Effects of aerosols on precipitation from orographic clouds

Spectral (bin) microphysics was coupled to the Weather Research Forecast model to investigate the effect of aerosols (i.e., air pollution) on precipitation in the Sierra Nevada Mountains. Two-dimensional simulations were produced using either maritime ("clean-air'') or continental ("dirty-air'') aerosols. The simulation with clean air produced more precipitation on the upwind mountain slope than the simulation with continental aerosols. After 3 hours of simulation time, the simulation with maritime aerosols produced about 30% more precipitation over the length of the mountain slope than the simulation with continental aerosols. Sensitivity tests demonstrated the importance of relative humidity and vertical velocity on cloud microphysical structure and precipitation amount. Greater differences in precipitation amounts between simulations with clean and dirty air were obtained when ice microphysical processes were included in the model simulations.