Sensitivity of the hydrological cycle to aerosol type and amount using high-resolution weather research and forecasting model over India

The present study is aimed to provide a quantitative prediction of the comparative impact of aerosol types on the changing pattern of the Indian summer monsoon (ISM) rainfall over recent years. Specific regions with contrasting features of aerosol loading and changes in ISM rainfall pattern relationship were identified through the comparative analysis of long-term (2000 to 2019) spatial distribution of observed AOD and rainfall over the Indian subcontinent. The spatial distribution of aerosol species were estimated using constrained aerosol estimation which could well represent the measured values. Spatial concordances were identified between the contrasting spatial features of changes in ISM rainfall pattern and the spatial distribution of pre-monsoon (March, April, May) anthropogenic and dust aerosols. Optical parameters consisting of aerosol optical depth (AOD), single scattering albedo (SSA), angstrom exponent (AE) corresponding to the estimated spatial distribution of aerosol types were simulated using optical simulation (OPTSIM) and further used in an aerosol radiative feedback simulation (ARFS) to evaluate the impact on ISM rainfall. The positive and negative radiative effect at the top of the atmosphere (TOA) were identified over the anthropogenic and dust aerosols dominated areas respectively. Although surface cooling was caused by both anthropogenic and dust aerosols, dust aerosols contributed to significantly higher surface cooling than the anthropogenic aerosols over the northwestern India (NWI) region. However, over the Indo-Gangetic plain (IGP) region, higher surface cooling was caused by the anthropogenic aerosols. A significant increase in rainfall with respect to no aerosol scenario was identified along the western coast of India due to combined aerosols (both anthropogenic and dust), which is notably in line with the current observations of high rainfall incidents over the region. Overall an increase and decrease in the ISM rainfall was observed over the NWI and IGP region respectively, which is strongly correlated to the spatial distribution of aerosol types over the Indian subcontinent. Production of less but heavier cloud droplets, leading to an enhanced condensation and increased rainfall over the NWI region was attributed to the comparative enhancement of regional evaporation rate due to a weaker surface cooling and atmospheric warming effect of dust aerosols than in the case of anthropogenic aerosols. Reduction in the ISM rainfall over the IGP region was attributed to the enhanced surface cooling due to anthropogenic aerosols (mostly dominated by sulphate aerosols), potentially suppressing the effective evaporation from the region.