Trade wind cumuli statistics in clean and polluted air over the Indian Ocean from in situ and remote sensing measurements

Abstract

Relationships between trade wind cumuli coverage and aerosol concentration Na, and between cloud optical thickness τ c and Na are examined using in situ and remote sensing observations acquired on 4 days within and downwind of the Indo-Asian haze. Cloud top height and cloud coverage decrease as N a increases, whereas τc increases, which could be due to aerosol or meteorological effects. Clouds with horizontal sizes less than 2 km, neglected in prior studies, contribute up to 40% to cloud area, and must be considered in studies of aerosol indirect and semi-direct effects. When radiative transfer models are applied to the observations, a regional change in top of the atmosphere TOA (surface) 0.64 μm radiative forcing of approximately +6 (+7) W m -2 μm-1 associated with changes in cloud properties is found. To the degree that synoptic-scale forcings do not vary systematically as air flows south, the results may be interpreted as the dominance of semi-direct over indirect forcing within the Indo-Asian haze for the times sampled. Copyright 2004 by the American Geophysical Union.