 0.15
shown in Plate 14. Typically, two types of aerosol vertical
distributionswere observed(Plates9a and 9b): one in which
the aerosol layer is concentrated in the boundary layer
(Plate 9a) and another in which an elevated layer is observed near 3 km (Plate 9b). The elevated aerosol vertical
structurewas also observedover India (from an INDOEX
lidar in Goa) containinghalf 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
at each altitude with ,• (obtained from vertically integrating
the profile) and then multiply it with the ,a from Plate 5. We
includeboth the profilesshownin Plates9a and 9b, weighting our forcing estimateswith the frequency of occurrence,
2/3 for the elevated layer aerosol profile and 1/3 for the
boundary layer aerosol profile.
The cloud fractions
are retrieved
from
METEOSAT
data
(IR brightnesstemperature).In general,the atmosphericconditionsare classifiedinto eight as follows:clear sky;clear sky
with cirrus; broken low clouds;broken low clouds with cirrus;
low clouds;low cloudswith cirrus;anvils;and deep cumulus.
The eightcategoriesare subgrouped
into three:clearsky;total
low clouds;and high clouds.We usedthree typesof skyconditions,specifically,clear skies,low clouds(includingcirrus),
and convectivecloudsincludinganvils.
Acknowledgments. INDOEX was funded by numerous national
agenciesincludingthe NationalScienceFoundation(lead agencyfor
g m-2; Zt, = 600m;andAZ = 300m. TheN• valuesat cloud the U.S. program),the Departmentof Energy,NASA, NOAA, Indian
SpaceResearchOrganization,European Union for Meteorological
base are used for (A2) and (A4). Using the vertical distri- Satellites(EUMETSAT), Max Planck Institute, French Centre Nabution of aerosols shown in Plate 9, the relationships be- tional d'EtudesSpatiales(CNES), and CentreNational de la Rechertween the various parameters as obtained by MACR are cheScientifique(CNRS). Seedfundsfor INDOEX were providedby
28,396
the Vetlesen
RAMANATHAN
foundation
ET AL.: INDIAN
and the Alderson
foundation.
OCEAN EXPERIMENT--AN
We thank EU-
METSAT for movingMETEOSAT to the Indian Ocean;the Government of Maldivesfor hostingINDOEX; J. Fein for his keen interest
and supportfor INDOEX; E. Friemanand C. Kennelfor their enthusiasticsupport;the UniversityCorporationfor AtmosphericResearch
for conductingthe field operations;andthe crewof the aircraftandthe
ships.We thank M. Lawrence,J. Williams, and the two JGR referees
for their comments
on an earlier
draft.
INTEGRATED
ANALYSIS
Collins,W. D., P. J. Rasch,B. E. Eaton, B. Khattatov,J.-F. Lamarque,
and C. S. Zender, Simulatingaerosolsusinga chemicaltransport
modelwith assimilationof satelliteaerosolretrievals:Methodology
for INDOEX, J. Geophys.Res., 106, 7313-7336, 2001.
Conant, W. C., An observationalapproachfor determiningaerosol
surface radiative forcing: Results from the first field phase of
INDOEX, J. Geophys.Res., 105, 15,347-15,360, 2000.
Cooke, W. F., C. Liousse, H. Cachier, and J. Feichter, Construction of
a 1ø x 1ø degree fossil fuel emissiondata set for carbonaceous
aerosoland implementationand radiativeimpactin the ECHAM4
model,J. Geophys.Res., 104, 22,137-22,162, 1999.
Finlayson-Pitts,B. J., and J. N. Pitts Jr., Chemistryof the Upperand
Ackerman, A. S., O. B. Toon, D. E. Stevens,A. J. Heymsfield, V.
LowerAtmosphere:Theory,Experiments,and ,4pplications,969 pp.,
Ramanathan,and E. J. Welton, Reduction of tropical cloudinessby
Academic,San Diego, Calif., 2000.
soot, Science,288, 1042-1047, 2000.
Grassl,H., Albedo reductionand radiativeheatingof cloudsby abAlbrecht, B. A., Aerosols,cloud microphysics,
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28,398
RAMANATHAN
ET AL.: INDIAN
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INTEGRATED
ANALYSIS
A. Jayaraman,
PhysicalResearchLaboratory,Ahmedabad380-009,
D. Althausenand J. Heintzenberg,Institutefor TroposphericRe- India.
search,Leipzig D-04318, Germany.
T. N. Krishnamurti,
Departmentof Meteorology,FloridaStateUniJ. Anderson,Departmentof Geologyand Chemistry,Arizona State
versity,Tallahassee,FL 32306.
University,Tempe, AR 85287.
A. P. Mitra, National PhysicalLaboratory,New Delhi 110 012,
M. O. AndreaeandJ. Lelieveld,Max PlanckInstitutefor Chemistry, India.
Mainz 55020, Germany.
T. Novakov,LawrenceBerkeleyNationalLaboratory,Berkeley,CA
W. Cantrell, Departmentof Chemistry,Indiana University,Bloom- 94720.
ington, IN 47401.
J. A. Ogrenand P. Sheridan,ClimateMonitoringand Diagnostics
G. R. Cass,Schoolof Civil andEnvironmentalEngineering,
Georgia
Laboratory,NOAA, Boulder, CO 80303.
Institute of Technology,Atlanta, GA 30332.
K. Prather,Departmentof Chemistry,Universityof California,RivC. E. Chung,W. C. Conant,P. J. Crutzen,D. Lubin,I. A. Podgorny,
erside, CA 92521.
V. Ramanathan(corresponding
author), S. Rupert, S. K. Satheesh,
K. Priestley,NASA LangleyResearchCenter,Hampton,VA 23665.
and F. P. J. Valero, Center for AtmosphericSciences,ScrippsInstituJ. M. Prospero,RosenstielSchoolof Marine andAtmosphericScition of Oceanography,
Universityof California,SanDiego, CA 92037.
ence,Universityof Miami, Miami, FL 33149.
(ram@fiji.ucsd.edu)
A.D. Clarke and S. Howell, Departmentof Oceanography,
Univer-
sity of Hawaii, Honolulu, HI 96822.
J. A. Coakley,Collegeof Oceanicand AtmosphericSciences,Oregon StateUniversity,Corvallis,OR 97331.
W. D. Collins,A. J. Heymsfield,J. T. Kiehl, G. McFarquhar,and P.
Rasch, National Center for Atmospheric Research, Boulder, CO
80307.
F. Dulac, Laboratoire des Sciencesdu Climat et de l'Environment,
Gif sur Yvette 91191, France.
B. Holben, NASA Goddard SpaceFlight Center, Greenbelt, MD
20771.
J. Hudson, Desert Research Institute, Reno, NV 89506.
P. K. Quinn, PacificMarine EnvironmentalLaboratory,NOAA,
Seattle, WA 98115.
K. Rajeev,SpacePhysicsLaboratory,VSSC, Thiruvananthapuram
695022, Kerala State, India.
R. Sadourny,Laboratoirede MeteorologieDynamique,Paris75005,
France.
G. E. Shaw,Geophysical
Institute,Universityof AlaskaFairbanks,
Fairbanks, AK 99775.
(ReceivedOctober3, 2000;revisedFebruary27, 2001;
acceptedFebruary27, 2001.)