Annales Geophysicae, Vol. 22, pp 1421-1434 (2004).

Impact of Absorbing Aerosols on the Simulation of Climate over the Indian Region in an Atmospheric General Circulation Model

by

Arindam Chakraborty, S K Satheesh, Ravi S Nanjundiah and J Srinivasan

Centre for Atmospheric and Oceanic Sciences
 Indian Institute of Science
 Bangalore 560012
INDIA

Abstract

The impact of anthropogenic absorbing aerosols (such as soot) on the climate over the Indian region has been studied using the NCMRWF general circulation model. The absorbing aerosols increase shortwave radiative heating of the lower troposphere and reduce the heating at the surface. These effects have been incorporated as heating of the lower troposphere (up to 700 hPa) and cooling over the continental surface based on INDOEX measurements. The heating effect is constant in the pre-monsoon season and reduces to zero during the monsoon season. It is shown that even in the monsoon season when the aerosol forcing is zero, there is an overall increase in rainfall and a reduction in surface temperature over the Indian region. The rainfall averaged over the Tropics shows a small reduction in most of the months during the January to September period. The impact of aerosol forcing, the model s sensitivity to this forcing and its interaction with model-physics has been studied by changing the cumulus parameterization from Simplified Arakawa-Schubert (SAS) scheme to Kuo scheme. During the pre-monsoon season the major changes in precipitation occur in the oceanic Inter Tropical Convergence Zone (ITCZ) where both the schemes show an increase in precipitation. This result is similar to that reported in Chung et al. (2002). On the other hand, during the monsoon season the changes in precipitation in the continental region are different in SAS and Kuo schemes. It is shown that the heating due to absorbing aerosols changes the vertical moist-static stability of the atmosphere. The difference in the precipitation changes in the two cumulus schemes is on account of different responses in the two parameterization schemes to changes in vertical stability.

Keywords: 3374 Tropical Meteorology, 0305 Aerosols and Particles, 3354 Precipitation

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