1. Mayurakshi Dutta Department of Atmospheric Sciences March 20, 2003
Impact of Aerosols on the Global Climate and Hydrological Cycle The INDOEX endeavor
Mayurakshi Dutta
Department of Atmospheric Sciences
March 20, 2003

2. Acknowledgements
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

3. What are AEROSOLS ?
Suspended particles in the atmosphere which range in size from about 10-3 µm to 20 µm
Anthropogenic - inorganic /organic
sulphates
carbonaceous aerosols [black carbon(BC)] and organic carbon (OC)
Dust
Sea-salt
Natural sources
Volcanic dust
Sea-spray
Wind-generated dust
Forest fire smoke

4. Contribution to 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.

5. Global distribution of Aerosols
Biomass burning + fossil fuel combustion – S/SE Asia
Biomass burning – Africa + S. America
Fossil fuel combustion- mid and high latitude

6. Effects 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

7. Effects 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

8. Effects of aerosols on climate
Indirect 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 effect
Increase in cloud lifetime
Semi direct effect - solar heating of the boundary layer by BC evaporate some clouds  more solar radiation reach the surface

9. Effects of aerosols on Radiative Forcing
INDOEX
Global

10. INDOEX 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

11. South Asian Brown Haze

12. Regional Distribution of Anthropogenic Aerosol Climate Forcing (Wm-2)
January- March 1999

13. Impact on Global Surface Temperature
TOA forcing determines global average surface temperature change…..
Global surface warming due to GHGs  global cooling effect of the aerosols
Climate models suggest – global surface would warm by about 0.5 to 0.8 K per 1 Wm-2
2.4 Wm = ~ 0.4 Wm-2  2.0 Wm-2
20th century Stored in should have
anthropogenic GHG oceans caused a K
forcing warming
But the observed warming between 1900 – 2000 is only
about 0.6 K . The estimated global mean TOA aerosol
forcing of –10.5 Wm-2 sufficient for the missing cooling

14. Impact on Global Hydrological Cycle
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

15. Impact on Global Hydrological Cycle
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

16. 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

17. ……..and Future Plans
A reliable global inventory of aerosol emission rates, lifetimes and concentrations needed
Understanding BC transport and how carbonaceous aerosols regulate microphysical processes in clouds
More reliable measurements of AOD over land needed
Careful designing of OC-GCM studies to better determine how hydrological cycle responds to the microphysical and radiative effects of aerosols

18. Now we can have Questions
Thanks very much
Now we can have Questions