Simulation of aerosol distributions and radiative forcing

for INDOEX: Regional climate impacts

William D. Collins, Phillip J. Rasch, Brian E. Eaton, David W. Fillmore,

and Jeffrey T. KiehlNational Center for Atmospheric Research, Boulder, Colorado, USA

C. Trevor BeckClimate Prediction Center, National Oceanographic and Atmospheric Administration, Camp Springs, Maryland, USA

Charles S. ZenderEarth System Science, University of California at Irvine, Irvine, California, USA

Received 13 October 2001; revised 16 July 2001; accepted 17 July 2001; published 18 September 2002.

[1] The direct radiative forcing by aerosols over the Indian Ocean region is simulated forthe Indian Ocean Experiment (INDOEX) Intensive Field Phase during Spring 1999. Theforcing is calculated for the top-of-atmosphere (TOA), surface, and atmosphere bydifferencing shortwave fluxes computed with and without aerosols. The calculationincludes the effects of sea-salt, sulfate, carbonaceous, and soil-dust aerosols. The aerosoldistributions are obtained from a global aerosol simulation including assimilation ofsatellite retrievals of aerosol optical thickness (AOT). The time-dependent, three-dimensional aerosol distributions are derived with a chemical transport model driven withmeteorological analyses for this period. The surface albedos are obtained from a land-surface model forced with an identical meteorological analysis and satellite-derived rainfalland insolation. These calculations are consistent with in situ observations of the surfaceinsolation over the central Indian Ocean and with satellite measurements of the reflectedshortwave radiation. The calculations show that the surface insolation under clear skies isreduced by as much as 40 W/m2 over the Indian subcontinent by natural and anthropogenicaerosols. This reduction in insolation is accompanied by an increase in shortwave fluxabsorbed in the atmosphere by 25 W/m2. The inclusion of clouds in the calculationschanges the direct effect by less than 2 W/m2 over the Indian subcontinent, although thereduction is much larger over China. The magnitude of the difference between all-sky andclear-sky forcing is quite sensitive to the three-dimensional spatial relationship between theaerosol and cloud fields, and other estimates of the difference for the INDOEX IntensiveField Phase are as large as 5 W/m2. INDEX TERMS: 0305 Atmospheric Composition and Structure:

Aerosols and particles (0345, 4801); 0345 Atmospheric Composition and Structure: Pollution—urban and

regional (0305); 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and

chemistry; 3337 Meteorology and Atmospheric Dynamics: Numerical modeling and data assimilation

Citation: Collins, W. D., P. J. Rasch, B. E. Eaton, D. W. Fillmore, J. T. Kiehl, C. T. Beck, and C. S. Zender, Simulation of aerosol

distributions and radiative forcing for INDOEX: Regional climate impacts, J. Geophys. Res., 107(D19), 8028,

doi:10.1029/2000JD000032, 2002.

1. Introduction

[2] The Indian Ocean region includes some of the mostdensely populated countries on Earth. The pollution fromfossil-fuel and biomass burning in these countries is alreadyhaving a significant influence on the regional atmosphericchemistry. The anthropogenic aerosols released from thisregion are projected to become the dominant componentof anthropogenic aerosols worldwide in the next 25

years [Nakicenovic and Swart, 2000, available at http://www.grida.no/climate/ipcc/emission/index.htm]. The IndianOcean Experiment (INDOEX) is designed to measure thecharacteristics, distribution, and radiative effects of theseaerosols. Measurements have been collected during thenorth-east winter monsoon each year from 1996–1999,with the main deployment of instrumentation during theIntensive Field Phase (IFP) in 1999. Several years ofchemical, meteorological, and radiative data are availablefrom the Kaashidhoo Climate Observatory (KCO, at4.965�N, 73.466�E). In this paper, we use a modelwhich is consistent with the field observations to derive

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. D19, 8028, doi:10.1029/2000JD000032, 2002

Copyright 2002 by the American Geophysical Union.0148-0227/02/2000JD000032$09.00

INX2 27 - 1

the direct aerosol radiative forcing over the INDOEXregion.[3] The radiative forcing by aerosols over the Indian

Ocean and Arabian Sea has been examined in severalprevious aerosol studies. Although there are numerousglobal forcing estimates for sulfate, carbonaceous, or soil-dust aerosols [e.g., Penner et al., 1992; Kiehl and Briegleb,1993; Haywood and Ramaswamy, 1998], we summarize thestudies that have examined forcing specifically in the IndianOcean region. The first detailed calculations of the aerosoleffects use data collected during the First GARP GlobalExperiment (FGGE) and summer Monsoon Experiment(MONEX) [Ackerman and Cox, 1982; Ellingson and Ser-afino, 1984; Ackerman and Cox, 1987]. These studies focusexclusively on the forcing by soil-dust aerosols from theSaudi Arabian peninsula and Sahara Desert. The resultsshow that the dust aerosols enhance the shortwave heatingrates by up to 50% in the lower troposphere over some areasof the Arabian Sea [Ellingson and Serafino, 1984; Acker-man and Cox, 1987]. The climatic effects of aerosolsreleased by the Kuwait oil fires during the 1991 Gulf Warhave also been extensively modeled [Bakan et al., 1991;Ferretti and Visconti, 1993; Kao et al., 1994]. These aresome of the first detailed calculations of the regional impactof anthropogenic aerosols, although of course the source ofthe aerosols was a transient phenomenon.[4] The effects of anthropogenic aerosols on the climate

of the Indian Ocean region has been modeled for individualaerosol species. Various studies have examined the impactsof sulfate aerosols on the Indian summer monsoon [Lal et al.,1995; Boucher et al., 1998], continental diurnal temperaturecycle [Lal et al., 1996], and continental surface insolation[Venkataraman et al., 1999]. The surface radiative forcingby carbonaceous aerosols has been estimated by Reddy andVenkataraman [1999]. In contrast to these analyses, thepresent study includes the radiative forcing by all majornatural and anthropogenic aerosol species. In addition, wefocus specifically on characterizing the forcing and evaluat-ing our estimates using observations from INDOEX.[5] The most recent set of observations and modeling

studies have been conducted as part of INDOEX [Ramana-than et al., 1996, available at http://www-indoex.ucsd. edu/publications/proposal]. The direct aerosol radiative forcingin the region has been rigorously characterized using sur-face observations [Jayaraman et al., 1998; Krishnamurtiet al., 1998; Meywerk and Ramanathan, 1999; Conant,2000; Jayaraman, 1999] combinations of surface andsatellite data [Satheesh and Ramanathan, 2000; Ramana-than et al., 2001a], and Monte Carlo modeling [Podgornyet al., 2000]. These studies have demonstrated that theradiative forcing of anthropogenic aerosols extends overmost of the Arabian Sea and Bay of Bengal and that theforcing is a significant term in the surface, atmospheric, andtop-of-atmosphere (TOA) energy budgets. Additionalexperimental evidence and modeling of the forcing isdiscussed elsewhere in the INDOEX special issues.[6] The present study should be viewed as an extension of

the pioneering analysis by Ackerman and Cox [1987]. To thebest of our knowledge, the work by Ackerman and Cox[1987] is the first calculation of the surface radiative energybudget for the Indian Ocean including realistic specificationof land-surface albedos and the distributions, physical prop-

erties, and optical properties of aerosols and clouds. Theenhancements in our calculation include time-dependentdistributions of sea-salt, sulfate, carbonaceous, and soil-dustaerosols from a chemical transport model (section 2.1) andtime-dependent surface albedos from a land-surface model(section 2.3). In contrast to Ackerman and Cox [1987], ourcalculations include the significant effects associated withanthropogenic aerosols. We also focus on the change in thesurface and TOA fluxes (the ‘‘forcing’’) associated with theintroduction of these aerosols, while Ackerman and Cox[1987] address the magnitude and regional gradients in thetotal all-sky fluxes. They also focus on the summer mon-soon, while we study winter conditions which allow for alarger anthropogenic effect. Most of the INDOEX observa-tions have been collected from instruments on ships, islands,and coastal regions of India. The bulk radiative properties ofthe aerosols and the variation of aerosol forcing with aerosoloptical depth from the model are compared to these in situdata. After evaluation of our radiative calculations againstINDOEX observations, we calculate the forcing over theinteriors of continental regions bounding the Indian Ocean.Our calculations therefore complement the INDOEX dataover oceans and can be combined with these data to yieldcontinuous maps of aerosol forcing over land and ocean[Ramanathan et al., 2001a].[7] Two estimates of the forcing by aerosols are dis-

cussed. The forcing is first computed for a cloud-freeatmosphere (section 3.1). The aerosol forcing is then eval-uated including the radiative effects of clouds (section 3.2).As previous studies [Liao and Seinfeld, 1998; Ramanathanet al., 2001a] have shown, the aerosol radiative forcingunder all-sky conditions can be quite different than theforcing under clear-sky conditions. These estimates of theforcing are compared with other calculations and observa-tions for the region in the conclusion (section 4). Specifi-cally, our estimate of the difference between the all-sky andclear-sky aerosol forcing is approximately 40% of the differ-ence calculated by Ramanathan et al. [2001a]. The reasonsfor the disparity are examined in the conclusion.

2. Description of the Model

[8] The direct aerosol radiative forcing has been calcu-lated over the Indian Ocean and adjacent continental regionsbetween 40–120�E and 30�S–30�N for January throughMarch 1999. The forcing is defined as the difference in thenet shortwave fluxes and shortwave heating rates computedwith and without the effects of aerosols. This definitiondiffers from that used by the Intergovernmental Panel onClimate Change (IPCC ) [1995] since our forcing estimatesinclude the effects of natural (nonanthropogenic) aerosols.The aerosol forcing has been calculated by applying aradiation model to the aerosol distributions simulated usinga global aerosol assimilation model. The spectral land-surface albedos are obtained from a global land-surfacemodel forced with a meteorological analysis and satellite-derived precipitation. Each of these components is describedin greater detail below.

2.1. Aerosol Assimilation Model

[9] The model used to generate the time-dependent dis-tributions of aerosols for input to the radiative transfer

INX2 27 - 2 COLLINS ET AL.: SIMULATION OF RADIATIVE FORCING FOR INDOEX

calculations consists of a chemical transport model coupledto an assimilation package. This system is described in thework by Collins et al. [2001], and its simulation of theaerosol distributions during the INDOEX Intensive FieldPhase is described in the work by Rasch et al. [2001]. Thefidelity of the simulated long-range transport has assessedusing aircraft data in the work by Clarke et al. [2001]. Thebasis of the system is the Model for Atmospheric Transportand CHemistry (MATCH) [Rasch et al., 1997], a chemicaltransport model forced with meteorological fields from anexternal source. The aerosol species included in the modelare sulfate, soil dust, black carbon (BC) and organic carbon(OC), and sea salt.[10] In the current configuration of the model, the vertical

profiles of sea salt are computed from the 10m wind speed[Blanchard and Woodcock, 1980]. The daily surface massconcentrations predicted by this diagnostic parameterizationagree with measurements in the Maldives to within 10–15%[Rasch et al., 2001]. Since the sea salt contributes a smallfraction of the direct radiative forcing (Section 3.1), the useof a more detailed sea-salt parameterization would not alterthe conclusions of this study. The other aerosol species arecomputed from prognostic equations including surface sour-ces, wet and dry deposition, and transport by resolved andsubgrid-scale motion. The aerosols are assumed to beexternally mixed. The dust is resolved in 4 size bins span-ning 0.01–1 mm, 1–10 mm, 10–20 mm, and 20–50 mm, andeach bin is represented by a lognormal distribution. Resultsfrom the model of Tegen and Lacis [1996], which includesmultiple bins for submicron dust aerosols, show that char-acteristic lifetimes vary by only 5% over the submicron sizerange. Thus transporting all submicron mass together in onebin causes an insignificant bias compared to, for example,the uncertainty in the dust size distribution at the source. Theother aerosols are represented by a single mass-mixing ratiofor each species. The model saves the instantaneous aerosolmass-mixing ratios every 6 hours at the same horizontal andvertical resolution as the input meteorological fields.[11] The formulation of the sulfur cycle is described in

the works of Barth et al. [2000] and Rasch et al. [2000].The emissions inventory for SO2 is from Smith et al. [2001].The sources for mineral dust are based upon the approach ofTegen and Fung [1994] with modifications to the soilmobilization to improve agreement with the Nimbus-7absorptive-aerosol index from Herman et al. [1997]. Theemissions of carbonaceous aerosols include contributionsfrom biomass burning [Liousse et al., 1997], fossil fuelburning [Penner et al., 1993], and a source of naturalorganic aerosols resulting from terpene emissions (J. Pen-ner, personal communication, 1999). During the INDOEXIntensive Field Phase, these data sets imply that the carbo-naceous emissions for biomass burning, fossil fuels, andnatural sources between 30N to 30S and 40E to 120E are4.1�107, 1.4�107, and 2.3�106 kg/day, respectively. Therethe natural carbon particle emissions are approximately 4%of the total mass flux of carbonaceous aerosols.[12] In this simulation we have adjusted the partitioning

of the carbon emissions so that the ratio of BC to OC bymass is 1:2. The BC:OC ratio is very difficult to measureand is subject to large uncertainties [Huebert and Charlson,2000]. The BC:OC ratio of 1:2 is intermediate between theratios observed over the Indian subcontinent and the north-

ern Indian Ocean by Novakov et al. [2000] duringINDOEX. It is comparable to the ratios measured by Quinnet al. [2002] from the R/V Ron Brown and by Chowdhuryet al. [2001] at KCO during the IFP. Our ratio is larger thanthe global-mean ratio of 1:7 to 1:10 used in other modelingstudies [e.g., Liousse et al., 1996] but it is in good agree-ment with the global-mean ratio of 2:5 obtained by Cookeet al. [1999]. The adoption of a single value is an over-simplification of both the geographical variation in theBC:OC ratio and the differences in the ratio for differentcombustion processes and fuels [Cooke et al., 1999]. Inorder to test the sensitivity of the results to this assumption,the calculations have been repeated with BC:OC ratios of1:4 and 1:1.[13] The assimilation is used to adjust the model for

consistency with satellite-retrieved aerosol optical thick-nesses (AOTs). The column-integrated AOTs over cloud-free ocean surfaces are obtained from AVHRR imageryrecorded by NOAA polar-orbiting satellites. The retrievalsare based upon the NOAA Pathfinder 2 aerosol algorithm[Stowe et al., 1997] and cloud-clearing procedure [Stoweet al., 1998]. In the algorithm, the aerosol is assumed tohave optical characteristics similar to those of a marineaerosol [Stowe et al., 1997]. The assimilation is performedfor each daytime satellite overpass at the model time stepclosest to the time of the overpass. The model AOTs areestimated from the aerosol mass-mixing ratios and relativehumidities using an aerosol optics package appropriate forthe 630 nm wavelength of the satellite retrievals. The mass-mixing ratios are adjusted by the assimilation procedure tominimize the differences between the modeled andretrieved AOTs using optimal interpolation [Lorenc, 1986;Levelt et al., 1998; Lamarque et al., 1999]. There are 14satellite orbits per day, each of which yields a digital map ofAOT extending roughly 1100 km in the east-west direction.These maps are interpolated onto the spatial grid of theaerosol model before assimilation. The retrievals from allAVHRR images collected during the ascending (afternoon)NOAA-14 orbits for January through March 1999 are inputto the assimilation. The assimilation is equivalent to asource or sink of aerosol mass. Formally, the assimilationappears in the transport equations as an instantaneoussource or sink term which is calculated after the otherstandard physical parameterizations for wet removal, drydeposition, etc. For the present model, the assimilation‘‘source’’ is less than 20% of the physical sources of aerosol[Rasch et al., 2001]. The assimilation ‘‘sink’’ is less thanany of the physical sinks.[14] Like other aerosol retrieval schemes of its class, the

errors in AOT from the NOAA Pathfinder algorithm can belarge when the observed aerosol departs significantly fromthe properties adopted in the retrieval [Mishchenko andTravis, 1997]. Comparison of satellite retrievals based uponpurely scattering optics with coincident estimates of AOTfrom surface sunphotometers in the INDOEX region indi-cates that these retrievals underestimate the AOT by approx-imately 30% [Rajeev et al., 2000]. The relative error isnearly constant during several weeks of observations. TheNOAA Pathfinder retrieval is based upon optical propertiesfor a remote marine aerosol [Stowe et al., 1997] with a sizedistribution similar to that used in the work of Rajeev et al.,[2000]. Since the optics for maritime aerosols yield nearly

COLLINS ET AL.: SIMULATION OF RADIATIVE FORCING FOR INDOEX INX2 27 - 3

conservative scattering at the wavelength of the AVHRRvisible radiances, the 30% error estimate is applicable to theNOAA Pathfinder retrievals in the INDOEX domain(V. Ramanathan, private communication, 2000). Beforeassimilation, the NOAA AOTs are adjusted upward by afactor of 1/0.7 to eliminate the bias relative to surfaceinstruments. It should be noted that other investigators havedirectly compared the NOAA Pathfinder 2 retrieval againstAOTs derived from surface and shipboard sunphotometersdeployed during INDOEX [Coakley et al., 2002]. Thiscomparison does not show significant biases between thePathfinder algorithm and the sunphotometer data, and theauthors conclude that the Pathfinder algorithm performed aswell as other retrievals devised specifically for the INDOEXregion.[15] The model is integrated using meteorological fields

from the NCAR/NCEP reanalysis [Kalnay et al., 1996].The data is stored on a Gaussian T62 grid with 28 sigmalevels at 6-hourly intervals. At the equator, the horizontalresolution is 209 km. The fields are temporally interpolatedto each model time step. Although the aerosol forecasts andanalyses for the IFP were based upon a high-resolutionNCEP aviation analysis [Collins et al., 2001; Rasch et al.,2001], we do not have the high-resolution data for January1999. Our calculations of the direct aerosol forcing spanJanuary though March 1999 for consistency with otherestimates [Ramanathan et al., 2001a]. Since the aviationanalysis and NCAR/NCEP reanalysis are based upondifferent models, and since it is preferable to integratethe chemical transport model with a single continuousmeteorological record, the NCAR/NCEP reanalysis isused to simulate the aerosol distributions for these forcingcalculations.[16] The average AOT for January through March 1999

is shown in Figure 1, and the AOT for each aerosol speciesis shown in Figure 2. The estimation of AOT from themodel fields is described in the work of Collins et al.[2001]. The effect of assimilation is to increase the aerosoloptical depth in the northern Indian Ocean (Figure 1b).Corrections in coastal areas extend over a region deter-mined by an error correlation length scale of 100 km in theassimilation procedure, and this effect combined with trans-port leads to differences in AOT over land between modelsrun with and without assimilation [Collins et al., 2001]. Themodel reproduces the large meridional gradient in AOTobserved in satellite retrievals of AOT for the same period[Rajeev et al., 2000]. It should be noted that the corre-spondence between the modeled and satellite-derived AOTresults in part due to the application of assimilation. Whenthe AOT is decomposed by species (Figure 2), it is evidentthat the AOTs from sulfate and carbonaceous aerosols arethe dominant contributions to the AOT in the Indiansubcontinent and south-east Asia. The total AOT of theanthropogenic species is the principal component of theAOT over the southern Arabian Sea and Bay of Bengal.Figure 2 does not support the common assumption inmodeling studies that the mass concentrations of carbona-ceous and sulfate aerosols are linearly related. The soil-dustaerosols, which are the focus of the earlier MONEXstudies, contribute most of the AOT in the northern ArabianSea. The parameterization for soil dust tends to under-estimate the dry deposition. In comparisons with in situ

INDOEX measurements of surface aerosol concentrations,the largest errors are associated with the overestimate ofdust by factors of 2–3 [Rasch et al., 2001]. As a result, themodeled dust AOT is larger than the sulfate AOT in thesouthern Arabian Sea, which is not consistent with theINDOEX observations [Satheesh and Ramanathan, 2000].The sea-salt AOT is the main component of the total AOTin the southern Indian Ocean.

2.2. Radiation Model and Aerosol Optics

[17] The radiative fluxes are calculated using a version ofthe 2-stream delta-Eddington code used in the NCARCommunity Climate Model (CCM, version 3.6.6) and theNCAR Column Radiation Model (CRM, version 2.1.2)[Briegleb, 1992]. The fluxes and shortwave heating ratesare computed at each of the 28 levels in the aerosol-modeloutput. The fluxes and heating rates are calculated eachhour during January through March 1999 by interpolatingthe 6-hourly aerosol distributions and moisture profiles tohourly resolution. The principle modifications to the stand-ard CCM shortwave code are the introduction of anaccurate solar ephemeris based upon the Naval Almanac[Michalsky, 1988; Spencer, 1989], the inclusion of opticsfor sea salt, dust, and carbon, and a much more accuratetreatment of the effects of cloud geometrical overlap on all-sky fluxes [Collins, 2001]. The notation we use for thedaily mean radiative quantities is defined in Table 1. In thisstudy, we will not calculate the effects of aerosols on thelongwave fluxes, although these effects can be significantin regions with large dust loadings. Lubin et al. [2002]estimate that, for an aerosol visible optical depth of 0.3, thedownwelling longwave flux is increased by 7.8 W/m2 andthe outgoing longwave flux is decreased by 2.6 W/m2.[18] The optics for sulfate are the same as those used in

Kiehl et al. [2000]. The sulfate is assumed to be H2SO4

with a lognormal size distribution under dry conditions.The dry geometric radius is rg = 0.05 mm and the standarddeviation is sg = 2.03 mm. The hygroscopic growth iscomputed using the Kohler curve, and the indices ofrefraction for the wetted sulfate aerosol are calculated asmass-weighted averages of the indices for pure H2SO4 andwater. The optics for dust are the same as those used inthe work of Collins et al. [2001]. The size distributions ineach of the 4 dust size bins are lognormal with parametersgiven in Table 2. The index of refraction for dust is takenfrom measurements of Saharan dust [Patterson, 1981].[19] The optics for the sea-salt and carbonaceous aerosols

are derived from the Optical Properties of Aerosols andClouds (OPAC) data set [Hess et al., 1998]. FollowingCooke et al. [1999], the OPAC optics for soot aerosols areused for black carbon, and the OPAC optics for water-soluble aerosols are used for organic carbon. OPAC treatssoot as hydrophobic so the optics are not affected byambient humidity. The optics for hydrophobic and hydro-philic organic carbon correspond to the optics for dry andwetted water-soluble aerosols, respectively. The opticalproperties depend upon the chemical and physical proper-ties of the carbonaceous aerosols and can vary over a widerange [Liousse et al., 1996; Tegen et al., 1997]. The choiceof optics for the carbon species is one of the largest sourcesof uncertainty in the forcing calculations. The sea-salt opticsare treated using a mass-weighted combination of specific

INX2 27 - 4 COLLINS ET AL.: SIMULATION OF RADIATIVE FORCING FOR INDOEX

extinctions for sub and super-micron sea salt from theOPAC data set. The corresponding single-scattering albedos(wo) and asymmetry parameters are calculated using stand-ard formulae for combining optical properties of differentatmospheric constituents [e.g., Cess, 1985]. The ratio ofmass concentrations of super to submicron sea salt rangesfrom 6–16 [Haywood et al., 1999], and in our calculationwe have assumed that the ratio is approximately 6. Theresulting specific extinction at 70% relative humidity isintermediate between the values derived by Tegen et al.[1997] and Quinn and Coffman [1999]. The OPAC opticsare interpolated onto the 19 spectral intervals used by theCRM using a high-resolution model calculation for the

solar spectrum (W. Wiscombe, private communication,2000). The hygroscopic growth of the extinction, asymme-try parameter, and single-scattering albedo in each spectralinterval are fit as functions of relative humidity followingKiehl et al. [2000].[20] The time series of the modeled column-average wo

at the INDOEX KCO site during the INDOEX IFP isshown in Figure 3. The values of wo include all the aerosolspecies in the model and is evaluated at l = 500 nm. Theaverage of wo at KCO for the IFP is 0.877, which agreeswith the average of estimates from surface and aircraft datato within ±0.02 [Satheesh and Ramanathan, 2000]. Theexperimental values include estimates from surface radio-

Figure 1. The total aerosol optical thickness from the chemical transport model averaged over Januarythrough March 1999. The AOT is estimated at a wavelength of 630 nm. Panel A: model run withassimilation of satellite AOT; B: difference in AOT from model with assimilation minus AOT frommodel without assimilation.

COLLINS ET AL.: SIMULATION OF RADIATIVE FORCING FOR INDOEX INX2 27 - 5

meters; in situ chemical and optical characterization fromtwo ships, an aircraft, and fixed surface sites includingKCO; multiwavelength lidar; and combinations of surfaceradiometers and Monte Carlo models [Ramanathan et al.,2001a]. There is a small trend toward decreasing values ofwo during the experiment. During March 1999, the periodof the largest concentrations of anthropogenic aerosols overKCO [Satheesh and Ramanathan, 2000], the daily meanwo is always less than 0.9. The only major aerosol speciesin the model with wo < 0.89 at 500 nm is black carbon, andtherefore the range of wo between 0.8 and 0.9 duringMarch must result from a significant increase in thefraction of the total aerosol mass comprised of blackcarbon. In fact, the modeled mass of black carbon overKCO is 1.7 times larger in March compared to February.The change in the concentration of black carbon is coin-

cident with a shift in large-scale flow regimes at the end ofFebruary [Rasch et al., 2001]. A map of wo averaged overthe IFP is shown in Figure 4. The lowest values of wo

occur over central India and south-east Asia, while thelargest values occur over the southern Indian Ocean. Themeridional gradient in wo is a consequence of the highconcentrations of absorptive anthropogenic aerosols nearregions with significant biomass burning and fossil-fuelconsumption. In the 3-month mean, wo is less than 0.9 overthe entire northern Indian Ocean, Arabian Sea, Bay ofBengal, and surrounding continental areas. As we willshow in section 3.1, the low values of wo and large AOTsresult in significant heating of the atmosphere over thenorthern Indian Ocean.

2.3. Land-Surface Model for Land-Surface Albedos

[21] Calculation of the direct radiative forcing overcontinental regions adjacent to the Indian Ocean requires

Figure 2. The aerosol optical thickness from the assimilation model averaged over January throughMarch 1999. Panel A: sulfate; B: black and organic carbon; C: soil dust; and D: sea salt. The AOTs areestimated at a wavelength of 630 nm.

Table 1. Daily Mean Shortwave Forcing Terms

Symbol Definition

S(TOA) Forcing at TOA for clear-sky conditionsSk(TOA) Forcing at TOA for species k (clear sky)So(TOA) Forcing at TOA with ocean albedos used everywhereS(0) Forcing at surface for clear-sky conditionsSk(0) Forcing at surface for species k (clear sky)So(0) Forcing at surface with ocean albedos used everywhereS(ATM) Forcing on atmosphere (S(TOA) � S(0))So(ATM) Forcing on atmosphere with ocean albedos used everywhereQ Shortwave heating ratet AOT at l = 500 nm

Table 2. Optical Properties for Soil-Dust Aerosols

Size, mm rg,a mm ln sg

b c,c m2/g

0.01–1 0.4 0.7884 1.3031–10 2.0 0.7884 0.12410–20 15.0 0.7884 0.04020–50 25.0 0.7884 0.017

aGeometric mean radius.bWidth of log-normal distribution.cOptical extinction at l = 630 nm.

INX2 27 - 6 COLLINS ET AL.: SIMULATION OF RADIATIVE FORCING FOR INDOEX

estimates for the spectral surface albedos for diffuse anddirect radiation. The CRM uses two visible albedos fordiffuse and direct radiation at wavelengths less than 0.7mm and two near-infrared albedos for diffuse and directradiation at wavelengths greater than 0.7 mm. For oceanregions, the standard CCM albedo formula is used [Brie-gleb et al., 1986]. Albedos of land surfaces are much moreheterogeneous, and unfortunately visible and near-infrared

albedos were been measured directly over the region ofinterest during INDOEX. If measurements of TOA clear-sky fluxes are available, then these data can be used toestimate the broadband surface albedos with reasonableaccuracy [Staylor and Wilber, 1990]. Additional assump-tions are required to infer spectral land-surface albedos[Pinker and Laszlo, 1992]. However, only intermittentTOA flux data is available for 2 weeks during theINDOEX IFP. The spectral albedos for land surfaces haveinstead been calculated using the NCAR Land SurfaceModel (version 1) [Bonan, 1996]. The land-surface albe-dos are estimated from a radiative transfer code that treatsthe interaction of downwelling radiation with the vegeta-tive canopy, bare soils, lakes and rivers, and snow. LSMincludes estimates of spectral albedos for several differenttypes of bare soil, and it parameterizes the effects of soilmoisture on these albedos. Seasonal changes in vegetationcover and type are obtained from an annually cyclic dataset. The LSM calculates the land-surface albedos eachhour at 1-degree resolution for January–March 1999.[22] For this application, the LSM is integrated using a

global, multiyear atmospheric forcing data set extractedfrom the NCAR/NCEP T62 meteorological reanalysis[Kalnay et al., 1996]. Satellite retrievals of precipitation[Xie and Arkin, 1997] and calculations of downwellingsurface radiation based upon ISCCP cloud properties[Bishop et al., 1997] are used to correct known deficien-cies in the reanalysis. The meteorological fields are timeinterpolated from the 6-hourly resolution of the reanalysis.The initial state of the LSM is the last instantaneous stateof the model from an integration from 1979 through 1998[Bonan et al., 2001]. For consistency with the solargeometry in CRM, the calculation of surface albedos isbased upon the same ephemeris code [Michalsky, 1988;Spencer, 1989].

Figure 3. Time series of the simulated single-scatteringalbedo wo of the aerosol in a 3 � 3 grid-cell array centeredon the INDOEX KCO site for January–March 1999. Thecolumn-average wo is estimated at a wavelength of 500 nm.The averaging over model layers is weighted by the opticaldepth for each layer. Each point represents a daily mean forthe 3 � 3 array, and the vertical bars represent the spatialstandard deviation within that array.

Figure 4. Simulated single-scattering albedo of aerosol at a wavelength of 500 nm for January–March1999. Vertical averaging is weighted by the optical depth for each layer.

COLLINS ET AL.: SIMULATION OF RADIATIVE FORCING FOR INDOEX INX2 27 - 7

[23] The shortwave surface albedos are shown inFigure 5. The albedos are derived from ratios of thecorresponding time-mean downwelling and upwellingfluxes at each grid point in the model. The broadbandalbedo is less than 0.2 over the portions of the Indiansubcontinent, south-east Asia, and eastern Africa includedin the calculation. The only regions where the albedoexceeds 0.2 are over the Saudi Arabian peninsula and thesnow covered regions of the Tibetan plateau and surround-ing areas. Unfortunately it is not possible to evaluate themodeled surface albedos directly against observations.However, estimates of the surface albedo have beenderived from measurements of the clear-sky TOA fluxes[Staylor and Wilber, 1990; Pinker and Laszlo, 1992].Most of the estimates are based upon data from theNASA Earth Radiation Budget Experiment (ERBE) dur-ing 1985–1990 [Harrison et al., 1990]. The estimation ofsurface albedos from TOA fluxes requires a correction forthe radiative effects of the atmosphere. In the INDOEXregion, the correction process is complicated by the largeaerosol loadings and heterogeneous aerosol composition(Figures 1A and 2). The shortwave albedo calculated withthe LSM is compared against two estimates of the albedofrom the NASA Surface Radiation Budget (SRB) data setfor January–March 1986–1988 in Figure 6. The LSMalbedos are larger than the Pinker and Laszlo [1992]values, but the albedos from LSM and Staylor and Wilber[1990] agree to within ±0.05 (Figure 6). Over southernChina, the LSM albedos are larger than both empiricalestimates. In general, the LSM is consistent with the SRBdata to within the uncertainty in the techniques for derivingsurface albedo from satellite fluxes. As we will demon-strate in section 3.1, errors in the LSM surface albedos are

a secondary error term in the modeled direct aerosolforcing.

3. Forcing Calculations

3.1. Clear-Sky Radiative Forcing

[24] The surface and satellite measurements collectedduring INDOEX can be used to evaluate the modelcalculation. The derivatives of the surface and TOA forcingwith respect to AOT can be derived from simultaneousmeasurements of the fluxes and AOTs. In these estimates,the AOT is calculated at 500 nm. The modeled relation-ships between AOT and the surface and TOA forcing areshown in Figure 7. At TOA, the net flux decreases by�22 W/m2 per unit increase in AOT, while at the surface,the net flux decreases by �73 W/m2 per unit increase inAOT. These derivatives are only applicable over oceansurfaces for moderate values of the AOT. The correspond-ing derivatives have been calculated by Satheesh andRamanathan [2000] from NASA CERES data at TOAand INDOEX instruments at the surface. The observatio-nally derived derivatives are

d S TOAð Þdt

¼ �25 W=m2 ð1Þ

d S 0ð Þdt

¼ �71:5� 1:5 W=m2 ð2Þ

The modeled derivatives are consistent with the observedvalues to within 3 W/m2 per unit optical depth, and ourestimates are comparable to those derived from a radiativetransfer model with wo obtained from measured aerosol

Figure 5. Average surface shortwave albedo for January–March 1999 from the LSM calculation on a1 � 1 degree grid.

INX2 27 - 8 COLLINS ET AL.: SIMULATION OF RADIATIVE FORCING FOR INDOEX

optical properties [Satheesh and Ramanathan, 2000]. Theseresults suggest that the model is consistent with themeasured sensitivity of the TOA and surface forcing tochanges in AOT. The sensitivity dS(0)/dt of the surfaceforcing is approximately 3 times the sensitivity dS(TOA)/dtof the TOA forcing. This difference is characteristic of anaerosol which absorbs as well as scatters the incidentsunlight. At TOA, the effects of absorption and scattering aresubtractive, whereas at the surface the effects are additive.

The result is that the surface forcing is much greater than theforcing at TOA for a given aerosol loading. The slopes forthe two sensitivity experiments in which the BC:OC ratio isset to 1:4 and 1:1 are given in Table 3.[25] The time-mean forcing for the TOA, surface, and

atmosphere is shown in Figure 8. The correspondencebetween the forcing and AOT (Figure 1a) is particularlyevident for S(0) (Figure 8c). The regions with the largestabsolute values of S(TOA) are located in the northern

Figure 6. Comparison of modeled and satellite-derived surface albedos for January–March averagedover 1986–1988. All data is mapped onto a 2.5 � 2.5 degree grid. Panel A: LSM calculation; B: surfacealbedo from Pinker and Laszlo [1992]; and C: surface albedo from Staylor and Wilber [1990].

COLLINS ET AL.: SIMULATION OF RADIATIVE FORCING FOR INDOEX INX2 27 - 9

Arabian Sea, the western coast of India downwind of Bom-bay, and the northern China Sea. There is a clear meridionalgradient in S(TOA), with values ranging between �3 W/m2

in the southern Indian Ocean to �9 W/m2 in the northernocean regions. There are also reductions in the forcingmagnitude of 2 to 4 W/m2 at the continental boundaries,and the forcing changes sign over the Arabian Peninsula.Over the desert regions, the introduction of dust aerosolsincreases the shortwave radiation absorbed by the land-atmosphere column. These large gradients in the forcingare associated with the discontinuities in surface albedo atthe coastlines rather than discontinuities in the aerosolloading.[26] The effects of the land-surface albedos may be iso-

lated by calculating the forcing using ocean albedos in placeof LSM albedos over land surfaces. For an optically thinscattering aerosol, the TOA forcing should decrease quadrati-cally with increasing surface albedo [Charlson et al., 1992].

The differences between the forcing computed with LSM andocean albedos are shown in Figure 9. These differences areshown to demonstrate that the IFA-mean forcing calculations(Figure 8) over India and south-east Asia are not sensitive tolarge errors in the modeled surface albedo. With the excep-tion of the deserts, positive values for the TOA and surfaceimply that the magnitude of the forcings are reduced whenLSM albedos are used in place of ocean albedos. Positivevalues for the atmosphere indicate that the magnitude of theforcing is increased. The differences are identically 0 overocean regions by definition and exceed 10 W/m2 over theSaudi Arabian peninsula for the TOA and surface. In general,the largest relative changes in forcing associated with theland-sea albedo contrast occur in the TOA forcing. Overdeserts, the absorption of shortwave radiation is increased bymultiple reflections between the surface and dust aerosol.This process increases the average number of times theradiation traverses the layer of dust, which is moderatelyabsorbing, and the absorptive gases and water vapor in thelower atmosphere.[27] The surface forcing by aerosols (Figure 8c) is �35 to

�40 W/m2 over central India, south-east Asia, and southernChina. The maximum values near Myanmar are associatedwith the region of lowest wo (Figure 4). The forcing reducesthe surface insolation by 15% over central India and by 17–20% over south-east Asia and southern China. For compar-ison, estimates of the climatological surface shortwavecloud forcing are plotted in Figure 10 [Staylor and Wilber,1990; Pinker and Laszlo, 1992]. The aerosol surface forcingduring the INDOEX IFP is more than twice the climato-logical surface cloud forcing over central India, Myanmar,and Thailand. The two forcings are similar in magnitudeover the northern Arabian Sea and the Arabian Peninsula.The model results suggest that forcing by anthropogenicaerosols can exceed shortwave cloud forcing during theIndian winter monsoon.[28] The absorption by carbonaceous species results in

atmospheric forcing that exceeds 25 W/m2 over centralIndia, south-east Asia, and southern China (Figure 8). Thegeographic patterns in S(ATM) are closely correlated withthe AOT contributed by the black and organic carbon(Figure 2b). Since the shortwave heating rates are propor-tional to the shortwave absorption in the atmosphere, thechanges in column-integrated heating caused by aerosolsare proportional to S(ATM). The changes in the verticalprofile of shortwave radiative heating rates are shown inFigure 11. The figure shows a meridional cross-section ofthe perturbations in Q which includes the troposphereabove the Maldives and the region around Bombay. Theperturbations are calculated from averages of the dailymean heating rates during January through March 1999.The perturbations in Q are less than 0.01 K/day south of10�S. In the northern hemisphere, the perturbation exceeds0.7 K/day in the region around Bombay. This correspondsto an increase in the clear-sky heating rates in the lowertroposphere by 130%. Approximately 90% of the increasein Q is associated with the carbonaceous aerosols. Notethat the model does not reproduce the double-layeredvertical structure in aerosol profiles frequently observedduring INDOEX [Ramanathan et al., 2001a]. As a resultthe modeled heating rates decrease monotonically withaltitude.

Figure 7. Panel A: Variation in simulated TOA clear-skynet solar flux with 500 nm AOT in a 3 � 3 grid-cell arraycentered on the INDOEX KCO site. Each point represents adaily mean flux and AOT for February–March 1999. Solidline is the least-squares best fit to the model data. Panel B:Corresponding variation in simulated surface clear-sky netsolar flux with 500 nm AOT.

INX2 27 - 10 COLLINS ET AL.: SIMULATION OF RADIATIVE FORCING FOR INDOEX

[29] The aerosol forcing can be decomposed into theforcings from individual species in order to assess the relativecontributions of each aerosol type. To a good approximation,the total forcing is the sum of the forcings for each species.The relationship between the time-mean total forcing to thesum of forcings for carbon, dust, sulfate, and sea salt isshown in Figure 12. The difference between the total forcingat TOA and the sum of forcings for each species is 0.25 ±0.32W/m2, and the corresponding difference at the surface is0.30 ± 0.43 W/m2.

[30] The decomposition of the TOA forcing by speciesis shown in Figure 13. The largest forcing averaged overthe INDOEX domain is contributed by the dust aerosols,and the smallest is contributed by sea salt. The factthat the forcing by dust is the largest component of theTOA forcing is related to the overestimation of dustconcentrations in the model by factors of 2–3 [Raschet al., 2001]. The forcings from sulfate and dust aerosolshave opposite sign to the forcings from carbonaceousaerosols over all the land surfaces in the domain. The

Figure 8. Direct shortwave radiative forcing for clear-sky conditions averaged over January–March1999. Panel A: TOA; B: atmosphere; and C: surface.

COLLINS ET AL.: SIMULATION OF RADIATIVE FORCING FOR INDOEX INX2 27 - 11

difference in sign can result in partial cancellation betweenthe forcings by scattering and absorbing aerosols at TOAif the aerosols are co-located. The maximum values ofthe sulfate forcing occur over southern China, where thelargest quantities of sulfate in the INDOEX region aregenerated by combustion of high-sulfur coal. The largestforcings on the atmosphere (Figure 14) and on the surface(Figure 15) are contributed by black and organic carbon.

Over India, southeast Asia, and China, carbon contributes85–90% of the atmospheric forcing and at least 60% ofthe surface forcing.

3.2. Direct Aerosol Forcing Includingthe Effects of Clouds

[31] The direct aerosol forcing is affected by the presenceof clouds. Clouds above the aerosols usually reduce the

Figure 9. Differences in total direct aerosol forcing computed with land-surface albedos and oceanalbedos over continental regions. Forcings are calculated under clear-sky conditions and averaged overJanuary–March 1999. Panel A: difference S(TOA) � So(TOA) at TOA; B: difference S(ATM) �So(ATM) on atmosphere; and C: difference S(0) � So(0) at the surface.

INX2 27 - 12 COLLINS ET AL.: SIMULATION OF RADIATIVE FORCING FOR INDOEX

Figure 10. Mean surface shortwave cloud forcing for January–March. Panel A: Staylor and Wilber[1990]. Panel B: Pinker and Laszlo [1992].

Figure 11. Perturbation to clear-sky heating rates (in K/day) from introduction of aerosol averaged over70–75�E. Heating rates are averaged over January–March 1999.

COLLINS ET AL.: SIMULATION OF RADIATIVE FORCING FOR INDOEX INX2 27 - 13

amount of sunlight interacting with the aerosols. High-altitude clouds also reduce the TOA aerosol forcing becauseof the nonlinear relationship between TOA albedo and totalatmospheric optical depth. Clouds below the aerosols areequivalent to a highly reflective land surface and thereforereduce the TOA forcing (section 3.1). Since the relationshipbetween transmission and total optical depth is also non-linear, the low-altitude clouds generally reduce the surfaceforcing as well. Therefore the net result of including cloudsin the calculations should be to decrease the direct aerosolradiative forcing at TOA and the surface. There can beadditional effects from the indirect effects of aerosols oncloud radiative and physical properties [Ackerman et al.,2000], but we do not include the indirect effect in ourcalculations.[32] The forcing including clouds is defined as the all-sky

flux including aerosols minus the corresponding all-sky fluxwithout aerosols. Based upon the spatial relationship betweenclouds and aerosols during the Indian winter monsoon, it isunlikely that the all-sky aerosol forcing could differ signifi-cantly from the clear-sky values. Long-term satellite and

surface observations show that the amount of low-altitudeclouds in the Bay of Bengal and the Arabian Sea is less than30% during January to March [Bony et al., 2000]. Theabsence of low clouds is one of the reasons for choosingthe winter monsoon season for INDOEX. Removal of con-tinental aerosols by wet deposition in these areas reaches itsminimum during the winter monsoon season.[33] The calculation of all-sky aerosol forcing requires

information on the three-dimensional distribution of con-densedwater and cloud amount. These fields are typically notavailable in operational meteorological analyses and are notincluded in the NCEP/NCAR reanalysis. The condensedwater is derived from the reanalysis using ‘‘process recon-struction’’ [Rasch et al., 1997; Barth et al., 2000] using theprognostic cloud-water parameterization from CCM [Raschand Kristjansson, 1998]. The global-mean cloud liquid waterpredicted by this scheme in CCM is between 39–73% of theliquid water derived from satellite microwave data usingseveral retrieval techniques [Rasch and Kristjansson, 1998].The cloud amount is diagnosed from the relative humidityfield and other state variables using the standard cloud-amount parameterization from CCM [Hack, 1998]. Theglobal and zonal-mean low and middle cloud amounts fromthe diagnostic parameterization fall within the ranges of thesatellite and surface observations [Hack, 1998], although thehigh cloud amounts are 20% larger (absolute) than ISCCPretrievals. The distribution of low, middle, and high cloudsand the total condensed water path derived usingMATCH areshown in Figure 16. The maximum low and high cloud coverand condensed water are located at approximately 10�S, aregion where the primary component of the AOT is sea salt(Figure 2). With the exception of southern China, the mini-mum cloud amounts and water paths occur between 10�N–30�N. The 6-hourly cloud and condensed water fields outputby MATCH are interpolated to the hourly time resolution ofthe forcing calculations.[34] The difference between the all-sky and clear-sky

aerosol forcings are shown in Figure 17. The positive signof the changes at TOA and the surface show that themagnitude of the aerosol forcing is reduced as expected.The changes over most of the INDOEX domain rangebetween 0–4 W/m2 and represent a small perturbation tothe aerosol forcing in the northern Indian Ocean. The largestdifferences occur over southern China, where the forcing isreduced by as much as 20 W/m2. This reduction is theconsequence of 2 factors: the large sulfate loading in thatregion (Figure 2a), and the collocation of the sulfate andsignificant low cloud amount (Figure 16b). The absorp-tion of radiation is reduced by up to 2 W/m2 over the Indiansubcontinent and Arabian peninsula, a change of less thanapproximately 10% in the absorbed solar radiation. Thesecalculations suggest that the effects of clouds on the directforcing are a minor term in the insolation reaching land

Figure 12. Relationship of total shortwave forcing to sumof forcings by individual aerosol species. Panel A: TOA;and B: surface. The forcings are averages over January–March 1999 at each model grid point between 30�N–30�S,40�E–120�E.

Table 3. Radiative Forcing Sensitivity

BC:OC dS(TOA)/dta dS(0)/dta

1:2 �22.2 �73.11:4 �26.6 �61.21:1 �17.3 �86.8

aDaily average flux (in W/m2) per unit AOT.

INX2 27 - 14 COLLINS ET AL.: SIMULATION OF RADIATIVE FORCING FOR INDOEX

Figure 13. Decomposition of TOA shortwave radiative forcing shown in Figure 8 by aerosol species.Panel A: Sulfate; B: carbon; C: dust; and D: sea salt.

Figure 14. Decomposition of atmospheric shortwave radiative forcing shown in Figure 8 by aerosolspecies. Panel A: Sulfate; B: carbon; C: dust; and D: sea salt.

COLLINS ET AL.: SIMULATION OF RADIATIVE FORCING FOR INDOEX INX2 27 - 15

Figure 15. Decomposition of surface shortwave radiative forcing shown in Figure 8 by aerosol species.Panel A: Sulfate; B: carbon; C: dust; and D: sea salt.

Figure 16. Mean cloud properties used to compute effects of clouds on direct forcing during January–March 1999. Panel A: Low cloud amount (700 mb to surface, random overlap); B: midlevel cloudamount (400 mb to 700 mb); C: high cloud amount (10 mb to 400 mb); and D: grid-box averagedcondensed water path (g/m2).

INX2 27 - 16 COLLINS ET AL.: SIMULATION OF RADIATIVE FORCING FOR INDOEX

surface in the INDOEX region. Other studies [e.g., Ram-anathan et al., 2001a] have obtained larger differencesbetween the all-sky and clear-sky radiative forcing. Thereasons for the disparity between the estimates are discussedin the conclusions.

4. Conclusions

[35] The direct aerosol radiative forcing has been com-puted for the INDOEX Intensive Field Phase during January

through March 1999. The distribution of aerosols is obtainedfrom an aerosol assimilation model, and the backgroundatmospheric state is taken from the NCAR/NCEP meteoro-logical analysis. The surface boundary conditions for theradiative calculations are obtained from a land-surface modelforced with the same meteorological analysis and satelliteretrievals of insolation and precipitation. This analysisextends the radiative budget in the Indian Ocean basincalculated byAckerman and Cox [1987] to include the effectsof anthropogenic aerosols.

Figure 17. Difference between direct aerosol forcing computed under all-sky minus clear-skyconditions for January–March 1999. Panel A: TOA; B: atmosphere; and C: surface.

COLLINS ET AL.: SIMULATION OF RADIATIVE FORCING FOR INDOEX INX2 27 - 17

[36] The radiative properties of the simulated aerosols arein good agreement with the in situ observations. The modelreproduces the observed single-scattering albedo to within±0.02, and it reproduces the measured sensitivities of theTOA and surface forcing to AOT to within 3 W/m2 per unitt. The model also simulates the chemical speciation of theaerosol measured at INDOEX surface sites to within10–20% [Rasch et al., 2001]. Based upon the agreementwith the observations, we have used the model to calculatethe aerosol distributions and radiative effects over the IndianOcean and surrounding continental regions.[37] The modeled aerosols over India, southeast Asia, and

southern China are composed primarily of sulfate and blackand organic carbon. The combination of natural and anthro-pogenic aerosols reduce the surface insolation by up to40 W/m2 and increase the atmospheric shortwave absorptionby up to 35 W/m2. The aerosol surface forcing decreases thesurface insolation by 15–20% over the continental areas andtherefore significantly alters the surface energy budget. Theincreased shortwave absorption is accompanied by changesto the time-mean solar heating rates of up to 0.8 K/day. Themaximum relative perturbation to Q is 130% during theINDOEX IFP. Most of this perturbation is contributed bythe effects of carbonaceous aerosols. The relative change inthe heating rate from anthropogenic aerosols should becompared to the 50% change in Q calculated for the dustaerosols observed during MONEX [Ellingson and Serafino,1984; Ackerman and Cox, 1987]. The dynamical effects ofthis significant change in atmospheric heating are discussedin Ramanathan et al., [2001b].[38] Using sensitivity studies, we have examined whe-

ther these results have a large dependence on the specifi-cation of surface albedos and on the presence of clouds.The largest uncertainties in the forcing related to surfacealbedos are associated with the desert regions in theArabian peninsula. Over the Indian subcontinent and south-east Asia, the uncertainties in the forcing at TOA andsurface are generally between 0–4 W/m2 and represent asmall relative error. The differences between the directaerosol forcing computed with and without clouds are onthe same order except over southern China. The directforcing by aerosols over this region is reduced by approx-imately 50%, or 2 W/m2, by the introduction of clouds. It isimportant to note that other calculations show that theIFA-mean difference between all-sky and clear-sky aerosolforcing is 5 ± 2 W/m2 [Ramanathan et al., 2001a].Extensive comparisons between the radiation codes usedin the work of Ramanathan et al. [2001a] and this papershow that the instantaneous all-sky and clear-sky forcingagree to within 1–2 W/m2 when the input cloud amounts,condensed water paths, and aerosol optical depths areidentical. Further analysis has shown that the disparity inthe estimates of all-sky aerosol forcing results fromdifferences in the spatial relationships of the cloud andaerosol fields adopted in the work of Ramanathan et al.[2001a] and the present study.[39] The large magnitude of the forcing by aerosols has

several implications. First, the aerosol forcing is as large orlarger than other heat sources and sinks, for exampleshortwave cloud forcing, which are currently included inthe land-surface model and meteorological analysis system.Second, aerosol forcings of the magnitude observed during

INDOEX could have a significant impact on the regionalatmospheric and oceanic dynamics. Other studies usinggeneral circulation models [e.g., Ramanathan et al.,2001b] are underway to examine the atmospheric response.Studies of the coupled ocean-atmosphere response areneeded to understand the effects on the regional climatesystem. Finally, it is clear that anthropogenic aerosols havesignificantly altered the radiative transfer over India andChina. Projections of the aerosols released from thesecountries after further industrialization indicate that thisregion will be the dominant source of anthropogenic aero-sols by 2025 [Nakicenovic and Swart, 2000]. Modelingstudies of the aerosol forcing over the next century whichare consistent with INDOEX observations are urgentlyneeded for climate-change assessment.

[40] Acknowledgments. K. Oleson and S. Levis (NCAR) performedthe integrations of the NCAR Land Surface Model to provide hourlysurface albedos. The authors wish to thank V. Ramanathan and S.K.Satheesh (SIO/UCSD) for their forcing estimates. The authors are gratefulto L. Stowe and A. Ignatov (NOAA) for providing the code to the NOAAPathfinder aerosol retrieval algorithm. The assistance of S. Williams(UCAR) and T. Cremidis (NOAA) in acquiring NOAA AVHRR GAC datais gratefully acknowledged. This research has been supported by NASAGrant MTPE S-97889-F (WDC), NSF award ATM9405024 to the Centerfor Clouds, Chemistry, and Climate (PJR and BEE), and NSF INDOEXgrant ATM9612887 (PJR). CSZ was supported by NASA IDS grant S-87391-F. The computational resources were provided in part by NCAR.The surface albedo and surface cloud-forcing data were obtained from theNASA Langley Research Center Atmospheric Sciences Data Center. Thecomments and suggestions of two anonymous reviewers have been instru-mental in improving the manuscript. The National Center for AtmosphericResearch is sponsored by the National Science Foundation.

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�����������W. D. Collins, B. E. Eaton, D. W. Fillmore, J. T. Kiehl, and P. J. Rasch,

Climate and Global Dynamics Division, National Center for AtmosphericResearch, P.O. Box 3000, Boulder, CO 80307-3000, USA. (wcollins@ucar.edu; eaton@ucar.edu; fillmore@ucar.edu; jtkon@ucar.edu; pjr@ucar.edu)C. T. Beck, Climate Prediction Center, National Oceanographic and

Atmospheric Administration, World Weather Building, 5200 Auth Road,Camp Springs, MD 20746, USA. (tbeck@nesdis.noaa.gov)C. S. Zender, Earth System Science 216 PSRF, University of California,

Irvine, CA 92697-3100, USA. (zender@uci.edu)

INX2 27 - 20 COLLINS ET AL.: SIMULATION OF RADIATIVE FORCING FOR INDOEX