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

Relationships between trade wind cumuli coverage and aerosol concentration N_a, and between cloud optical thickness τ _c and N_a 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 τ_c 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.