Numerical Analysis of the Impact of Complex Urban Environment on a Snowfall Event in Beijing

The impacts of anthropogenic heat emissions, urban land use, and aerosols on a snowfall event in Beijing were investigated by using the Weather Research and Forecasting model. The results show that urban land use and anthropogenic heat emissions can reduce the accumulation of snowfall by 17%-22% within the fifth ring road, which depends on the aerosol number concentration and size. The increase in aerosols (same land use and anthropogenic heat emissions) can increase the accumulated snowfall within fifth ring road, especially for accumulated and coarse mode. Urban land use, anthropogenic heat emissions, and aerosols have different effect on snowfall, which depend mainly on relatively humidity. Urban land use and anthropogenic heat emissions lead to the formation of an "urban heat and dry island," which is not favorable to snowfall processes during the initial snowfall processes and leads to a decrease in ice crystals and snow by 47% averaged. At this time, the increase in aerosol number concentrations has less impact (increase by 5% averaged) on snowfall because of higher supersaturation required for activation. With the development of snowfall processes, the feedback of cloud microphysics decrease temperature and increase relative humidity, which is favorable to snowfall process and increase snowfall slightly by 5% averaged. Meanwhile, the critical radius of aerosols that can be activated decrease, smaller particle aerosols are activated to form supercooled cloud water and further to form ice crystals and snow, which leads to an increase in accumulated snowfall by 11% averaged.