A correlational study on size differentiated aerosols on monsoonal and pre-monsoonal cloud properties over the Indo Gangetic Basin

Abstract

Particle size is a critical parameter in deciding the cloud condensation nuclei activation efficiency and hence largely influential in the cloud formation and dynamics. Here we study the size differentiated influence of aerosol optical depth (AOD) on cloud properties for the densely polluted Indo-Gangetic Basin (IGB) for a period of 2000–2017 using satellite observations. Terra on board MISR derived AOD for two size ranges, small (aerodynamic radius < 0.35 μm) and large (aerodynamic radius > 0.7 μm) are correlated with MODIS derived cloud properties such as cloud fraction (CF), cloud optical thickness (COT), cloud top pressure (CTP) and cloud effective radius – liquid (CER-Liq) for monsoon and pre monsoon period, over the study area. Our study reveals that the size differentiated aerosols interacts differently with the cloud properties sometimes even contrarily. Small aerosol particles have shown better correlation with cloud properties than coarser particle. Furthermore, estimation of aerosol indirect effect has shown that small particles favour the Twomey effect while large particles follow the anti Twomey effect over the IGB. The anti Twomey effect for large particles i.e., positive AOD-CER relationship could be due to the elevated absorbing aerosol over the region causing aerosol induced heating of the cloud. In order to understand the aerosol characteristics over the IGB region, AERONET datasets from four IGB stations have been analysed, which indicated the presence of increased amount of coarser mode particles and absorbing aerosols over the study region. Aerosol-cloud interactions (ACI) are often overshadowed by internal cloud dynamics and upper air meteorology. Cloud segregated analysis using ISCCP classification suggests that the dominant clouds such as cirrostratus and altostratus influences the correlation statistics considerably. Partial correlation studies on four meteorological parameters (RH, vertical velocity, U and V component of wind) have revealed that ACI is also influenced by the upper air dynamics, particularly during monsoon.