A numerical sensitivity study of the impact of CCN spectra on the observed spatio-temporal distribution of convective rainfall during the Joint Aerosol Monsoon Campaign 2009 (JAMEX09) is presented. The numerical study was conducted using the Weather Research Forecasting Model (WRF V3.2) with Milbrandt and Yau (2005) double moment microphysics. CCN spectra (clean and polluted) for the case study were estimated from in-situ measured aerosol size distributions and bulk hygroscopicity during JAMEX09, before and after a regional-scale pollution event in the Indian Gangetic Plains (IGP). Typical continental and marine CCN spectra present as default in the WRF microphysics scheme were also used for control simulations. Results are discussed in terms of precipitation accumulation, as well as storm dynamics and microphysics.