一次超级单体的多普勒特征和数值模拟特征对比分析

COMPARISON BETWEEN DOPPLER AND SIMULATED FEATURES FOR A SUPERCELL

首先利用多普勒雷达分析了影响广州的超级单体的典型特征。从反射率因子产品中可以观测到指状回波、V型缺口、弱回波区、回波悬垂和回波墙。截取沿着风暴低层入流方向并通过反射率因子核区的垂直剖面,分析出云内存在两支对峙的上升和下沉气流。随后利用三维对流风暴云模式模拟了此次过程。模拟的最大反射率因子为75 dBZ,60 dBZ回波核区厚度超过了14km,比实际探测值偏大,这是模式本身的问题,其主要原因是没有考虑衰减的影响,也可能是模式初始扰动偏大和粒子非球形的影响。模式还给出了超级单体强盛时的流场结构：近地层以辐合气流为主,为雷暴出流；其上上升气流占据主导地位,为辐合风场；但在顶端没有给出携带降雨粒子的出流所产生的云砧。另外,模式较好地模拟了云内垂直运动的演变：开始时,地面辐合形成有组织的上升气流；随后上升运动加大,具有倾斜性,促使单体进一步发展；之后,单体内出现了下沉气流,与上升气流并存,但上升气流占主导地位；风暴强盛时近地层辐散加强；此后,下沉气流逐步控制云体,单体迅速减弱。

Firstly, typical features of a supercell, which occurred in Guangzhou on August 11 th, 2004, are discussed by using the new generation weather radar data. V-notch, finger-echo, weak echo region, overhang and echo-wall are observed from reflectivity products. A vertical cross section of the radial velocity is made along the direction of the low-level inflow and across the maximum reflectivity core, which displays a part of strong updraft and downdraft. Secondly, a 3-D convective storm model is used to simulate the sepercell. The maximum reflectivity and the core thickness of the simulated radar echo are 75 dBz and 14km, respectively. These values are more than the counterparts that are detected by radar. The reason is that attenuation is not calculated in the model. The wind field structure is also given when the storm is the strongest. Divergence,, caused by thunderstorm outflow, is in the low level. In the middle and high level, convergence is dominant, but the plume is not simulated at the top. Finally, the evolution of the simulated vertical motion is documented. The interaction between the environmental wind and the updraft, which is formed by the convergence on the ground at the beginning, makes the storm stronger. Then, downdraft occurs and grows. When it becomes dominant, the supercell collapses.