大连地区一次区域暴雪的特征分析和数值模拟

Characteristic Analysis and Numerical Simulation of a Snowstorm in Dalian Region

利用多普勒雷达资料和GTS1型数字式探空仪探测资料,分析了2006年2月6～7日大连地区暴雪过程的回波演变和要素分布特征。结果表明,降雪期间,多普勒雷达显示为20～25 dBz的层状云,回波高度不超过3 km,对流层中下层的高空冷暖平流是产生强降雪的主要原因。采用中尺度非静力模式MM5对暴雪过程进行了数值模拟,模式较好地模拟了这次过程强降雪中心的强度、位置及强降雪的变化时间。这次降雪的主要影响系统是高空槽和地面华北气旋。诊断分析了强降雪的动力和热力特征及降雪期间的中尺度特征和云物理过程。高空槽前的西南暖湿气流提供了有利的水汽条件,高空辐散和低层辐合相叠置及高空正涡度的下传,有利于垂直上升运动的加强和地面华北气旋的发展。降雪前的增暖增湿与北方冷空气的楔入使大连位于sθe能量锋区和水汽辐合区内,有利于强降雪的产生。中尺度气旋性涡旋系统的形成和发展是强降雪产生和维持的有利因素,中尺度系统维持时间相对短暂。降雪期间,云中水成物的相态分布与温度密切相关,这次降雪过程只有气、固两相粒子作用。冰相粒子主要维持在600～300 hPa,其下部与雪区相对应。冰相粒子发展加强,地面降雪增大;冰相粒子减弱消失,降雪减小停止。

The characters of radar echo and meteorological element of a heavy snowfall event which occurred in Dalian on 6～7 February 2006 are analyzed using the data of Doppler radar and radiosounding in Dalian.In the Doppler radar echo intensity field,the intensity of sheet cloud echo in the period of the heavy snowfall is about 20～25 dBz and the maximum echo depth is less than 3 km.In the velocity field,the cold and warm advection′s alternative change of the upper air in the middle and lower layers of troposphere are the major causes reducing heavy snow.Furthermore,the numerical simulation has been carried out using the mesoscale numerical model（MM5）.The model can well simulate the strength,position and time change of snowstorm center.The main effect systems of causing heavy snowfall process are high-level trough and Huabei cyclone.Then, the dynamic and thermal,mesoscale characteristics and microphysical processes during snowfall are diagnostically analyzed.The results indicate that the southwesterly warm and moist air at middle and lower layers of tropospheres provide favorable vapor for heavy snowfall.Superposition between the upper layer divergence and the lower layer convergence as well as the positive vorticity transported downward from the upper troposphere over cyclonic circulation are favorable to intensification of upward movement in vertical and generation of Huabei cyclone.Interaction between warm and cold air is prone to formation of θse frontal zone,Dalian locates in the area of energy front and water vapor convergence,the heavy snowfall appears in that area.The forming and strengthening of mesoscale cyclonic vortex is favorable factor of heavy snowfall occurring and its lasting impact time is relative to the shorter.During the snowfall process,the phase distribution of particle in cloud has closed relationship with temperature.The temperature of Dalian snowfall is under 0℃ and there exist vapor and solid phase particles.The ice-phase particles mostly exist between 600 hPa and 300 hPa and the snow is under it.The ice-phase particles increase,the snowfall enhances.The ice-phase particles gradually decrease and vanish,the snowfall weaken and stop.