The effects of aerosols on intense convective precipitation in the northeastern United States

A fully coupled meteorology-chemistry-aerosol mesoscale model (WRF-Chem) is used to assess the effects of aerosols on intense convective precipitation over the northeastern United States. Numerical experiments are performed for three intense convective storm days and for two scenarios representing 'typical' and 'low' aerosol conditions. The results of the simulations. suggest that increasing concentrations of aerosols can lead to either enhancement or suppression of precipitation. Quantification of the aerosol effect is sensitive to the metric used due to a shift of rainfall accumulation distribution when realistic aerosol concentrations are included in the simulations. Maximum rainfall accumulation amounts and areas with rainfall accumulations exceeding specified thresholds provide robust metrics of the aerosol effect on convective precipitation. Storms developing over areas with medium to low aerosol concentrations showed a suppression effect on rainfall independent of the meteorological environment. Storms developing in areas of relatively high particulate concentrations showed enhancement of rainfall when there were simultaneous high values of convective available potential energy, relative humidity and wind shear. In these cases, elevated aerosol concentrations resulted in stronger updraughts and downdraughts and more coherent organization of convection. For the extreme case, maximum rainfall accumulation differences exceeded 40 mm. The modelling results suggest that areas of the northeastern US urban corridor that are close to or downwind of intense sources of aerosols, could be more favourable for rainfall enhancement due to aerosols for the aerosol concentrations typical of this area. Copyright (C) 2009 Royal Meteorological Society