0679香山局地大暴雨的中小尺度天气分析

Mesoscale and Microscale Analysis on a Local Torrential Rain Event in Fragrant Hills Area of Beijing on July 9,2006

利用Doppler雷达、地面自动站、Profile垂直风廓线及GPS水汽分布等多种新型探测资料,对2006年7月9日夜间发生在北京西郊香山附近的局地大暴雨天气的影响系统和γ-中尺度强降雨落区形成的动力机理进行了精细分析。对雷达等本地多种探测资料的精细研究表明:地形辐合回波带是造成这次过程的主要影响系统。地形辐合回波带上中气旋回波块的滚动更迭是大暴雨落区形成的直接原因;近地面辐合对大暴雨落区强降雨的发生具有重要作用。大暴雨落区形成阶段近地面3种辐合同时存在:平原东南风与山区偏北风风向切变辐合、平原东南风在山脉阻挡作用下的抬升辐合、大暴雨落区中心的γ-中尺度气旋性辐合。研究还表明:山前近地面地形辐合扰动,向上传播,引发边界层扰动的动力过程是香山大暴雨落区形成的主要动力源,而来自东南方向近地面层的暖湿平流为大暴雨提供了有效的水汽和能量。

The local rainstorm in fragrant hills area of western Beijing at night on July 9, 2006 is an intense rainfall event, which covers only around 10 to 20 kilometers, and the severe rain lasts only 2 hours. Considering the temporal and spatial scale and the distribution of the rainfall, it is difficult to analyze and describe by regular meteorological data and synoptic-dynamic diagnosis methods even mesoscale dynamic models. Therefore, it is studied by intensified observation data including Doppler weather radar products, wind profile data, wind element of surface automatic weather stations and detection data of atmospheric water vapor by GPS, etc. It is found that development of the meso-γ scale heavy rain, even the disturbance in surface and boundary layer around the rainfall area, can be captured by the above mentioned data. Moreover, meso-scale convective system of the local heavy rain event and dynamic mechanism forming severe rain can also be explained by these data exactly. Fine research on the local heavy rain event by these observation data shows that topographic convergence line is proved to be the major system that causes the local torrential rainfall, and the formation and development of mesocyclone in the echo belt by topographic convergence line is the main factor causing the heavy rain. Convergence in atmospheric surface layer is very important in the local heavy rain event. Besides, the studies show that topographic convergence disturbance in surface layer in front of the mountain spreads upward giving rise to boundary layer disturbance which is the main dynamic factor of the heavy rain in fragrant hills zone. Sufficient water vapor and energy for the heavy rainstorm are provided by warm-moist advection of atmospheric surface layer from southeast. From the above analysis, it is concluded： Against the large scale background of warm-moist airflow ahead of long-wave trough in middle troposphere about 500 hPa in the daytime in summer and weak anti-cyclone wind in low layer in Beijing, topographic convergence echo belts can be formed by formation and development of southeast wind in surface layer at dusk on the plain. Meso-γ scale echoes with features of mesocyclone in the echo belts move and develop from southwest to northeast along southwest airflow ahead of 500 hPa long-wave trough. It is the major cause that brings the local rainstorm in fragrant hills area of Beijing at night on July 9, 2006. Convergence in atmospheric surface layer plays an important role in the local heavy rain event. There are three types of convergence simultaneously during the severe rain period of two hours： Wind shear convergence between the southeast wind from the plain and the north wind from the mountains, convergence and lifting of the southeast wind from the plain by mountains, and cyclonic convergence of meso-γ scale system in the center of rainfall region. Fine research on wind profile data around heavy rainfall region shows that there are disturbance in surface layer and it spreads upward, then boundary layer disturbance appears twice during the severe rain period during the two hours. Each propagation is about one hour and twenty minutes. It is one of the main dynamic mechanisms that cause the local heavy rain to occur. According to GPS water vapor data and Doppler radar velocity products, there warm-moist airflow in atmospheric surface layer from southeast to the core of the heavy rainstorm before and during the torrential rain.