Impact of aerosol and freezing level on orographic clouds: A sensitivity study

The response of clouds and precipitation to changes in aerosol properties is variable with the ambient meteorological conditions, which is important for the distribution of water resources, especially in mountain regions. In this study, a detailed bin microphysics scheme is coupled into the Weather Research and Forecasting (WRF) model to investigate how orographic clouds and precipitation respond to changes in aerosols under different thermodynamic profiles. The model results suggest that when the initial aerosol number concentration changes from a clean continental background (4679 cm− 3) to a polluted urban environment (23,600 cm− 3), the accumulated surface precipitation amount can be increased up to 14% mainly due to the enhanced riming process which results from more droplets of 10-30 μm in diameter. When the freezing level is lowered from 2.85 km to 0.9 km (above 1000 hPa level), the growth of ice-phase particles via riming process is enhanced, leading to more precipitation. However, the response of surface precipitation amount to increase in aerosol particle concentration is not linear with lowering freezing level, and there is a maximum precipitation enhancement caused by aerosols (about 14%) as the freezing level is at 1.4 km. Further sensitivity tests show that, the response of riming growth to increase in aerosol particle concentration becomes more significant with lowering the freezing level, but this effect becomes less significant as the freezing level is further lowered due to the limited liquid water. Moreover, the growth of raindrops through collision and coalescence is suppressed with lowering freezing level, due to the shorter distance between the melting level and the ground.