摘要
利用中尺度模式MM5分别模拟了中国北方地区两次不同类型的降雪过程:2001年12月7—8日的北京小雪和1999年11月23—24日的辽宁雨转雪过程。文中还从微物理角度研究了这两次降雪过程,分析不同天气条件下降雪的水成物相态及其源汇项分布特点,并初步探讨云物理过程对降水热力、动力过程的反馈作用。重点分析了云物理变量的模拟结果,计算时采用输出水成物源、汇项小时累计量的方法,讨论了最大降水时段内各源、汇项的分布特征,并通过敏感性试验,分析了水成物相变潜热作用和降水粒子的拖曳作用对云的反馈影响。结果表明,云中水成物相态分布与温度有密切关系,北京小雪过程为气、固两相粒子作用,辽宁雨转雪过程为气、液、固三相粒子相互作用;在最大降水时段内,雪的产生主要来自于水汽凝华增长和雪收集冰晶增长,过冷水对形成霰很关键,冰相粒子融化加强雨的形成;降雪过程对热力、动力过程具有一定的反馈影响,相变潜热对上升运动和降水有正反馈作用,降水粒子下落拖曳力对上升运动和降水有负反馈作用。辽宁降雪过程降水粒子丰富,云物理过程对降水热力动力过程的反馈作用比北京小雪要强一些。
Snowfall is very common in north China in winter and sometimes of negative effects on the traffic and power transmission in big cities. The weather systems inducing snowfall are various. Some are small systems, for example the shallow trough system causing light snowfall during 7 - 8 Dec 2001 in Beijing city. Some are largescale cold frontal system, for example the cold wave causing rainfall and snowfall during 23 - 24 Nov 1999 in Liaoning province. Many researches have been done about the weather and climate characteristics of snowfall. While the understanding of cloud processes of snowfall is also essential to the formation and development mechanism of snowfall. Two snowfall cases under different weather conditions are simulated using the mesoscale model MM5. Two-level nesting structure of domains is designed for each case. Among the expilcit schemes of MM5, the Reisner graupel scheme is selected to describe the microphysical process. The simulated snowbands of two cases are basically consistent with observation. Discussion is mainly focused on the simulated results of microphysical processes. The distribution of water substance and source and sinks of the two cases under different weather backgrounds is described. The feedback effects of microphysical processes on the thermal and dynamic processes are also discussed. Method that output the accumulative source and sinks per hour is used to analyze the distribution characteristics of water substance during the strongest snowfall period. Two sensitive tests （called heat test and drag test） are conducted to examine the effects of microphysical processes on cloud produced by the latent heat and drag force. Results show that the distribution of particles has close relationship with temperature. The temperature of Beijing snowfall is under 0 ℃ and there exist vapour and solid phase particles, while Liaoning snowfall has vapour, liquid and solid phase particles due to the warm temperature. The distribution of these particles is not the same at different development stage. From the analysises of the characteristics of source and sinks, we can find that snow is mainly produced by the deposition and accretion with ice. Cloud water is crucial to graupel. The melting of ice-phase particles enhances the rain production. The results of heat tests and drag tests reveal that the microphysical processes have interacted with the dynamic and thermal processes. Latent heat of water substance feeds back positively on snowfall while the drag force not. At last comparisons of simulated results have been done between the two different kinds of snowfall cases. The microphysical processes of Liaoning snowfall case is more complicated than that of Beijing snowfall case. The values of the cloud variables are larger and the interactions between the microphysical processes and the thermal and dynamic processes of Liaoning snowfall case are stronger than that of Beijing snowfall case.
出处
《气象学报》
CAS
CSCD
北大核心
2007年第1期29-44,共16页
Acta Meteorologica Sinica
基金
国家科技部科研院所社会公益专项(2002DIA20013
2004DIB3J116)
国家自然科学基金课题(40305001)
