A modeling study of the direct effect of aerosols over the tropical IndianOcean

The microphysical, chemical, optical, and lidar data collected during the I ndian Ocean Experiment (INDOEX) resulted in a self-consistent aerosol formu lation for a multiple-scattering Monte Carlo radiation model. The model was used to simulate the direct aerosol radiative forcing, cloud radiative for cing, and heating rates for typical winter monsoon conditions over the trop ical Indian Ocean. The focus of the study is to understand how the anthropo genic and natural aerosols partition the incoming solar energy between the ocean mixed layer and the overlying cloudy atmosphere. The observed aerosol single-scattering albedo, omega, was in the range 0.8-0.9 at 500 Tim, mean aerosol visible optical thickness, tau (A), was in the range 0.1-0.8 at 50 0 nm, and the low-level clouds had horizontal scales of few kilometers and a cloud fraction of about 25%, typical of low-level clouds in the tropical oceans. The aerosol layer extended well above the low-level clouds in many instances, which has a significant impact on the radiative forcing. Althoug h contributing only about 10% to the aerosol optical thickness, the soot tr ansported from Asia and the Indian subcontinent significantly affects the a erosol direct forcing of the cloudy atmosphere. For monthly mean conditions (tau (A) = 0.4, omega = 0.9 and 25% low-cloud fraction), the diurnal mean surface radiative forcing is about -25 W m(-2) and the atmospheric forcing ranges from +22 to +25 W m(-2). The top-of-the-atmosphere direct aerosol fo rcing is in the range of zero to -3 W m(-2). The aerosol enhances the cloud atmospheric forcing by 0.5 and by 2.5 W m(-2) when aerosol is mostly below and above the clouds, respectively. Furthermore, the trade wind boundary l ayer is subject to a heating of about 1 to 1.5 K/d which might burn off the trade cumulus themselves. Thus the major impact of the predominantly anthr opogenic aerosol over the tropical Indian Ocean is a substantial redistribu tion of the solar energy between the atmosphere and the ocean mixed layer.