Aerosol Effects on Urban Convection and Precipitation

The role of aerosols in urban-induced convection and precipitation is investigated using a spectral-bin cloud model [Hebrew University Cloud Model (HUCM)]. For this purpose, extensive numerical experiments with various aerosol concentrations in urban areas are performed under different environmental moisture conditions. To take into account the effects of urban heat island and urban air pollution, it is assumed that there is low-level heating in urban areas and that aerosol concentration in urban areas is higher than that in surrounding rural areas. Results from our numerical experiments show that an urban heat island-induced low-level updraft cell, which is nearly stationary on the downwind side after some time, leads to periodic generation of deep convective clouds on its downwind side. Higher aerosol concentration in urban areas generally results in the development of stronger convective clouds, leading to a larger amount of surface precipitation on the downwind side of urban areas under all moisture conditions considered. This is due to the release of extra latent heat of condensation and the enhancement of the riming process, which result from the generation of a higher number of smaller cloud drops with increasing aerosol concentration. More detailed results and discussion about the interaction between aerosol-induced changes in cloud dynamics and microphysics will be given in the presentation.