The Impacts of Gravity Wave Drag and Urban Heat Island on Snowfall in Beijing

The combined effects of topography and the urban heat island in Beijing create a complex thermodynamic and dynamic structure in the environmental field, leading to significant uncertainty in snowfall forecasting. This study examines a snowfall event that occurred on January 25–26, which was notably absent over the urban plain, with precipitation instead observed in the surrounding mountains and plains. Based on numerical simulations and observational data, the roles of gravity wave drag and the urban heat island effect are investigated through sensitivity experiments. Results indicate that the unique spatial distribution of snowfall over Beijing is primarily attributable to gravity wave drag, while the urban heat island effect played only a minor role. Gravity wave drag influenced the formation and distribution of snowfall primarily through the following dynamic and moisture-related mechanisms. It reduced near-surface wind speeds and shifted the dominant wind direction over the plain from southwesterly/southerly to northwesterly, resulting in a substantial decrease in the southerly wind component. By weakening the southerly moisture transport in the lower atmosphere, it diminished the water vapor flux. This reduced moisture supply, in turn, hindered the development and intensification of cloud systems and snow formation. Secondly, gravity wave drag induced warming in the lower atmosphere and cooling in the upper atmosphere, which stabilized the atmospheric stratification and suppressed turbulent development. This lower-level warming led to a decrease in relative humidity, which on one hand suppressed ice-phase processes such as condensation and sublimation, and on the other hand promoted evaporation. This combined effect prevented snowflakes from reaching the ground, resulting in the virga distribution in Beijing.