The influence of aerosol hygroscopicity on clouds and precipitation over Western Ghats, India

We examine the role of aerosol hygroscopicity (kappa) on the formation of clouds and precipitation over the Western Ghats (WG) in India using various numerical model simulations (i.e., particle-by-particle-based small-scale, high-resolution mesoscale model). For the diffusional growth of cloud droplets, the size-dependent hygroscopicity is used in the kappa-Kohler equation of direct numerical simulation. The results of the small-scale model reveal that the distribution of cloud drop size varies from the initial mixing state to the well-mixed state due to variation in kappa. The value of kappa is obtained from Humidified Tandem Differential Mobility Analyzer (HTDMA) instruments at the High Altitude Cloud Physics Laboratory, India. The idealized and real simulations using the Weather and Research Forecasting (WRF) model with the aerosol-aware Thompson microphysics scheme are conducted by changing kappa values. Depending on the type of clouds (shallow or deep), different kappa values determine the mass and number of cloud and rain droplets. Low hygroscopicity (organics) simulates more and smaller drops, as well as uplifts below freezing level, resulting in more ice-phase hydrometeors. Organic aerosols have a significant impact on the formation of more snow and graupel hydrometeors. As compared to high kappa, low hygroscopicity weakens updrafts at the intermediate level and strengthens them at the upper level in the deep cloud region. The intensity of precipitation varies due to low and high kappa. The findings indicate that aerosol composition has a significant impact on the activation of cloud condensation nuclei. This study suggests that aerosol hygroscopicity is essential in weather prediction models to integrate aerosol chemical compositions.