Aerosol indirect effect over Indo-Gangetic plain

Moderate resolution imaging spectroradiometer (MODIS) data are analyzed over the Indo-Gangetic plain (IGP) to study the effect of aerosol optical depth (AOD) on the water (Reff,w) and ice (Reff,i) cloud effective radius for the period 2001–2005. The temporal variation of Reff,w and Reff,i shows reverse trend as that of AOD for most of the time. The intensity of positive indirect effect (i.e. increase of Reff,w/i with decrease of AOD and vice versa) is the highest in winter (ΔReff,w/ΔAOD∼−9.67 μm and ΔReff,i/ΔAOD∼−12.15 μm), when the role of meteorology is the least. The positive indirect effect is significant in 43%, 37%, 68% and 54% of area for water clouds in winter, pre-monsoon, monsoon and post-monsoon seasons, respectively, whereas the corresponding values for ice clouds are 42%, 35%, 53% and 53% for the four seasons, respectively. On the contrast, Reff,i in some locations shows increment with the increase in AOD (negative indirect effect). The negative indirect effect is significant at 95% confidence level in 7%, 18%, 9% and 6% grids for winter, pre-monsoon, monsoon and post-monsoon seasons, respectively. The restricted spatial distribution of negative indirect effect in IGP shows that the cloud microphysical processes are very complex. Our analyses clearly identify the contrasting indirect effect, which requires further in situ investigations for better understanding of the aerosol–cloud interaction in the region.