Impact of Aerosols on the Impact of Aerosols on the Global Climate and Global Climate and Hydrological CycleHydrological Cycle

The INDOEX endeavorThe INDOEX endeavor

Mayurakshi DuttaDepartment of Atmospheric Sciences

March 20, 2003

AcknowledgementsAcknowledgements

Ramanathan,V, P.J. Crutzen,J.T Kiehl and D. Rosenfeld, 2001, Aerosols, Climate, and the Hydrological Cycle, Science, 294, 2119 pp.Ramanathan et.al., 2001,The Indian Ocean Experiment: An Integrated Analysis of the Climate Forcing and Effects of the Great Indo-Asian Haze., J. Geophys. Res.-Atmos.,106, 2837 pp.Lelieveld,J.et al.,The Indian Ocean Experiment: Widespread Air pollution from South and Southeast Asia,Science, 291, 1031 pp

What are AEROSOLS ?What are AEROSOLS ?Suspended particles in the atmosphere which range in size from about 10-3 µm to 20 µmAnthropogenic - inorganic /organicsulphatescarbonaceous aerosols [black carbon(BC)] and organic carbon (OC)DustSea-saltNatural sourcesVolcanic dustSea-sprayWind-generated dustForest fire smoke

Contribution toContribution to Aerosol Optical Depth - AOD Aerosol Optical Depth - AOD

AOD is an index of the attenuation of radiation as it passes through the atmosphere due to the presence of suspended particles.

Global distribution of AerosolsGlobal distribution of Aerosols

Biomass burning + fossil fuel combustion – S/SE Asia

Biomass burning – Africa + S. America

Fossil fuel combustion- mid and high latitude

Effects of aerosols on climateEffects of aerosols on climate

Direct radiative forcing (RF) - increase the reflection of solar radiation to space. Global estimates of TOA RF due to anthropogenic aerosols ~ -0.5 to –2.5 Wm-2.The reflected solar flux due to anthropogenic sulphate averaged over NH ~ -1.1 Wm-2 ~ 50% (but opposite sign) of the RF due to anthropogenic CO2 (Jayaraman et. Al. 1998)Both the surface and the atmosphere will cool

BUT

Effects of aerosols on climateEffects of aerosols on climate

Carbonaceous aerosols (BC and organic) absorb and scatter solar radiation…..thus decreases the solar radiation reaching the surface.

In addition, carbonaceous aerosols absorb the upward solar radiation reflected by the surface and clouds

At the TOA - BC effect opposes the cooling effect of sulphates At the surface - all aerosols reduce solar radiation

Effects of aerosols on climateEffects of aerosols on climateIndirect radiative forcing-Aerosols serve as Cloud Condensation Nuclei (CCN) and ice nuclei aerosols [droplet number] reflection to space of solar radiation from clouds cooling effect in precipitation efficiency due to microphysical effectIncrease in cloud lifetimeSemi direct effect - solar heating of the boundary layer by BC evaporate some clouds more solar radiation reach the surface

Effects of aerosols on Effects of aerosols on Radiative ForcingRadiative Forcing

INDOEX

Global

INDOEX EndeavorINDOEX Endeavor

International field experiment over North Indian Ocean

Intense field phase b/w Jan-Mar,1999

First in situ observation of aerosols + air chemistry

Location unique

South Asian Brown HazeSouth Asian Brown Haze

Regional Distribution of Anthropogenic Regional Distribution of Anthropogenic Aerosol Climate Forcing (WmAerosol Climate Forcing (Wm-2-2))

January- March 1999

Impact on Global Surface Impact on Global Surface TemperatureTemperature

TOA forcing determines global average surface temperature change…..Global surface warming due to GHGs global cooling effect of the aerosolsClimate models suggest – global surface would warm by about 0.5 to 0.8 K per 1 Wm-2

2.4 Wm-2 = ~ 0.4 Wm-2 2.0 Wm-2

20th century Stored in should have anthropogenic GHG oceans caused a 1-1.6 Kforcing warmingBut the observed warming between 1900 – 2000 is only about 0.6 K . The estimated global mean TOA aerosol forcing of –10.5 Wm-2 sufficient for the missing cooling

Impact on Global Hydrological Impact on Global Hydrological CycleCycle

80% of the net radiative heating of the tropical oceans is balanced by evaporation

So in RF in evaporation tropical rainfall thus perturbation of the water budget

in rainfall latent heat released to the middle and upper troposphere implications for the lapse rate and Hadley and Walker circulations

Again

Impact on Global Hydrological Impact on Global Hydrological CycleCycle

Between 0-3 km the low level air temperature increased where the aerosol heating was imposed

Response to this low level heating in moist convection + strengthen rainfall along the ITCZ

Subsidence away from the region of enhanced precipitation in rainfall north and south of ITCZ

Future Implications……..Future Implications……..

Important to differentiate the decadal to centennial time scales of the GHG warming from the time scale of aerosol lifetimes

Regional aerosol effects will continue to play a major role as long as air pollution continues

…………..and Future Plans..and Future PlansA reliable global inventory of aerosol emission rates, lifetimes and concentrations neededUnderstanding BC transport and how carbonaceous aerosols regulate microphysical processes in cloudsMore reliable measurements of AOD over land neededCareful designing of OC-GCM studies to better determine how hydrological cycle responds to the microphysical and radiative effects of aerosols

Thanks very much Thanks very much