Simulation of aerosol effects on mixed-phase orographic clouds using the WRF model with a detailed bin microphysics scheme

The Weather Research Forecast (WRF) mesoscale model coupled with a detailed bin microphysics scheme was used to investigate the impact of aerosol serving as cloud condensation nuclei (CCN) and ice nuclei (IN) on orographic clouds and precipitation. Clouds developed over idealized mountains with peak height of 1000 m and 1500 m were simulated. In the low mountain case with peak height of 1000 m, warm-phase microphysical processes dominated the cloud microphysical processes and rain formation. The precipitation amount is reduced by up to 18% when the environmental concentration of hygroscopic aerosol particles is increased by a factor of 20, but it is almost unchanged when IN concentration is increased. In the higher mountain case, a mixed-phase orographic cloud formed. Sensitivity tests show that increase in the concentration with similar amount as former tests produced little change on the amount of precipitation, but there are significant changes in the microphysical processes. A detailed analysis of the microphysical processes indicated that the Wegener-Bergeron-Findeisen (WBF) process and the riming growth of ice particles became more effective under conditions with higher aerosol concentration, due to the relative large amount of cloud water, leading to suppressing the growth of snow deposition. It is also shown that the increase in ice water due to the WBF process completely offsets the decrease of snow growth caused by snow deposition with increasing hygroscopic aerosol loading resulting in 1.5% more precipitation. As ice nuclei concentration increases, the contributions of snow deposition and the WBF process to ice water growth increase. However the loss of snow deposition far outweighs the increase of the WBF process, leading to 1.8% reduction of total precipitation under polluted conditions. Finally, the signs of the impacts of hygroscopic aerosol on precipitation mainly depend on the changes of WBF process, riming growth and snow deposition in mixed-phase orographic clouds.