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1/48iLEAPS Newsletter Issue No. 5xApril 2008 1

iLEAPS isa core project

of IGBP

Aerosols Clouds

Precipitation Climate

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I

Cover photo

Haboob (dusty convective outflow) in Hombori,Mali, 23 August 2005. Photo taken duringthe AMMA (Analyses Multidisciplinaires de laMousson Africaine) field campaign by FranoiseGuichard and Laurent Kergoat, CNRS copyright.

iLEAPS Newsletter

SSN Printed version 1796-0363ISSN On-line version 1796-0401For submissions and subscriptionsplease contact [email protected]

Publisher

iLEAPSInternational Project OfficeGustaf Hllstrminkatu 2BP.O. Box 68FI-00014 University of HelsinkiTel: +358 (0)9 191 50571Fax: +358 (0)9 191 [email protected]://www.ileaps.org

Editorial PaneliLEAPS Executive Committee

Editor-in-Chief

Anni Reissell

Executive Editor

Sanna Sorvari

Circulation

4000

Printed by J-Paino Oy, Helsinki

Layout Kimarko Ky, Espoo

CONTENTS

Editorial 4

Science on aerosol, cloud, precipitation, climate interactions

ACPC The evolution of a joint initiative 6

IGBP and WCRP on ACPC initiative 8

Aerosols, Clouds, Precipitation, Climate (ACPC):

Outline for a new joint IGBP/WCRP initiative 10

Aerosol pollution impact on precipitation 16Why is Earths albedo 29% and was it always 29%? 18

Absorbing aerosols enhance Indian summer monsoon rainfall 22

What are the aerosol-precipitation effects at larger scales? 26

Can the slowing of auto-conversion result in increasing precipitation? 30

Toward the understanding of indirect aerosol effects and precipitation 34

Spatial and temporal variability of aerosol physical and chemical

properties and effects on cloud microphysics and precipitation 36

Science from the iLEAPS recognized projects

Getting hold of terpene emissions from vegetation 40

Observing soil moisture rainfall feedbacks 44

Activities

National activities: iLEAPS Japan 46Meetings 47

iLEAPS IPO IS SPONSORED BY

q University of Helsinkiq Finnish Meteorological Instituteq Ministry of Education



EditorialGuest editor Meinrat O. Andreae

Aerosolcloudprecipitationclimate interac-tions are the focus of this issue of the iLEAPSNewsletter. As in previous issues, however,the science articles are not limited to thistheme but illustrate the importance of land-atmosphere interactions showing the widespectrum of iLEAPS-related research.

The Intergovernmental Panel of ClimateChange (IPCC) published the Fourth Assess-ment report in 2007, stating that aerosols,clouds and precipitationand their com-plex and tightly coupled interactionsre-

main as the largest uncertainties in our cur-rent understanding of the climate system.A recent publication by the International

Aerosol Precipitation Science AssessmentGroup (IAPSAG), a panel of international ex-perts appointed by the World Meteorologi-cal Organization (WMO) and the Interna-tional Union of Geodesy and Geophysics(IUGG), focused on reviewing the state of thescience and indentifying the areas requiringfurther study. The panel was lead by ZevLevin and William Cotton, co-authors of anarticle in this issue summarizing the resultsof the report.

The Aerosols, Clouds, Precipitation, Cli-mate (ACPC) initiative by iLEAPS in collabo-ration with the International Global Atmos-pheric Chemistry (IGAC) core project of IGBPand the Global Energy and Water Experi-ment (GEWEX) of WCRP organized a special-ist workshop in October 2007 in Boulder,Colorado, USA. The aim of the workshop wasto develop the ACPC research agenda andstrategy in the short and long run.

In this newsletter, Bjorn Stevens and co-authors summarize the outcome of the

workshop, the issues of importance, and astrategy for a research program.The main authors of the ACPC-related

scientific articles from among the invitedspecialists and planning group members: V.Ramanathan, Bjorn Stevens, Bill Lau, UlrikeLohmann, Danny Rosenfeld, Maria AssunoF. Silva Dias, Cynthia Twohy and Sandro Fuzzi.Other invited speakers were Graeme L.Stephens and Brian Soden.

The workshop hosted by Guy Brasseurand the National Center for Atmospheric Re-

search gathered 57 participants (mainly fromthe US, but also from Europe, Brazil, Israel

and Australia), specialists in aerosol physicand chemistry, cloud dynamics and microphysics, atmospheric radiation and climatedynamics.

iLEAPS has established web pages and amailing list for the ACPC initiative. The ACPCweb pages http://www.ileaps.org/acpc/ canbe reached through the iLEAPS website ahttp://www.ileaps.org and the members othe mailing list through the email [email protected]

Volatile organic compounds from veg

etation surpass the anthropogenic emissionof volatile hydrocarbons on a global scaleplaying an essential part in atmosphericchemistry as well as in the formation oaerosol particles. lo Niinemets stresses thathe reliable prediction of biogenic emissionsor deposition onto vegetation, based onknowledge of the processes involved andalso the effects of environmental conditionsis essential in predicting atmospheric constitution on a regional scale and imperative toimprove emission and atmospheric chemis

try models.In this fifth issue, an article by Chris Tayloelaborates the importance of feedbacks between soil moisture and precipitation in ahotspot, the drought-prone Sahel at thesouthern border of the Sahara, where anegative feedback was observed locally withdevelopment of storms over dry soil. This isof considerable interest to the weather andclimate modelling communities; any feedbacks between soil water and precipitationcould either amplify or suppress climaticanomalies such as drought, depending onthe sign of the feedback.s

Dust storm over Marocco, 20 April 2004.Photo provided by Meinrat O. Andreae.

A low-level cloud line formed in the late afternoon hour above Agoufou, Mali 15 August 2005,this line then dissipated quickly at sunset. Photo by Franoise Guichard & Laurent Kergoat, CNRS copyright.



Training course Marie Curie iLEAPS,Model-Data AssimilationPhysics and chemistry of air pollution and their effects:field course and data analysis1019 March 2008, Hyytil Field Station, Finland. 3 ECTS.

Organizers: Prof. Timo Vesala, Prof. Markku Kulmala and

Prof. Pertti Hari from the University of HelsinkiDuring the course following subjects will be covered

q Introduction to SMEAR II station, www.atm.helsinki.fi/SMEAR/q Introduction to atmospheric physics and chemistry as well as boundary layer meteorologyq Biosphere - atmosphere interactionsq Formation of atmospheric aerosol particlesq VOC emissionsq Modelling the nitrogen cycleqTropospheric transport and inversion modelsq Analysis of data from SMEAR II stationq Data analysis and assimilation into global modelsq Statistical methods

The course is organized in cooperation between

q iLEAPS (Integrated Land Ecosystem-Atmosphere Processes Study)qThe Graduate School Physics, Chemistry, Meteorology and

Biology of Atmospheric Composition and Climate Changeq Atmosphere-Biosphere Studies (ABS) Masters Degree Programmeq EUCAARI (European Integrated project on Aerosol Cloud Climate and Air Quality Interactions)q EUSAAR (European Supersites for Atmospheric Aerosol Research).

The course involves 23 lectures per day, butthe main emphasis is on intensive groupwork, and writing a scientific report on the

group work after the course. The course isaimed to MSc and PhD students of atmos-pheric sciences. In the course the studentswill utilize 12 year aerosol, gas and meteoro-

logical data measured at Hyytil station tolearn some basic data treatment and dataanalysis methods.

The main tools are MATLAB and IDL pro-gramming language.Deadline for application submission to

this event has expired.

Conference Marie Curie iLEAPS,Feedbacks Land-Climate Dynamics Key GapsCurrent understanding of how integrated land ecosystematmosphere processes influence climate dynamicsFall 2008, La Badine, France.

Organizer: Dr. Nathalie de Noblet, CEA-CNRS, France

Marie Curie Actions are part of the EuropeanCommission initiatives to facilitate mobilityof researchers and Marie Curie iLEAPS ispart of this programme. A number of travelgrants are available for each event. Read cri-teria for eligibility, detailed information andupdates at

www.atm.helsinki.fi/ileaps/marie-curie-ileaps

MarieCurie

q

iLEAPS



Scienceon aerosols, clouds, precipitation, climate

ACPC The evolution of a joint initiativeRecognition of the potential climate impactsof anthropogenic aerosols has led to a largebody of research assessing the aerosols rolein the Earths radiative balance. Much less isknown about their effects on precipitation,and the consequences for the climate sys-tem and the water cycle.

This is also stressed in the 4th Assess-ment report of the Intergovernmental Panelof Climate Change (IPCC 2007, see Noone etal, in this issue). Existing knowledge and re-search needs have been summarized in thereport by the WMO/IUGG International Aero-sol Precipitation Science Assessment Group(IAPSAG; see Cotton and Levin, in this issue).

The initiative on Aerosols, Clouds, Pre-cipitation and Climate (ACPC) is intended todevelop an integrated research program toinvestigate the interactions and feedbacksamong aerosols, cloud processes, precipita-tion, and the climate system (ACPC BoulderSpecialist Workshop report, see Stevens at al,in this issue).

Ideas for a broad project to study thelinkages between aerosols, clouds, precipita-tion and climate had been circulating infor-

mally though the community for a fewyears, and in fact were part of the originaliLEAPS Science Plan.

They came into a clearer focus duringdiscussions at the 3rd iLEAPS Scientific Steer-ing Committee meeting in Boulder, Colo-rado, 2021 January 2006, where it also be-came obvious that better coordination withcurrent and proposed WCRP/GEWEX aero-sol-cloud-climate activities was warranted.

Just prior to the SSC meeting, DannyRosenfeld had given the presentation Ther-

modynamic responses to precipitationchanges as induced by surface and aerosolimpacts on cloud processesImplicationsfor the Earths energy budget and the hydro-logical cycleat the 18th GEWEX SSG meet-ing in Dakar, Senegal (913 January 2006),which stimulated lively discussions in thatcommunity.

Subsequently, a first outline describingthe initiative was presented at the joint IGBPand WCRP Steering Committee meeting inPune, India, (27 March 2006), where thespecial focus was to advance collaborationand discussion on common scientific ques-

tions and new initiatives between the twoprograms and also within ESSP.The discussions indicated that, while we

are far from understanding aerosol-climateconnections, there is already underused datathat can still provide new information.

This can be addressed by better coordination with current and proposed WCRP/GEWEX aerosol-cloud-climate activitiewithin GRP (GEWEX Radiation Panel), GCSS(GEWEX Aerosol and Cloud ResearchGEWEX Cloud Systems Studies), HAP (Hydro

logical Application Project), and other programs such as THROPEX (Predictability andmitigation of natural disasters) and GWSP(Global Water System Project).

But besides mining existing informationand gaining access to data from ongoingprograms, the importance of the impact oaerosols on precipitation patterns - andtherefore on the thermodynamic andradiative energy budget of the Earth - callfor an international, coordinated project odedicated field campaigns.

Following the joint IGBP/WCRP meetingin Pune, the scientific steering committees o

Anni Reissell



LEAPS, IGAC and GEWEX appointed ACPCepresentatives for each core project to pro-uce separate white papers describing thecientific issues of relevance to the foci of therojects.

After the 4th iLEAPS SSC meeting on 178 January 2007 in Wageningen, the Nether-

ands, a small ACPC planning group consist-ng of representatives from iLEAPS, IGAC andGEWEX was established.

The planning group representatives from

LEAPS are M.O. Andi Andreae, MarkkuKulmala and Daniel Rosenfeld, from IGACandro Fuzzi, Graciela Raga, and Colin

ODowd (also representing SOLAS), and fromGEWEX Tom Ackerman, Bill Lau, Ulrikeohmann and Pier Siebesma.

The group met at the end of March toompile a joint white paper and to plan aarger science workshop to take place in Oc-ober 2007. The joint white paper is pub-shed in iLEAPS Newsletter (iLEAPS Newslet-er 4/2007) and also available at the ACPC

website (http://www.ileaps.org/acpc/).This white paper formed the startingoint for the iLEAPS-IGAC-GEWEX ACPCpecialist Workshop, which was held 810

October 2007, at NCAR in Boulder, Colorado,USA. The meeting was hosted by GuyBrasseur, Director of Earth and Sun Systemsaboratory, and the National Center for At-

mospheric Research (NCAR).The workshop was sponsored also by

he European Network of Excellence on At-mospheric Composition Change (ACCENT).

The workshop was organized by iLEAPS,GAC (International Global AtmosphericChemistry) and GEWEX (Global Energy andWater Cycle Experiment), core projects ofGBP and WCRP.

In all, 57 participants attended from theUS (41 participants), Australia (1), Brazil (2),nland (2), France (1), Germany (1), Israel (2),aly (1), Mexico (1), Netherlands (1), Switzer-

and (2), and United Kingdom (2). The eventathered specialists from aerosol physicsnd chemistry, cloud dynamics and micro-hysics, atmospheric radiation, and climateynamics.

The workshop presentations (available athttp://www.ileaps.org/acpc/) were as follows:

Andi Andreae,Background for ACPC initiative, part I

Markku Kulmala,Background for ACPC initiative, part II

V. Ramanathan,Aerosol-Cloud-Climate Interactions:

Outstanding IssuesBill Cotton,Results of the WMO/IUGG internationalAerosol Precipitation Science AssessmentGroup (IAPSAG)

Graeme L. Stephens,Remote sensing of aerosol, cloud,precipitation, climate interactions:A-Train capabilities

Bjorn Stevens,Quantifying aerosol effects

on cloudiness, as mediated by rainSandro Fuzzi,Aerosols as CCN,IN and their effects on CDNC

Maria Assuno F. Silva Dias,Can the slowing of auto-conversionresult in increasing precipitation? part I

Danny Rosenfeld,Can the slowing of auto-conversionresult in increasing precipitation? part II

Brian Soden,

Aerosols, Precipitation andthe Energetics of the Climate System

Cynthia Twohy,How do Aerosol-Precipitation Interactionsaffect the Energetics of the Climate System

Bill Lau,What are aerosol-precipitationimpacts on the larger scales?Aerosol-climate and water cycle(precipitation, clouds, winds) interactions

Ulrike Lohmann,How do Aerosol-Precipitation Interactionsaffect the Energetics of the Climate System?

We submitted a proposal to the Interna-tional Space Science Institute, ISSI (http://www.issibern.ch/), to provide support for thefurther development of ACPC. The main taskof ISSI is to contribute to the achievement ofa deeper understanding of the results fromspace-research missions, adding value tothose results through multi-disciplinary re-search in an atmosphere of international co-operation.

Our proposal was approved, and theplanning group morphed into an ISSI Team,

getting together at ISSI in Bern, Switzerlandfor three meetings in 20089 to write scien-tific articles and an ACPC Science and Imple-mentation Plan. The first such meeting wasin 2830 January 2008 and the second onewill be on 79 October. Bjorn Stevens willjoin the Team at the second meeting.

The ACPC mailing list (email address [email protected]) was established in the be-ginning of 2008 and is aimed at those inter-ested in and working on issues related toaerosol-cloud-precipitation-climate interac-tions.

This mailing list was proposed at theACPC workshop in Boulder by JingfengHuang from the Rosenstiel School of Marineand Atmospheric Science (RSMAS), Univer-sity of Miami, Florida.

The aim is to strengthen the communi-cation among researchers from a variety ofdisciplines and to encourage collaborationson ACPC-related research worldwide.

Two moderators are taking care of themailing list:Jingfeng Huang and Amato Evanfrom the Cooperative Institute for Meteoro-logical Satellite Studies at the University ofWisconsin-Madison, Wisconsin, USA.

The purpose of this mailing list is to facili-tate discussions and debates on scientificquestions, to provide a means for the rapidexchange of each others publications withinthe ACPC community, to give informationabout upcoming events and conferences,and to establish a weblog.

By spreading the word quickly throughthe mailing list, it will help to stimulate re-search interests, attract global attention, andencourage potential collaborations.s

ain shower from a cumulonimbus praecipitionver downtown Helsinki as seen from the Kumpulaampus, University of Helsinki. The mast of themear III station can be seen at the right. Photo bymo Nousiainen.

Arrival of gust front associated with a decayingmesoscale convective system in the late morning,in Agoufou, Mali, 11 August 2006. Photo byFrancoise Guichard and Laurent Kergoat. CopyrightCNRM-GAME & CESBIO (CNRS).



IGBP and WCRP onACPC initiative

Kevin Noone1, Carlos Nobre2,John Church3and Ann Henderson-Sellers4

1. International Geosphere Biosphere Programme Secretariat,Royal Swedish Academy of Sciences, Stockholm, Sweden

2. Weather Research and Climatic Studies Center (CPTEC) ofthe National Space Research Institute (INPE),Cachoeira Paulista, So Paulo, Brazil

3. Antarctic Climate & Ecosystems Cooperative Research Centre (CRC) andAustralian Commonwealth Scientific and Industrial Research Organisation (CSIRO)Marine Reseach, Hobart, Tasmania, Australia

4. World Climate Research Programme,World Meteorological Organization, Geneva, Switzerland

Once again, the Fourth Assessment Reportof the Intergovernmental Panel on ClimateChange (IPCC) has identified aerosols, cloudsand precipitation as one of the largest un-certainties in our current understanding ofthe climate system. As shown in the Table 1,this same conclusion has extensive historicalbaggage.

One reason that aerosol and cloud proc-esses remain so uncertain is because theprocesses linking aerosol-cloud interactions,precipitation development, and the dynami-cal processes that drive convection are com-plex and tightly coupled.

This issue was also highlighted at a re-cent workshop in Sydney, Australia jointly or-ganized by the Global Climate ObservingSystem (GCOS), the World Climate ResearchProgramme (WCRP) and the InternationalGeosphere-Biosphere Programme (IGBP) asone that is central to the research and obser-vational strategies for all three organizations.

As with any complex issue, there is nosingle approach that by itself will lead torapid advances in understanding. Indeed theIPCC Assessments have always highlightedthe physics of convection and convectivecloud formation as being a serious chal-

lenge. Improving understanding of the importance of CCN couple these outstandingphysics questions with newer and equallychallenging chemistry ones.

In this regard, the Aerosols, Clouds, Precipitation and Climate (ACPC) initiative is avery exciting opportunity to gather togethethe breadth and depth of expertise that wibe needed to make progress in this area.

ACPC brings together scientists fromseveral IGBP and WCRP projects, led byiLEAPS and IGAC within IGBP and GEWEXwithin WCRP. This initiative will create a focufor a new and expanded community of sci

ProfessorKevin Nooneis Executive Directorof the International Geosphere Biosphere

Programme (IGBP) SecretariatProfessorCarlos Nobreis Chair ofthe IGBP Steering Committee

Dr.John Churchis Chair of the World ClimateResearch Programme (WCRP) Joint SteeringCommittee

ProfessorAnn Henderson-Sellerswas Directorof WCRP Joint Planning Staff in 20062007



ntists to look at all the various dynamical,microphysical and chemical processes thatnderlie ACPC.

As with any endeavor to address a diffi-ult issue, there are debates within the scien-

fic community about what issues are ofreatest importance in terms of determininghe climatic effects of aerosols and clouds,nd about how to prioritize research effortso address them.

Besides the inherent scientific difficultyf the subject, another reason that aerosolsnd clouds remain the dominant uncer-ainty in radiative forcing is because ourommunities are still working out the mostffective ways to collaborate on this issue.

This new constellation will enable a great

tep forward to be taken in our ability toook at these processes in an integratedashion, instead of within separate com-munities with different priorities and ap-roaches.

ACPC links directly with the joint IGBP-WCRP Atmospheric Chemistry and Climatectivities (AC&C) and with our joint climate

modeling work conducted by IGBPs AIMESnd WCRPs Working Group on Coupled

Modelling (WGCM).Beyond addressing key science issues, an

dditional positive result of this new initia-ve is the benefit it will have for the parentrogrammesIGBP and WCRPin terms ofhowing concrete ways in which the pro-rammes can pursue a closer working rela-onship across the board to address com-lex scientific issues of common interest.

IGBP and WCRP are fully supportive ofhe ACPC initiative, and are looking forwardo exciting progress and results. We are con-dent that because of the work done in

ACPC, the conclusions about uncertainty inhe next IPCC Assessment Report will beery different! s

Many factors currently limit our ability to project and detect future climate change.In particular, to reduce uncertainties further work is needed on the followingpriority topics:

q Estimation of future emissions and biogeochemical cycling (including sources andsinks) of greenhouse gases, aerosols and aerosol precursors and projections of futureconcentrations and radiative properties.

q Representation of climate processes in models, especially feedbacks associated withclouds, oceans, sea ice and vegetation, in order to improve projections of rates andregional patterns of climate change.

q Systematic collection of long-term instrumental and proxy observations of climatesystem variables (e.g., solar output, atmospheric energy balance components, hydro-

logical cycles, ocean characteristics and ecosystem changes) for the purposes of modeltesting, assessment of temporal and regional variability, and for detection and attribu-tion studies.

IPCC Second Assessment Report (1995)

Table 1. Compilation of statements from the summaries for policymakers

of the Second (SAR), Third (TAR) and Fourth IPCC Assessment Reports.

Since the Second Assessment Report, significant progress has been achieved in bettercharacterising the direct radiative roles of different types of aerosols. Direct radiativeforcing is estimated to be 0.4 W m2 for sulphate, 0.2 W m2for biomass burningaerosols, 0.1 W m2for fossil fuel organic carbon and +0.2 W m2for fossil fuel blackcarbon aerosols. There is much less confidence in the ability to quantify the total aerosoldirect effect, and its evolution over time, than that for the gases listed above. Aerosolsalso vary considerably by region and respond quickly to changes in emissions.

IPCC Third Assessment Report (2001)

IPCC Fourth Assessment Report (2007)

Anthropogenic contributions to aerosols (primarily sulphate, organic carbon, blackcarbon, nitrate and dust) together produce a cooling effect, with a total direct radiativeforcing of 0.5 [0.9 to 0.1] W m2and an indirect cloud albedo forcing of 0.7[1.8 to 0.3] W m2. These forcings are now better understood than at the time ofthe TAR due to improved in situ, satellite and ground-based measurements and morecomprehensive modelling, but remain the dominant uncertainty in radiative forcing.Aerosols also influence cloud lifetime and precipitation.

Decaying remnants of a cumulonimbus mammahat slowly drifted over eastern Helsinki, 3 Septem-er, 2007. Photo by Timo Nousiainen.



Bjorn Stevensis a Professor of dynamic meteorologyat the University of California Los Angeles. In 1996 hereceived his PhDfromColorado State University, andin collaboration with WilliamR. Cotton, his advisor, andGrahamFeingold he used and developed large-eddysimulation with explicit microphysics to elucidate themechanisms whereby precipitation suppresses thegrowth of the marine boundary layer. He was Post-doctoral Fellow of the Advance Study Programat the

National Center for Atmospheric Research, where heworked with Chin-Hoh Moeng and Peter Sullivan anddeveloped interests in both larger-scale modeling, andfield measurements.

His large-scale interests were nurtured byHumboldt Fellowship spent visiting the group of ErichRoeckner at the Max Planck Institute for Meteorologyin Hamburg Germany. His field interests were explored

further after arriving as an assistant professor at UCLAin 1999. At UCLA he developed and led the DYCOMSII (in collaboration with Don Lenschow and Gabor Valand the RICO(in collaboration with Robert RaubeCharles Knight and Harold Ochs) field studies. ThDYCOMS-II measurements placed the first definitivbounds on entrainment at the top of stratocumulustopped boundary layers as well as quantified the rateand effects of precipitation fromstratocumulus.

The RICOdata are providing new insights into throle precipitation plays in the development of shallowcumulus, and the possible role (or lack thereof) of thatmospheric aerosol. In recognition of his pioneerinadvances in understanding and modeling of cloudtopped boundary layersDr. Stevens was awarded th2002 Clarence Leroy Meisinger Award of the AmericaMeteorological Society.

Bjorn Stevens, the ACPC Planning Team and

participants of the ACPC workshop in Boulder

Aerosols, Clouds, Precipitation, Climate (ACPC): Outline for

a new joint IGBP/WCRP initiativeBackground

Environmental hazards in the form of air pol-

lution, floods and droughts affect a largeportion of the worlds population. Increasesin atmospheric aerosol associated withindustrialization have caused health-relatedproblems associated with worsening airquality, while floods and droughts result inmajor losses in lives and property, displacingmillions from their homes every year.

Recent studies suggest that the in-creased aerosol loading may have changedthe energy balance in the atmosphere andat the Earths surface [1] and altered the glo-

bal water cycle in ways that make the cli-mate system more prone to precipitation ex-tremes.

As yet, we do not fully understand howthe aerosol affects the development of pre-cipitation, nor the extent to which it affectsthe cycles of water and radiant energy in theclimate system as a whole. Hence, achievinga better understanding of how the aerosolaffects clouds and precipitation, and conse-quently large-scale circulations is not only amajor scientific challenge, but is also impor-tant to policy makers and stakeholders formaking decisions on mitigation strategies.

Over the past decades new measure-ments from satellite-based remote sensorshave revealed conspicuous associations

amongst aerosols, clouds and precipitation[2, 3]. In accord with expectations based onin situ measurements, clouds forming in apolluted environment appear to havesmaller droplets, which, in the absence ofother effects, may suppress the formation ofrain by shallow clouds [4].

Correlations between aerosol and cloudfraction and cloud liquid-water (both posi-tive and negative) have been demonstrated([5], and ref therein), with commensurate ef-fects on the amount of solar radiation re-flected back to space.

In deep clouds the low-level suppressionof warm rain appears to be associated withmodified microphysical pathways, enhancedice precipitation aloft, and invigoration of theconvection.

It appears that both the radiative andcloud-mediated aerosol effects induce largechanges in precipitation patterns, which inturn may change not only regional waterresources, but also the horizontal and verticaldistribution of diabatic heating that propelsthe regional and global circulation systemsthat constitute the Earths climate.

This has been the impetus for large observational, modeling, and theoretical effortthat have focused mainly on the radiative

components of these aerosol-cloud interactions and their impact on the climate system[e.g., 6]. While this has led to a much deepeunderstanding of the interactions betweenaerosols, radiation, and precipitation, muchremains still to be learned about theseradiative effects.

On the other hand, there has not yebeen a comparable research effort concerning the microphysical effects of aerosols onclouds and precipitation, and the consequences for the water cycle and large scale

circulation. Scientific understanding in thirealm therefore remains very incompletehampering our ability to make projections ofuture climate change and to propose mitigating actions.

Much of the energy that drives the climate system is infused into the troposphereby deep convective rain clouds. Howevethere are no quantitative measurements othe impact of the aerosol on deep convective clouds, their propensity to precipitatethe vertical distribution of heating, and thesubsequent modulation of circulation systems and rainfall distribution.



For this reason, and because the effectsf the aerosol on shallow cloud systems is aopic of much existing effort, it makes sense

o initially focus any proposal for a new pro-ram on questions that affect deep, precipi-ating convection. Such a program, whichwe call the Aerosols, Clouds, Precipitationnd Climate (ACPC) program, is outlinedelow.

A first step towards this program wasaken with a previous article in this Newslet-er [7]. The present paper outlines the scien-fic justification, fundamental questions, andeneral implementation concepts for ACPC.

Adetailed science and implementation planunder development by the ACPC planning

eam and will be presented in the near fu-ure.

Scientific Issues

Many of the important questions can beast in broad categorical terms:

q What is the relationship between surfacerain and cloud microstructure, i.e., thenumber of activated cloud droplets ornucleated ice crystals? To the extent thatthey exist, are such relationships modu-lated by meteorological conditions?

q Looking toward smaller scales, what is therelationship between cloud microstruc-ture and the ambient aerosol? How will

aerosol dynamics together with atmos-pheric chemistry affect cloud microphys-ics?

q If changes in the development of precipi-tation can be attributed to perturbationsto the aerosol, how do changes in thevertical structure of latent heating affectthe subsequent development of circula-tions systems on scales ranging from thatof the cloud or cloud-system to the re-gional and beyond?

q Likewise, to what extent do radiative per-

turbations (including those mediated bythe surface) attributable to the varyingphysicochemical and optical properties ofthe aerosol, impact circulation systemsacross these many scales?

q In closing the loop, to what extent dochanges in clouds, precipitation, and cir-culation systems regulate the distributionof the aerosol itself?

q And ultimately, how well can (or do) werepresent existing understanding of theabove respects in our theories and mod-els of the climate system?

It has proven difficult to extract a harmo-nized view of the relationships of the typearticulated above for a variety or reasons [8].

Foremost, in many situations we simply havenot been listening, which is to say that sig-nificant gaps in the observational record areapparent.

But even when we have been payingattention, it has been difficult to extract asignal, with confounding factors being: thebackground roar of the ambient meteorol-ogy; a narrow focus on individual clouds forwhich the noise of circumstance is oftenloudest; consonance between the back-ground aerosol and the meteorological con-

ditions making it difficult, and often impossi-ble, to isolate one from the other; and disso-nance amongst myriad processes embodiedby any categorical relationship.

As an example of this dissonance wenote that the relationship between cloudmicrostructure and rain may depend onmany factors, ranging from microscale proc-esses such as the local intensity of turbu-lence in the cloud, to larger-scale processessuch as the efficacy of cold-pool develop-ment, distribution of cloud-top heights, orthe relative humidity of the cloud environ-ment.

The SMOCC92 campaign in the Amazon BasinM O Andreae and D Rosenfeld with the research aircraftat Cruzeiro do Sul airport. Inthe back a deforestation fire with a pyrocumulus cloud building above it.



Likewise, the various ice-nucleation proc-esses and the myriad microphysical path-

ways via which rain forms in mixed-phaseclouds have thwarted attempts to quantita-tively relate the character of the aerosol tothe microphysical properties of glaciatedclouds, and surface rainfall.

Given the history of attempts to drawdemonstrable links of the type we seek, itseems natural to ask, what is new? To someextent we are more knowledgeable. In part,because the maturation of Earth System sci-ence has increased the discourse amongstdiverse intellectual communities, questions

such as ours are no longer the domain ofscientists drawn from a single discipline, butare now being embraced by a diverse com-munity.

This discourse has been facilitated andaccelerated by the relatively recent realiza-tion that climate change, and associated ef-fects on the hydrological cycle, are a prob-lem of immense importance. In the presentcontext these include experts in land-surfaceprocesses, aerosol and cloud physics, chemis-try, radiative transfer and remote sensing,fluid simulation, and large-scale modeling,among others.

Such intermingling is giving birth to newexperimental designs. In particular our un-

derstanding of the aerosol, and its relation-ship to the microstructure of warm-phaseclouds has advanced significantly [9, 10] overthe past decade, both as a result of advancesin aerosol instrumentation, but also as aproduct of the deployment of such instru-ments in a series of field studies spanningthe globe. Some of these experiments havealso helped expand our imagination interms of where to look for relationships be-tween aerosols, clouds and precipitation.

Deliberate burnings in near tropical re-

gions such as the South African Savannah,rain forests of the Amazon, or the sugar canefields on subtropical islands provide oppor-tunities for exploring these questions; theseare natural laboratories in which one canhope, for the first time, to separate the ef-fects of meteorology and the aerosol onlarge scales.

The empirical and conceptual progressin our understanding parallels other ad-vancements in measurement science. Tre-mendous advances in remote sensing, usingboth space- and surface-based sensors,means that characterization of cloud micro-

physical and optical properties fromspaceborne active and passive sensors, together with surface networks of multi-parameter and multi-wavelength radars, lidarand microwave radiometers, now provide anunprecedented opportunity to quantifyboth cloud structure, water content, and surface rain rates at a range of scales.

Such opportunities are amplified by a

number of other developments: the emergence of new classes of autonomous vehcles (unmanned aerial vehicles, UAVs); newradar-scanning technologies, such as phasedarrays; a satellite observing system in theform of the A-Train (the satellite constellation; e.g., CALIOP/CALIPSO and CPRCloudSat), providing data of unprecedentedquality, whose capabilities are unlikely to beimproved upon in the coming decade; anew fleet of modern aircraft capable of deploying large-payloads with great endurance

and at tremendous altitude; and modelingsimulation/assimilation systems capable osynthesizing diverse and rich data streamon the one hand, and parsing the conceptual landscape on the other.

These developments lead to new opportunities. For instance the emergence of strategies for untangling aerosol perturbationfrom meteorological ones, e.g., by focusingon areas well downwind of deliberately sefires.

The articulation of questions, such as

how do clouds and precipitation modify thesize and composition of aerosol? Canchanges in profiles in the convective heating/drying, long measured by soundingarrays, be related to aerosol effects? Howdoes aerosol modification of the surfaceheat budget affect convection and precipitation?

Likewise, advancements in measuremenscience offer the possibility to statisticallycharacterize differences in the life-cycle oconvective complexes as a function of theaerosol, or to begin thinking about parsimo

nious descriptions of the joint distribution oaerosol properties and cloud microphysicastructure.

Strategy

To realize these opportunities and makeprogress in understanding physical relationships among aerosols, clouds and precipitation we propose a coordinated internationaeffort encompassing a strategy that embodies the following elements: (i) isolation ofand focus on systems where there arestrong indications of aerosol effects on deep

MODIS image of mixture of smoke, haze, fog and bright cloudstrapped by the topography of the Indo-Gangetic Plain.



onvection; (ii) an emphasis on statisticalharacterizations of aerosol-cloud-precipita-on interactions; (iii) the development of ap-roaches that leverage past and ongoing ac-vities; (iv) thorough integration of modelingnd observational activities; (v) a hierarchicalpproach to both modeling and data collec-on/analysis.

ystems with strong indications oferosol effects on deep convection

em (i) above, focuses attention on tropicalr monsoonal regions over the Amazon, Af-ca, South and East Asia and the Maritime

Continent. These regions are known for veryigh anthropogenic aerosol loading from

mega-cities and industrial complexes, and/oriomass burning aerosols, and dust, which inhe case of the latter can sometimes also beraced to land-use practices.

Regions of biomass burning, particularlywithin a season, are a particularly attractivearget for more intensive study; because, tohe extent fires reflect seasonal (rather thanay-to-day) variations in the meteorology,

one stands a chance of decorrelating thebiomass burning aerosol and the meteorol-ogy.

In this respect the Amazon is particularlyattractive as past experience (The LargeScale Biosphere-Atmosphere Experiment inAmazonia, LBA field study) suggests thatmeteorology and aerosol loading may in-deed decorrelate on subseasonal timescales[4].

Already planned activities in South andEast Asia such as JAMEX (Joint Aerosol-Monsoon Experiment [11) as part of theplanned Asia Monsoon Year (AMY 2008-2012), and the recent AMMA2006 (AfricanMonsoon Multidisciplinary Analyses [12])program over West Africa also provideunique opportunities that should be furtherdeveloped prior to renewed commitment ofresources elsewhere.

While AMMA concentrated mainly onthe radiative and dynamic aspects andSMOCC [4] on the microphysical impacts ofthe aerosols, an integrated approach isneeded to make further progress.

Statistical characterizations ofaerosol-cloud-precipitation interactions

The second element of the strategy, whichemphasizes the development of statisticalrelationships, can be implemented by stud-ies that consider extended ranges in spaceand/or time, such as remote sensing studiesat basin scales and long term measurementsat potentially sensitive locations worldwide.

Statistical relationships are most likely toarise from large samples in homogeneous

conditions, thus favoring observational re-gions that maximize the observational areawhile minimizing the heterogeneity in boththe underlying surface and the expectedcomposition of the aerosol.

Of the monsoon and tropical regionsdiscussed above, the Amazon and EquatorialAfrica satisfy this constraint better, both be-cause of more homogeneous land-surfacefeatures, but also because of the perceptionthat they are more locally forced, i.e., large-scale factors associated with the reorganiza-tion of monsoonal circulations are still poorlyunderstood [13].

Passage of a line of low-level cumulus clouds on the Mont Hombori, in the morning of 16 August 2005. These clouds are the last remnants ofan MCS which propaged throughout the previous night. Photo by Francoise Guichard and Laurent Kergoat. Copyright CNRM-GAME & CESBIO (CNRS)



Also, past experience suggests that it isimportant to first evaluate the likelihood ofobservations being able to test a particular

statistical hypothesis. Such an evaluation iscritical to both the framing of the hypoth-eses, and the design of the observationalnetwork intended to test them. Hence initialwork incorporating simulation studies ofcases from past, and ongoing, field workshould focus on such an evaluation.

Preparatory work of this type may use-fully be performed in the context of theGEWEX Cloud Systems Studies (GCSS) work-ing group activities, although it could also becarried out by individual groups.

Development of approaches thatleverage past and ongoing activities

The importance of leveraging past and on-going work is essential to both the observa-tional and the simulation components of theACPC program.

In terms of simulations, GCSS has a richhistory of using observations to evaluatemodels and, in cases where existing observa-tions have been insufficient, designing entirefield campaigns (for instance the phase twoof the Dynamics and Chemistry of MarineStratocumulus, DYCOMS-II and Rain in Cu-

mulus over the Ocean, RICO) centered onspecific questions that emerge from themodeling.

In terms of ongoing and past field work,AMMA2006 provides a unique opportunityto explore how enhancements to the obser-vational network better constrain tropicalclouds.

Likewise, AMY2008-2012 provides newchances to test components of an emergingexperimental strategy, and perhaps, throughmodest augmentation of the alreadyplanned resource deployment, begin devel-oping the basis for better exploring the af-fects of the aerosol on deep convection inthe context of the Asian Monsoon [14].

Apart from the deployment of additionalinstruments during the AMY2008-2012, oneidea that addresses many aspects of theabove, would be to use GCSS to developcase studies from the AMMA2006 orAMY2008-2012 field data, with an eye to-ward better framing the issues for a possiblefuture Amazon field study.

So doing would ensure the thorough in-tegration of simulations and observations,thereby addressing both the third andfourth elements of the strategy outlinedabove.

Thorough integration of modelingand observational activities

The integration of models and measurements also places demands on the modelsThe physics of current models, particularlygeneral circulation models (GCM), inadequately (if at all) represent relevant processes for aerosol effects on precipitation.

Traditionally, cloud microphysical processes are only included in GCM parameterizations of stratiform clouds but not in convective clouds [15]. Also, most GCMs onlyallow one convective cloud type per grid ceinstead of the whole spectrum of convectiveclouds.

Options to be exploited include the development and implementation of differen(more detailed) representations of parameterized convective clouds, the multiscalemodel framework (MMF), and global (or verylarge-scale) cloud resolving models.

In terms of the latter, the emphasis onAMMA2006 by the Cascade project (at theUniversity of Reading), provides a unique opportunity to explore issues pertaining toaerosols, clouds, and precipitation in the context of very large-scale cloud resolvingmodeling studies.

Smallish late-summer cumulonimbus as seen from Viikki, Helsinki, Finland, 30 August 2007. Photo by Timo Nousiainen.



1. Denman KL, Brasseur G, Chidthaisong A, Ciais P, CoxPM, Dickinson RE, Hauglustaine D, Heinze C, Hol-land E, Jacob D, Lohmann U, Ramachandran S, daSilva Dias PL, Wofsy SCand Zhang X2007. Cou-plings between changes in the climate systemandbiogeochemistry. In Climate Change 2007: ThePhysical Science Basis. Contribution of WorkingGroup I to the Fourth Assessment Report of the

Intergovernmental Panel on Climate Change. Solo-mon S, Qin D, Manning M, Chen Z, Marquis M,Averyt KB, Tignor M and Miller HL (eds.). Cam-bridge University Press, Cambridge, United King-domand New York, NY, USA.

2. Lin JC, Matsui T, Pielke Sr RA and Kummerow C2006. Effects of biomass burning-derived aerosolson precipitation and clouds in the Amazon basin:asatellite-based empirical study. Journal of Geo-physical Research 111, D19204, doi: 10.1029/2005JD006884.

3. Rosenfeld D1999. TRMM observed first direct evi-dence of smoke fromforest fires inhibiting rainfall,Geophysical Research Letters 26, 31053108.

4. Andreae MO, Rosenfeld D, Artaxo P, Costa AA, FrankGP, Longo KM and Silva-Dias MAF 2004. Smokingrain clouds over the Amazon. Science 303, 13371342.

5. Myhre G, Stordal F, Johnsrud M, Kaufman YJ,Rosenfeld D, Storelvmo T, Kristjansson J E, BerntsenTK, Myhre A and Isaksen ISA 2007. Aerosol-cloudinteraction inferred fromMODISsatellite data andglobal aerosol models. Atmospheric Chemistry andPhysics 7, 30813101.

6. Ramanathan V, Chung C, KimD, Bettge T, Buja L,Kiehl JT, Washington WM, Fu Q, Sikka DRand WildM 2005. Atmospheric brown clouds:Impacts onSouth Asian climate and hydrological cycle. Pro-ceedings of the National Academy of Sciences ofthe United States of America 102, 53265333.

7. iLEAPS-IGAC-GEWEXjoint white paper on Aerosols,Clouds, Precipitation and Climate Initiative (ACPC).iLEAPSNewsletter 4, 5253.

8. IAPSAG2007. WMO/IUGG International AerosolPrecipitation Science Assessment Group (IAPSAG)

Report:Aerosol Pollution Impact on Precipitation:AScientific Review. World Meteorological Organiza-tion, 482 pp.

9. Andreae MO and Rosenfeld, D 2008. Aerosol-cloud-precipitation interactions. Part 1. The natureand sources of cloud-active aerosols, Earth ScienceReviews, in press.

10.McFiggans G, Artaxo P, Baltensperger U, Coe H,Facchini MC, Feingold G, Fuzzi S, Gysel M,Laaksonen A, Lohmann U, Mentel TF, Murphy DM,ODowd CD, Snider JRand Weingartner E2006.The effect of physical and chemical aerosol proper-ties on warmcloud droplet activation. Atmos-pheric Chemistry and Physics 6, 25932649.

11. Lau KM, Ramanathan V, Wu G-X, Li Z, Tsay S-C, HsuC, Sikka R, Holben B, Lu D, Tartari G, Chin M,Koudelova P, Chen H, Ma Y, Huang J, Taniguchi Kand Zhang R2008. The Joint Aerosol-Monsoon Ex-periment (JAMEX):a new challenge for monsoonclimate research. Bulletin of the American Mete-orological Society 89.

12.Redelsperger J-L, Thorncroft CD, Diedhiou A, LebelT, Parker DJ and Polcher J 2006. African MonsoonMultidisciplinary Analysis: An International Re-search Project and Field Campaign. Bulletin of the

American Meteorological Society 12, 17391746,doi:10.1175/BAMS-87121739.13.Bell TL, Rosenfeld D, KimK-M, Yoo JM, Lee MI and

Hahnenberger M 2008. Midweek increase in USsummer rain and stormheights suggests air pollu-tion invigorates rainstorms. J. Geophysical Research107, D20, 8072, doi: 10.1029/2001JD000225.

14.Lau KM and KimKM 2006. Observational relation-ships between aerosol and Asian monsoon rainfall,and circulation. Geophysical Research Letters 33,L21810, doi: 10.1029/2006GL027546.

15.Lohmann U and Feichter J 2005. Global indirectaerosol effects: a review. Atmospheric Chemistryand Physics 5, 715737.

Hierarchical approach to bothmodeling and data collection/analysis

nally, the development of a systematicallymulti-tiered, or hierarchical approach is ofaramount importance.

In terms of the modeling this involveshe careful design of test cases that link theull range of relevant models: (1) detailed

hysicochemical models of aerosol-cloud in-eractions (perhaps in parcel models); (2) mi-rophysical models of the precipitation for-

mation process; (3) fine-scale models that ex-lore cloud and boundary layer-scale inter-ctions associated with the effects of ahanging aerosol on cloud radiative or mi-rophysical properties; (4) cloud-system re-olving models that can explore the effectsf aerosol mediated changes to the radiativeorcings and/or precipitation on the scale ofoud systems; and (5) regional or global

cale models capable of both feeling thelobal constraints that may restrict the ef-ects of perturbations seen on smaller scales,nd extending the effects of such per-urbations remotely (through teleconnec-ons).

In terms of observations, a multi-tieredpproach means that the observationaltrategy must be refined through the explo-ation of existing data-sets, particularly fromwhole range of satellite sensors and con-

olidated meteorological data sets (e.g.,AMMA2006), strategic partnerships in ongo-ng or planned experiments (e.g., JAMEX/AMY, but ongoing plans for a year of tropicalonvection provide another opportunity),nd the development of base-line longer-erm measurements in an area targeted formore intensive study.

Additionally, periods of more intensivetudy should be repeated in two or moreeasons and in ways that afford the bestpportunity to extrapolate local findings toegional and even global scales using theurrent generation of earth observing satel-tes.

In this context advanced data assimila-on techniques are essential, which in prac-ce means active collaboration with the Eu-opean Center for Medium Range Weatherorecasting (ECMRF), whose state of the artata assimilation system is essential to inte-rated assessment activities spanning a

wide range of scales.

Summary

We encourage the development of a newinitiative focused on quantifying aerosol-cloud-precipitation interactions in regions ofdeep tropical convection.

Such an initiative should build on ongo-ing and past activities, and include long-term monitoring in strategic locations (per-haps the Amazon), the enhancement of ob-servational networks for already plannedfield studies, further analysis of existing andemerging data sets, coordinated modelingstudies that span a range of scales (perhapsin the context of GCSS), building toward aperiod of more intensive field work span-ning one or more seasons. Preliminary worksuggests that the Amazon has many fea-tures that would lend itself well to such astudy.

At the very least, such a study stands to

significantly advance our understanding ofdeep, precipitating, convective systems, longa meteorological enigma. There is also rea-son to believe that such an effort could pro-vide bounds on the susceptibility of suchsystems to changes in the atmospheric aero-sol; both would be a great leap forward.

The transfer of knowledge gained fromsuch an effort to regional weather forecast-ing centers to improve weather and climateprediction will be of substantial benefit tosociety. s

[email protected]



William R. Cotton1and Zev Levin21. Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, USA2. Department of Geophysics and Planetary Science, Tel Aviv University, Tel Aviv, Israel

This is a summary of the report by the WorldMeteorological Organization (WMO) and theInternational Union of Geodesy and Geo-physics (IUGG) International Aerosol Precipi-tation Science Assessment Group (IAPSAG)entitled Aerosol pollution impact on precipi-tation: a scientific review[1].

Increasing concentrations of anthropo-genic aerosol particles have an effect on theamount as well as the spatial and temporal

distribution of clouds and precipitation af-fecting the hydrological cycle. The complexinteractions between meteorological param-eters, aerosols, cloud microphysics and dy-namics make it difficult to assess the effecton precipitation change and climate.

Clouds are known to play a major role inclimate through their direct interactions withsolar radiation. In addition, precipitation fromclouds is the only mechanism that replen-ishes ground water and completes the hy-drological cycle. Changes in either theamounts and/or the spatial and temporaldistribution of rainfall will have dramatic im-

pacts on climate and on society. Increases ordecreases in rainfall in one region could af-fect rainfall downwind. Similarly, changes inrainfall distribution will strongly affect semiarid regions that are of dire need of water.

Among the factors that could contributeto cloud and rain modification are the ef-fects of air pollution from various sourcessuch as urban air pollution and biomassburning. In 2003 the WMO and the IUGG

recognized the potential danger from sucheffects and passed resolutions aimed at fo-cusing attention to this issue. As a follow upto this resolution the WMO formed an inter-national forum composed of a number ofexperts to review the state of the scienceand to identify areas that need further study.

The IAPSAG panel was chaired by ZevLevin and William Cotton served as vicechair. The manuscript is 482 pages and wasprepared by 13 lead authors, 27 contributors,and 17 reviewers. The report has now gonethrough a technical editing stage and is be-ing published by the WMO (Fig. 1). During

Aerosol pollutionimpact on precipitationthe three-year process of compiling this report, Peter Hobbs who served as the originachair of the panel, Yoram Kaufman whoserved as a lead author, and Brian Ryan whowas one of the reviewers passed away.

The report begins with an overviewchapter of the principles of cloud and precipitation formation. This is followed by achapter on the sources and nature of atmospheric aerosols, and then followed by a

chapter on the distribution of aerosols, including their transport, transformation, andremoval. There is also a chapter on in situand remote sensing techniques for measuring aerosols, clouds, and precipitation andtwo chapters that review the state of knowledge on the effects of pollution and biomass aerosols on clouds and precipitationone from an observational perspective andthe other a modeling perspective. These arefollowed by a chapter on parallels and contrasts between deliberate cloud seeding andaerosol pollution effects. A summary andconclusions is given in the final chapter.

Figure 1. Thunderclouds protruding abovevast smoke cover in the Amazon Basin inBrazil near the Bolivian border (lat. 12.5 S,

long. 64 W). Photograph taken during thespace shuttle mission (STS 026-43-080) in1988. [Suggested cover photo for IAPSAGreport, photo not yet approved by WMO].



WilliamR. Cottonis a Professor in the Department ofAtmospheric Science, Colorado State University, FortCollins, Colorado. He holds a BSin mathematics and aMSc in atmospheric science, both fromthe State Uni-versity of New York, Albany, in 1964 and 1966, respec-tively. He obtained a PhDdegree in meteorology fromPennsylvania State University in 1970. He is a Fellow ofthe American Meteorological Society and the Coop-erative Institute for Research in the Atmosphere (CIRA)at Colorado State University. He has published over160 papers in peer-reviewed journals, eight chaptersin books, authored one book, and co-authored twobooks. He has supervised 36 PhDand 38 MSc stu-dents. Prof. Cotton was the Vice-Chair of the WMO/IUGGInternational Aerosol Precipitation Science As-sessment Group.

Zev Levinis a Chair Professor in Atmospheric Physicsat Tel Aviv University where he has been since 1971.He received his PhDfromthe University of Washing-ton in 1970 and spent one year at University of Cali-fornia, Los Angeles (UCLA), working on experimentalcloud physics. He was the Chairman of the Depart-

ment of Geophysics and Planetary Science, then theDean of Research and Vice President for Research andDevelopment of Tel Aviv University. He was nominatedas the first director of the newly formed Porter Schoolof Environmental Studies of Tel Aviv University. Prof.Levin is the President of the International Commissionon Clouds and Precipitation, ICCP. He is the author ofabout 150 peer reviewed papers. He was the Chair ofthe WMO/IUGGInternational Aerosol Precipitation Sci-ence Assessment Group.

It is concluded that both observationsnd modeling studies show that pollutionerosols increase cloud drop concentrationsnd reduce average drop size. However, theffects on precipitation on the ground are

much more difficult to quantify as once therecipitation process is altered, the dynamicsf clouds and mesoscale systems also

hange in a nonlinear way. Thus the effectsf aerosols on precipitation become muchess predictable.

The clouds exhibiting the strongest sig-al are orographic clouds where both obser-ations and modeling studies suggest anppreciable reduction in precipitation byerosol pollution. These clouds are more sus-eptible to aerosol pollution owing to their

modest liquid water contents, the limitedme that drops and ice particles stay withinhem, and if they are downwind of majorollution sources their temporal and spatialersistence makes them highly vulnerableo pollution. However, orographic precipita-on downwind of polluted urban regions

may also be modified by changes in urbanand-use (e.g. urban heat island, changes inurface roughness) and thus both increasesr decreases in precipitation are possible.

In the case of deep convective clouds,models give mixed evidence whether aero-ol pollution increases or decreases precipi-ation. For less intense cumulonimbusouds, aerosol-pollution appears to decrease

recipitation. For more intense multicelltorms, the suppression of warm cloud pre-pitation processes can lead to more super-ooled water thrust aloft where it can freeze,hereby releasing more latent heat of freez-ng and increasing the storm intensity andainfall. However, the situation gets moreomplicated when one considers the im-acts of secondary convection forced byold-pools. There, increased primary stormntensities can lead to weaker or strongerecondary convection and therefore increase

r decrease rainfall on the ground depend-ng on stability, windshear, and interactionswith larger-scale circulations like sea-breezer urban heat island circulations.

In the immediate vicinity of polluted ur-an areas, observations suggest increased

precipitation downwind of cities in several

regions. Modeling studies, however, indicatethat urban land-use is the dominant sourceof those precipitation anomalies. Aerosolpollution, however, can still have importantimpacts but those impacts will vary depend-ing on the background aerosol supply andprobably the specific locations of the urbancenters relative to local physiography (Fig. 2).

Atmospheric general circulation models(AGCMs) coupled to mixed-layer oceanmodels suggest that the direct and indirecteffects of aerosols can cool ocean sea sur-

face temperatures and thereby slow downthe hydrological cycle in contrast to green-house warming. The implication is that pol-lution aerosols could have far greater conse-quences to precipitation than previouslythought and that there are many knowl-edge gaps and uncertainties related to aero-sol pollution effects on precipitation. s

gure 2.MODIS (Moderate Resolution Imagingpectroradiometer) satellite image of a dust stormteracting with a cold front over the Mediterra-ean. Image taken on January 28, 2003 during thest flight of the Colombia space shuttle. Photo

om the National Aeronautics and Space Adminis-ation (NASA).

[email protected]

1. Levin Zand Cotton WR2007. Aerosol pollutionimpact on precipitation:a scientific review. WMO/IUGGInternational Aerosol Precipitation ScienceAssessment Group (IAPSAG), pp 482.



V. Ramanathan is Distinguished Professor of

Atmospheric and Climate Sciences and the foundingdirector of the Center for Clouds, Chemistry andClimate at the Scripps Institution of Oceanography,University of California, San Diego, California, USA. Inthe mid 1970s he discovered the greenhouse effect ofCFCs and numerous other manmade trace gases.Among others, he has shown that the greenhouseeffect can be monitored fromspace, and clouds havea large natural cooling effect on the planet. He has led

Why is Earths albedo 29%and was it always 29%?

Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, USA

V. Ramanathan

the Indian Ocean Experiment (INDOEX) with Dr P

Crutzen; the project discovered the wide spread SoutAsian Atmospheric Brown Clouds (ABCs). He currentlchairs the United Nations Environment Programm(UNEP) sponsored ABCProject and the USacadempanel that provides strategic advice to the USClimatChange Science Program. His current research focuseon the use of miniaturized instruments on unmanneaircraft to understand long range transport of aerosoand how they regulate the planetary albdo.

Earth scientists have elegant and testabletheories for the evolution of CO2 and other

greenhouse gases over the Earths history.Earth scientists can also explain the amountand distribution of the dominant naturalgreenhouse gas, water vapor, in the atmos-phere from thermodynamics and dynamics.

Using Newtonian dynamics, scientistscan account for the large scale circulation ofthe atmosphere. Given the large scale circu-lation, scientists can understand the geo-graphical locations of deserts, rainforests, etc.

From the solar insolation at the top-of-the-atmosphere, these theories enable re-

searchers to determine the global meanclimate of the Earth-atmosphere system, butfor one missing link. Earth scientists have nosound physical basis or theory for constrain-ing the influence of clouds on the amountof solar radiation reflected by the planet, theplanetary albedo.

Modern day satellites have constrainedthe Earths albedo to be about 29% (2%).The albedo of the Earths clear sky region isabout 15% (2%). Thus the presence ofclouds enhances the albedo of a cloud-freeEarth by about a factor of about two, from15% to 29%. Why is the lack of a theory for

constraining the role of clouds in planetaryalbedo a major problem and why is this

relevant to iLEAPS Aerosols, Clouds, Precipita-tion, Climate initiative?

I The albedo questions

Why is the global albedo about 29%?

Two following examples illustrate why this isan important question. A global albedo of32% would plunge the Earth into a climatesimilar to that of the last ice-age; while analbedo of 27% would be comparable to aseven-fold increase in the CO2 concentration,

close to the values required to bring theplanet to the warm cretaceous. Given thisstate of the field, and given the fact thatclouds exert a large global cooling effect(about 15 to 20 W m2 globally), scientistsneed to understand the processes that con-strain the Earths albedo to be around 29%.

There is no obvious physical constraintfor the albedo to be 29%. For example, isthere a microphysical or optical constrainton the cloudy sky albedos to limit the plan-etary albedo to be about 29%? There is nosuch constraint, because for an overcast skywith very deep clouds, the limit on the

albedo is 70% to 80%, as indeed, observedover the deep cumulonimbus regimes o

the western Pacific warm pool or othewarm oceans in the tropics.

Was the global albedo always 29%?

Most if not all geological and geochemicastudies on the evolution of the Earthclimate have implicitly or explicitly assumedthe albedo to be 29%. While some haveaccounted for ice sheet and land surfacevariations on the albedo, the clouds haveimplicitly been assumed to be the same atoday. Is there any justification for this assumption?

The albedo of our neighboring planeVenus is about 70% or more, while that oMars, the other neighboring planet, is abou17% (Fig. 1). Venus is overcast, i.e, 100%cloud-covered but Mars is nearly cloud-freeIt is remarkable that although the solar insolation at top-of-the-atmosphere of Venus isabout twice as large as that on Earth, theactual solar energy absorbed by Venus isslightly less than that of Earth.

Clearly, clouds and albedo feedbackhave played a major role in the evolution othe climates of these three planets. There i



ne hypothesis, however, which proposeslbedo-climate feedback as a regulator ofhe planetary climate. The GAIA hypothesis1] invokes black daisies (greenhouse gases)nd white daisies (clouds and aerosols [2])

Why is this issue relevant to iLEAPS issuef aerosol-cloud-precipitation interactions?

While the atmospheric circulation deter-mines the location and extent of clouds andwater content, aerosols are the nuclei foroud drops and thus determine the size

nd number distribution of cloud drops. Theerosol properties, in turn, are determined byhe chemistry (e.g. oxidation of sulfur diox-de) and the biology (dimethyl sulfide andrganics). The aerosol-cloud interactions inurn determine the precipitation efficiency ofouds and thus regulate lifetimes and spa-al extent of clouds. All of these parameters

ncluding the aerosol concentration andomposition undergo significant temporalminutes to years) and spatial (meters tolanetary scales) variations.

It is remarkable that general circulationimate models (GCMs) are able to explainhe observed temperature variations duringhe last century solely through variations inreenhouse gases, volcanoes and solar con-tant. This implies that the cloud contribu-on to the planetary albedo due toeedbacks with natural and forced climatehanges has not changed during the last00 years by more than0.3%; i.e, the cloudorcing has remained constant within1 W

m2. If indeed, the global cloud propertiesand their influence on the albedo are thisstable (as asserted by GCMs), scientists needto validate this prediction and develop atheory to account for the stability.

Largest source of uncertainty inpredict-ing anthropogenic climate change.

The link between aerosols and cloud albedodetermines the so-called indirect effect ofanthropogenic aerosols. Many models andsome field observations, for example, the In-dian Ocean Experiment (INDOEX [3]) hasshown that an increase in anthropogenicaerosols can nucleate more cloud drops andenhance the cloud albedo and lead to acooling effect.

The IPCC 2007 report [4] shows that thiscooling effect may be large enough to offset50% of the radiative heating due to thebuild up in greenhouse gases. This indirecteffect is acknowledged to be the largestsource of uncertainty in understanding thehuman impact on the global climate.

II Cloud Radiative Forcing:Regional Puzzles

Cloud radiative forcing

Cloud radiative forcing (CRF) is defined asthe difference between the radiation budget(net incoming solar radiation minus the out-going long wave) over a cloudy (mix of clearand clouds) sky and that over a clear sky. Ifthis difference is negative clouds exert acooling effect, while if it is positive, it denotesa heating effect. Five-year average of thecloud radiative forcing [5] is shown in Fig. 2.The global average forcing is about 15 to20 W m-2 and thus clouds have a majorcooling effect on the planet. Two major puz-zles posed by this data are germane to thetopic of this paper.

gure 2. Net cloud radiative forcing from the Earthadiation budget experiment, ERBE [5].

650 Wm-2

341 Wm-2

150 Wm-2

240 Wm-2

288 K125 Wm-2

220 K

200 Wm-2

700 K

70 %

29 %

18 %

pace Images http://solarviews.com/ Photo copyrights:NASA/MODIS/USGS and Calvin J. Hamilton

Figure 1. Solar radiation budget of Venus, Earthand Mars. For each of the 3 planets, the numbernext to the arrow pointing towards the planetshows the incoming solar radiation; the numbernext to the arrow pointing away from the planetshows the percent of incoming solar radiation thatis reflected by the surface-atmosphere system, i.e,albedo; the numbers in red below each planet arethe absorbed solar radiation and the surface tem-perature. All of the numbers are globally-annually-

diurnally averaged values. The figure is adaptedfrom Ramanathan 2006 [8].



1. Lovelock JE1979. Gaia:A new look at life on EarthOxford University Press. ISBN 0192862189.

2. Charlson RJ, Lovelock JE, Andreae MOand WarreSG1987. Oceanic phytoplankton, atmospheric suphur, cloud albedo and climate. Nature 326, 655661.

3. Ramanathan V, Crutzen PJ, Lelieveld J, Mitra AAlthausen D, Anderson J, Andreae MO, Cantrell WCass GR, Chung CE, Clarke AD, Coakley JA, CollinWD, Conant WC, Dulac F, Heintzenberg JHeymsfield AJ, Holben B, Howell S, Hudson J

Jayaraman A, Kiehl JT, Krishnamurti TN, Lubin DMcFarquhar G, Novakov T, Ogren JA, Podgorny IAPrather K, Priestley K, Prospero JM, Quinn PKRajeev K, Rasch P, Rupert S, Sadourny R, SatheeshSK, Shaw GE, Sheridan P and Valero FPJ 2001. ThIndian Ocean Experiment: an integrated assessment of the climate forcing and effects of theGreat Indo-Asian Haze. Journal of GeophysicaResearch Atmospheres 106, 28,37128,399.

4. Forster P, Ramaswamy V, Artaxo P, Berntsen T, BettR, Fahey DW, Haywood J, Lean J, Lowe DC, MyhreG, Nganga J, Prinn R, Raga G, Schulz M and VaDorland R 2007. Radiative forcing of climat

change (pp. 129234). In Climate Change 2007The Physical Science Basis. Contribution of WorkingGroup I to the Fourth Assessment Report of thIntergovernmental Panel on Climate Change. Solomon S, Qin D, Manning M, Chen Z, Marquis MAveryt KB, Tignor M and Miller HL (eds.Cambridge University Press, Cambridge, UniteKingdomand New York, NY, USA.

5. Ramanathan V, Cess RD, Harrison EF, Minnis BarkstromBR, Ahmad Eand Hartmann D1989Cloud-radiative forcing and climate:results fromthe Earth radiation budget experiment. Scienc

243, 5763.6. Weaver CP and Ramanathan V1997. Relationshipbetween large-scale vertical velocity, static stabilityand cloud radiative forcing over Northern Hemsphere extratropical oceans. Journal of Climate 1028712887.

7. Crutzen PJ 2002. Analysis of the Gaia hypothesis aa model for climate/biosphere interactions. GAIA2/2002, 96103.

8. Ramanathan V 2006. Global dimming and itmasking effect on global warming (Bjerkenelecture, AGU 2006 fall meeting). EOS, TransactionsAmerican Geophysical Union 87(52), Fall Meeting

Supplement, Abstract A23D01.

The enormous cooling effect of extra-tropical storm track cloud systems

Extra-tropical storm track cloud systems pro-vide about 60% of the total cooling effect ofclouds [6]. The annual mean forcing fromthese cloud systems is in the range of 45 to55 W m2 and effectively these cloud sys-tems are shielding both the northern and

the southern polar regions from intenseradiative heating. Their spatial extent to-wards the tropics moves with the jet stream,extending farthest towards the tropics(about 35 deg latitude) during winter andretreating polewards (polewards of 50 deglatitude) during summer. This phenomenonraises an important question related to pastclimate dynamics.

During the ice age, due to the large polarcooling, the northern hemisphere jet streamextended more southwards. But have the

extra tropical cloud systems also movedsouthward? The increase in the negativeforcing would have exerted a major positivefeedback on the ice age cooling. There is acurious puzzle about the existence of thesecooling clouds. The basic function of the ex-tra tropical dynamics is to export heatpolewards.

While the baroclinic systems are efficientin transporting heat, the enormous negativeradiative forcing (Fig. 2) associated withthese cloud systems seems to undo the

poleward transport of heat by the dynamics.The radiative effect of these systems is work-ing against the dynamical effect. Evidently,we need better understaning of the dy-namic-thermodynamic coupling betweenthese enormous cooling clouds and theequator-pole temperature gradient, andgreenhouse forcing.

Land-atmosphere-oceancoupling in the Tropics

Another major interesting feature in Fig. 2 is

the smaller forcing over the highly reflective(high albedo) tropical cloud systems. It iswell known that over the western tropicalPacific, equatorial Africa, South America, andthe monsoon regions of Asia, deep cumulo-nimbus-cirrus systems give rise to extensivehigh albedo clouds, but Fig. 2 does not showthis cooling effect.

The fundamental reason is that thegreenhouse effect of these cloud systemsare very large (because their tops are locatedin the cold upper troposphere) and the largepositive forcing from the cloud greenhouseeffect nearly cancels out the large negative

forcing from the high albedo of these

clouds. There is one intriguing feature of theequilibrium albedo of these deep cumulo-nimbus-cirrus cloud systems.

The frequency distribution of clear skyalbedos over Sahara and the clear andcloudy sky albedos over the Amazon regionin South America are shown in Fig. 3. Thestriking feature is the near similarity in thealbedo between the all-sky (clear andclouds) albedo over the Amazon (magenta)and the clear sky albedo of Sahara. The allsky value of Saharan albedo is not shown

because it is very similar to its clear skyalbedo values.Although the clear sky albedo of Ama-

zon is less than 15% (compared with Saharavalue of 34%), the clouds even out thealbedo differences and hence Amazon allsky values are close to Saharan values. Thissimilarity cannot be attributed to a limitingvalue of Amazon albedo to, say, a value closeto 30%. The annual mean albedo of Amazonis most likely determined by land-atmos-phere-ocean interactions limiting the cloud

fraction and its thickness by the availabilityof cloud water and nuclei. However, it is in-triguing why the mean albedo of this regionover a longer time period is close to 30%.Similar annual mean and large scale averagevalues are found over the other tropicalcloud systems as well.

The GAIA hypothesis has remaineduntestable [7]. We need a more testable hy-pothesis that not only addresses the ques-tions raised here but also explains thealbedo puzzles posed in this paper. s

[email protected]

Figure 3. Frequency distribution of top-of-atmosphere broad band albedo based on data fromEarth radiation budget experiment, ERBE.



BG2.1 Interactions of Land Cover and Climate

Convener: Reissell, A.

Co-Convener: ; Kabat, P.Andreae, M.

AS1.14 African MonsoonMultidisciplinary Analysis (AMMA)

(co-listed in BG, CL, OS & SSS)

Convener: Taylor, C.

Co-Convener: Janicot, S.;

Marticorena, B.

AS3.21 The dry deposition

process at the substrate- to global

scale (co-listed in BG & OS)

Convener: Ganzeveld, L.

Co-Convener: Altimir, N.

CL22 Land-climate interactions

from models and observations:

Implications from past to future

climate

Convener: Seneviratne, S.

Co-Convener: van den Hurk, B.;

Ciais, P.

CL15 Physical andbiogeochemical feedbacks in the

climate system

Convener: Jones, C.

Co-Convener: Alexeev, V.

European Geosciences Union General Assembly, 13-18 April, 2008. Vienna, Austria

Integrated Land Ecosystem Atmosphere Processes Study

www.iLEAPS.org

BG2.7 Land - atmosphereInteractions and human activity in

Monsoon Asia

Convener: Fu, C.

Co-Convener: Kabat, P.; Reissell, A.

BG2.8 Land-atmosphere

interactions in Northern Eurasia (co-

listed in CR)

Convener: Groisman, P.

Co-Convener: Kabat, P.; Hibbard, K.

BG2.11 Synthesis efforts from the

global network of ecosystem-

atmosphere CO , water and energy2

Exchange (FLUXNET)

Convener: Reichstein, M.

Co-Convener: Papale, D.

BG4.1 Biogeochemical feedbacks

on global climate change

Convener: Friend, A.

Co-Convener: Betts, R.; Poulter, B.;

Lenton, T.

BG2.6 Methane fluxes and carbon

cycle of permafrost ecosystems

Convener: van Huissteden, K.Co-Convener: Christensen, T.

iLEAPS Session

iLEAPS organized/co-sponsored/related sessions

Land cover both responds to the climate and affects the climate, and these interactions are amajor focus of the IGBP core project Integrated Land Ecosystem - Atmospheric Processes Study(iLEAPS). Changes in land cover are now recognized as a factor that has contributed to changesin climate on all scales from local to regional and even global. This symposium invites papers onthe full range of topics relating to the interactions of changes in land cover and condition and inclimate, including changes relating to deforestation, agriculture, and other development; feedbacksrelating to albedo, roughness, carbon storage and fertilization, trace gas fluxes, and otherbiogeochemical cycles; and impacts relating to ecosystem shifts, melting of permafrost,and water resources and soil moisture.

We warmly welcome you to participate in the sessions!

iLEAPS Newsletter Issue No. 5xApril 2008 2



William K. M. Lau1and Kyu Myong Kim21. NASA/Goddard Space Flight Center, Greenbelt, Maryland, USA2. University of Maryland at Baltimore County, Baltimore, Maryland, USA

WilliamK. M. Lau is chief of Laboratory for Atmos-

pheres, NASA, Goddard Space Flight Center, and anAdjunct Professor at Department of Meteorology, Uni-versity of Maryland. His research work spans threedecades covering a wide range of topics in climatedynamics, tropical and monsoon meteorology, ocean-atmosphere interaction, climate variability and change.He is a Goddard Senior Fellow, a fellow of the Ameri-can Meteorological Society and American Geophysical

Absorbing aerosols enhanceIndian summer monsoon rainfall

Union. He is currently lead scientist for the Joint Aerosol-Monsoon Experiment (JAMEX)a core elemenof the World Climate Research Programme, AsiaMonsoon Years Initiative. Dr. Lau has published ove190 refereed papers, articles in encyclopedias anbook chapters, and is the principal author of the booIntraseasonal variability of the tropical ocean-atmosphere climate system.

Aerosols can affect precipitation throughradiative effects of suspended particles in

the atmosphere and/or by modulating thecloud formation processes. This article dealsonly with precipitation and large-scalechanges induced by radiative effects of aerosols.

Based on radiation properties, aerosolscan be classified into two types: those thaabsorb solar radiation, and those that do notBoth types of aerosols scatter sunlight andreduce the amount of solar radiation thareaches the Earths surface, causing it to cooHowever, absorbing aerosols, in addition to

cooling the surface, can heat the atmosphere. The heating of the atmosphere produces a rising motion and low-level moisture convergence, which lead to increase inrainfall. The latent heating from enhancedrainfall may produce feedback processes inthe large-scale circulation, further amplifyingthe initial response to aerosol radiative forcing.

Figure 1. A space shuttle view of the IndoGangetic Plains shrouded in haze, against the

partially snow-covered foreground of the TibetanPlateau.



(A) June 21, 2006

Coordinates (North Latitude, East Longitude)

55.4989.22

47.6385.66

39.6982.89

31.7180.57

23.7078.54

15.6776.68

7.6374.91

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ltitude,

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532 nm Total Attenuated Backstatter /km/sr Begin UTC: 2006-06-21 20:34:15.5002 End UTC: 2006-06-21 20:47:34.4772 Version 1.05, image date:09/05/2006

Altitude,k

m

0

5

10

15

20

25

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55.4878.40

47.6174.84

39.6872.07

31.7069.76

23.0967.72

15.6565.86

7.6164.09

532 nm Total Attenuated Backstatter /km/sr Begin UTC: 2006-06-22 21:17:32.0652 End UTC: 2006-06-22 21:30:51.0412 Version 1.05, image date:09/05/2006

(B) June 22, 2006

1.0 x 10-1

5.0

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5.0

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1.0 x 10

-3

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Coordinates (North Latitude, East Longitude)

The aerosol-induced atmospheric feed-ack effects are likely to be most effective in

erosol hotspots which are characterizedy heavy aerosol loading over large areaswith abundant atmospheric moisture. Thendo-Gangetic Plains (IGP) in northern India

an aerosol super-hotspot due to highopulation density and concentration of in-ustrial plants. Most of the aerosols in the

GP region are absorbing speciesblack car-on from coal, biofuel, and biomass burning,s well as dust transported from the Middleast and the Thar Deserts. During the north-rn spring and early summer, these aerosols

re found in large quantities piling upgainst the southern slopes of the Tibetanlateau, visible in the form of a thick layer ofaze shrouding the entire region (see Fig. 1).

Recent studies have shown that theadiative heating perturbations induced bybsorbing aerosols, i.e., dust and black car-on, in the atmosphere-land-ocean system

may, through dynamical feedback processes,ncrease or decrease monsoon rainfall de-ending on the relative importance of thetmospheric heating and the surface cool-

ng effects [1, 2, 3, 4]. With regard to the rolef topography in further enhancing the ini-

tial response to aerosol radiative forcing, Lauet al [3] introduced the Elevated HeatPump(EHP) hypothesis, which suggests that

dust and black carbon accumulated over theIndo-Gangetic Plains (IGP), by wind transportagainst the foothills of the Himalayas, act asan elevated heat source that spurs anoma-lous warming of the middle and uppertroposphere over the southern Tibetan Pla-teau. The tropospheric heating subsequentlyleads to an enhancement of monsoon rainover northern and central India. Lau and Kim[4] have provided preliminary observationalevidence of the EHP effects.

CloudSat and CALIPSO satellites, whichwere launched in 2006, have pro

Albedo_atmosphere

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