Seasonal variation in satellite-derived effects of aerosols on clouds in the Arabian Sea

[1] Aerosols act as cloud condensation nuclei for cloud water droplets, with changes in aerosol concentrations having significant impacts on the corresponding cloud properties. An increase in aerosol concentration may lead to an increase in CCN, with an associated decrease in cloud droplet size for a given cloud liquid water content. Smaller droplet sizes may then lead to a reduction in precipitation efficiency and an increase in cloud lifetimes. However, these effects are highly dependent on the aerosol concentration, aerosol species, and the meteorological conditions. In the Arabian Sea (10–20 N, 62–72 E), prevailing aerosol type transitions from mostly small-mode anthropogenic aerosols during the winter months to mostly coarse-mode mineral dust and sea salt during the summer due to a change in prevailing wind speed and direction, is likely to impart substantial variability on any associated indirect effects. To examine this variability, we use one year (2004) of MODIS derived aerosol optical thickness (AOT) and cloud products over the Arabian Sea to quantify aerosol indirect effects. Results show that indirect effects in the Arabian Sea are a strong function of season, which is a result of the changing aerosol and moisture (humidity) concentrations during the course of the year. During the winter months (DJF), cloud-droplet size and AOT were found to have a weak positive correlation (r = 0.12), opposite of the expected effect. The low atmospheric humidity coupled with wide-spread subsidence and other dynamical factors may prevent these aerosols from being activated. During the summer months (JJA), AOT increases with the addition of mineral dust and sea salt aerosols and the correlation between AOTand cloud droplet size becomes negative (r = 0.22). The magnitude of the first indirect effect corresponds to an increase in low level wind speeds, increasing the concentration of hygroscopic sea salt into the atmosphere. For both periods, a positive correlation (r = 0.16, 0.32) was found between AOT and LWP indicating a reduction in precipitation efficiency.