北京城市化对一次降雪过程影响的数值模拟研究

A numerical study of urbanization impacts on a snowfall event in Beijing area

中文关键词:  降雪  城市化  降水类型  中尺度数值预报模式(WRF)

英文关键词:Snowfall  Urbanization  Precipitation type  WRF

中文摘要:

      利用中尺度数值预报（Weather Research and Forecast，WRF）模式，针对2018年3月17日05—17时（北京时）北京地区的一次降雪过程模拟分析了城市化对降雪的主要影响机制。结果表明，城市化使得北京五环以内降雪量减少，降雨量增加，这主要是由于城市化低层增温效应加强了雪的融化过程，产生混合型降水，距离市中心越近越容易发生混合型降水。城市化对降雪的总降水量和降水的时、空分布也存在一定的影响。降水初期，城市化造成的“城市干热岛”效应不利于水汽的水平和垂直输送，不利于云的形成，地面总降水量减小。随着降水过程的发展，部分冰相粒子融化，使近地面水汽增多，“城市热岛效应”的热力抬升作用有利于水汽的垂直输送和云的发展，部分云滴或水汽抬升进入云中，增强冷云过程，使雪和霰粒子含量增大，地面总降水量增加。城市化产生的“城市效应”对低层大气温度和云微物理过程产生影响，而云微物理过程的非绝热过程反过来又影响低层大气温度和大气层结，影响能量和水汽输送，进而对云和地面降水产生影响。

英文摘要:

      The snowfall that occurred in Beijing from 05:00 to 17:00 BT 17 March 2018 is simulated using the Weather Research and Forecasting (WRF) model to study the main influencing mechanism of urbanization on snowfall. Results show that urbanization in the metropolitan area of Beijing can reduce snowfall and increase rainfall, which is largely attributed to the fact that the low-level warming effect of urbanization strengthens the melting process of snow, producing more mixed-phase precipitation. Areas closer to the city center are prone to more mixed-phase precipitation. Urbanization also affects spatial and temporal distribution of total precipitation. At the beginning of the precipitation process, the "urban dry heat island" effect induced by urbanization is not favorable for horizontal and vertical transport of water vapor and the formation of clouds, and thereby the total precipitation would decrease. As the precipitation process develops, some ice phase particles melt and the water vapor in the near-surface layer increases. The thermal uplift effect of the "urban heat island effect" is conducive to vertical transport of water vapor and the development of clouds. Cloud droplets and/or water vapor rise into the clouds, enhancing the cold-cloud process. The cold-cloud process leads to more snow and graupel particles and increases total surface precipitation. The "urban effect" generated by urbanization has impacts on low-level atmospheric temperature and cloud microphysical processes. The non-adiabatic cloud microphysical process in turn affects lower atmospheric temperature and atmospheric stratification as well as the energy and water vapor transport, which subsequently influence cloud and precipitation.