Pre-Monsoon Aerosol-Cloud-Rainfall Interactions in the Central Himalayas due to Regional-Scale Pollution in the Indo-Gangetic Plains

Aerosol size distribution (ambient and dried) and chemical composition were measured during the pre-monsoon season (April – June 2009) during the Joint Aerosol Monsoon Campaign 2009 (JAMEX09) at two distinct locations in Central Nepal. Diameter growth factors (DGF) were estimated using the larger mode (around 100 nm) of the lognormal distribution fitted to the measured ambient and dried distributions. The DGF and chemical composition in combination with a hygroscopic growth model were used to determine the contribution of organic compounds to aerosol hygroscopic growth. It was found that assuming ideal behavior of water-soluble organic compounds and using their fraction as determined by water extraction of filters produces the best agreement with the observed DGFs. This finding together with the DGF and chemical composition data was used to determine the aerosol bulk hygroscopicity parameter, . The parameter suggests that aerosols in this region are less hygroscopic than aerosols reported in previous studies from other regions. The critical supersaturation for droplet activation was estimated based on the k-Köhler theory using the estimated parameter. Cloud Condensation Nuclei (CCN) spectra were obtained by integrating the fitted aerosol size distribution from the dry diameters corresponding to each critical supersaturation value. The two CCN spectra are the first estimates from ground-based observations for the Central Himalaya region. Next, aerosol-cloud-rainfall interactions for a pre-monsoon storm in the Central Himalayas associated with a regional-scale pollution event in the IGP (Indo-Gangetic Plains) during JAMEX09 were investigated using the Weather Research Forecasting Model with the Milibrandt and Yau double moment microphysics. A significant shift in the maxima of the event cumulative precipitation was observed between the simulations conducted with control continental aerosol spectra and the JAMEX09 polluted spectra. Differences in the simulated vertical profile of temperature, water vapor mixing ratio and hydrometeor distributions and associated latent heating differences lead to changes in local circulations, which in turn are modulated by orographic effects. In particular, the simulations suggest strong coupling between the relative contribution/role of ice processes (H-M process, and graupel production), the number of CCN activated, and the slope of the CCN spectra and the evolution of convective storm cells. Overall, the range of spatial variability of cumulative rainfall due to CCN sensitivity at the ridge-valley scale typical of mountain catchments is on the order of 50-200% for this study, which can have major implications for freshwater harvesting. Additional modeling studies of climatologically relevant hydrometeorological regimes for different conditions will be examined.