Atmos. Chem. Phys. Discuss., 13, 32885-32923, 2013
© Author(s) 2013. This work is distributed
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Modeling the influences of aerosols on pre-monsoon circulation and rainfall over Southeast Asia
D. Lee1,2,3, Y. C. Sud2, L. Oreopoulos2, K.-M. Kim2, W. K. Lau2, and I.-S. Kang3
1GESTAR, Morgan State University, Baltimore, Maryland, USA
2Earth Sciences Division, NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
3School of Earth and Environmental Sciences, Seoul National University, Seoul, South Korea

Abstract. We conduct several sets of simulations with a version of the GEOS-5 Atmospheric Global Climate Model (AGCM) equipped with a two-moment cloud microphysical scheme to understand the role of biomass burning (BB) aerosol emissions in Southeast (SE) Asia in the pre-monsoon period February–May. Our experiments are designed so that both direct and indirect aerosol effects can be evaluated. For climatologically prescribed monthly sea surface temperatures, we conduct sets of model integrations with and without biomass burning emissions in the area of peak burning activity, and with direct aerosol radiative effects either active or deactivated. Taking appropriate differences between AGCM experiment sets we find that BB aerosols affect liquid clouds in statistically significantly ways, increasing cloud droplet number concentrations, decreasing droplet effective radii (i.e., a classic aerosol indirect effect), and locally suppressing precipitation due to a decelerate of the autoconversion process, with the latter effect apparently also leading to cloud condensate increases. Geographical re-arrangements of precipitation patterns, with precipitation increases downwind of aerosol sources are also seen, most likely because of advection of weakly precipitating cloud fields. Somewhat unexpectedly, the change in cloud radiative effects (cloud forcing) is in the direction of less cooling because of decreases in cloud fraction. Overall, however, because of direct radiative effect contributions, aerosols exert a negative forcing at both the top of the atmosphere and, perhaps most importantly, the surface, where decreased evaporation triggers feedbacks that further reduce precipitation. Invoking the approximation that direct and indirect aerosol effects are additive, we estimate that the overall precipitation reduction is about 40% due to the direct effects of absorbing aerosols which stabilize the atmosphere and reduce surface latent heat fluxes via cooler land surface temperatures. Further refinements of our two-moment cloud microphysics scheme are needed for a more complete examination of the role of aerosol-convection interactions in the seasonal development of the SE Asia monsoon.

Citation: Lee, D., Sud, Y. C., Oreopoulos, L., Kim, K.-M., Lau, W. K., and Kang, I.-S.: Modeling the influences of aerosols on pre-monsoon circulation and rainfall over Southeast Asia, Atmos. Chem. Phys. Discuss., 13, 32885-32923, doi:10.5194/acpd-13-32885-2013, 2013.
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