Role of aerosols in modulating cloud properties during active–break cycle of Indian summer monsoon

Abstract

In this study, the weather research and forecast model coupled with chemistry (WRF-Chem), is used to understand the impact of aerosol-cloud interaction during the active-break cycles of the Indian summer monsoon. Two sets of simulations are performed, one with a fixed aerosol concentration ( ConstantAero) and the other with an observation-based prescription of the rate of change of aerosol concentration as a function of precipitation ( VaryingAero). This prescription is derived based on satellite-retrieved daily rainrate and concurrent observations of aerosol optical depth from aerosol robotic network. The proposed modification is necessitated by the lack of realistic emission estimates over the Indian region as well as the presence of inherent biases in monsoon simulation in WRF-Chem. In the VaryingAero simulation, unlike in the ConstantAero run, we find that the break-to-active monsoon phase has more cloud liquid water (CLW) and less rain efficiency than in the active-to-break phase. This is primarily due to the indirect effect of increased aerosol loading in the break phase. This result is in accordance with the observed behaviour of CLW estimtes from microwave imager (TRMM 2A12) and radar reflectivity (TRMM precipitation radar). We also find that the proposed interactive aerosol loading results in higher spatial variability in CLW and enhances the likelihood of increased cloud cover via formation of larger clouds. The modification also alters the diurnal cycle of clouds in break and break-to-active phases as compared to other phases due to aerosol loading, with a stronger diurnal cycle of upper level clouds in these phases in the VaryingAero model as compared to ConstantAero model.