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北京2004年“7.10”突发性对流强降水的雷达回波特征分析
Radar Echoes Characteristics of the Sudden Convective Rainstorm over Beijing Area on July 10, 2004
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摘要: 利用新一代天气雷达回波资料和一个雷暴单体识别、追踪和分析算法, 对2004年7月10日下午造成北京局地短时强降水的雷暴特征进行了初步分析。在偏南暖湿气流中生成的对流云团, 在北京上空迅速发展, 逐渐形成了一个覆盖城区的β-中尺度对流超级复合体, 导致了这次强降水过程。详细分析表明, 强对流主要是来自城区西南和东南两个方向生成和发展起来的雷暴。在北京西南部的雷暴逐渐向东北的城近郊区移动和发展, 并与新生成的雷暴合并加强, 造成了石景山、门头沟和海淀部分地区的大雨。在北京东南部逐渐形成的两个小雷暴单体迅速增长并向西北方的城区移动, 在到达城区时合并且迅速加强, 但移速缓慢, 在北京城区维持了两个多小时, 造成了城区的大暴雨过程, 降水量大但空间分布不均匀。雷暴顶高度和最大反射率因子的关系呈反位相变化, 雷暴最大反射率因子出现的高度均位于0 ℃等温线之下 (≥0 ℃) 或其附近, 雷暴的中心和反射率因子权重质心也基本位于0 ℃等温线之下, 均证实了这是一个典型的液态强降水对流系统。分析还表明, 20:00 (北京时) 左右的超强雷达回波是由大气异常传播造成的虚假超折射回波。Abstract: An intense thunderstorm attacks Beijing and brings sudden severe rainstorm over local Beijing urban area in the afternoon of July 10, 2004. An analysis of the paroxysmal rainstorm characteristics is made using weather observation data, radar echo data of CINRAD/SA S-band at Tanggu of Tianjin and a thunderstorm identification, tracking and analysis algorithm. The analysis illuminates the clear characteristics of the strong convective rainstorm which is of small extent, high precipitation, paroxysmal initiation and development, and slow motion. The evolution and motion properties indicate that the convection has direct and close relation to the large scale warm southerly airflow with abundant water vapor characteristics. The convective cloud clusters are gestated and given birth in warm and wet southwestern air stream. The cloud clusters rapidly intensify when they arrive in Beijing area, then gradually a super cell storm comes into being with meso-β scale over Beijing urban zone and result in a heavy rainfall. The detailed analysis results show the thunderstorm cells come from two different directions into the city area, southwest and southeast respectively, produce the strong convective rainfall. The southwest thunderstorms gradually move and develop northeastward near the urban zone, and merge and reinforce with newly inspired storm cells, bring a heavy rainfall in Shijingshan, Mentougou and Haidian districts of Beijing. The two small cells born at southeast of Beijing develop rapidly and gradually move northwestward into urban area. They merge into a storm when reaching urban zone, then the storm quickly strengthens. However, the storm has very low velocity and hovers and maintains near 2 hours just over Beijing urban zone. The storm pours a great deal precipitation into urban zone in only 2 hours. But the precipitation is not evenly distributed over urban zone. Statistical analyses and storms tracking on the evolution characteristics of the convection are performed by using Tianjin radar data and automatic rain gauge data of Beijing area. During the course of the heavy rain, the convection has an evolution characteristic from intensifying to weakening, then intensifying again. There is an evidently out-of-phase between the top of the thunderstorms and the height of maximal reflectivity. The height of maximal reflectivity is less than or close to the height of 0 ℃ isotherm, and the heights of centroid and reflectivity-weighted centroid for the whole storm are also less than the height of 0 ℃ isotherm on the whole. Therefore, the results confirm the convection can only give birth to liquid state rainfall without any hail. That is consistent with the fact that there is not any surface hail log. Another analysis, based on relative humidity from soundings at Beijing Observatory and radar data comparison between Tianjin S-band and Beijing C-band, indicats that these strong radar echoes of Tianjin radar at about 20:00 Beijing local time are resulted from anomalous propagation (AP), so they are inveracious superrefraction echoes.
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Key words:
- sudden severe rainstorm;
- radar echo;
- analysis of characteristics
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图 1 天津雷达 (CINRAD/SA) 探测示意图 (a) 和2004年7月10日下午北京城区和城近郊降水记录的自动站站点分布图 (b)
(b中观象台站点分布为探空站点)
图 6 TITAN对造成“7.10”强降水的雷暴发展演变和合并分裂的分析
(图中彩色阴影区为PPI格式的雷达反射率因子资料，仰角为0.5°, 蓝色粗等值线包含的区域其反射率因子≥35 dBz，为TITAN识别的雷暴在水平面上的投影，图中标值为最大反射率因子, 单位: dBz; A，B和C，D分别标示出引起强降水的雷暴单体在合并前的发展阶段，图h中黑色虚线为文中所述剖面的位置；雷达距离圈间隔为20 km)
图 7 TITAN追踪的图 6中蓝色粗等值线包含区域的雷暴在其不同高度上最大反射率因子 (a) 和平均反射率因子 (b) 随时间的变化特征
(图中数字为每个时刻雷暴最大反射率因子的值及其位置，褐色曲线为雷暴中心随髙度和时间的变化，紫色曲线为反射率因子权重质心随高度和时间的变化；黑色粗虚线为0 ℃等温层所在的近似高度）
图 8 2004年7月10日天津雷达观测的反射率因子阈值≥35 dBz的雷暴面积增长率
(图中雷达距离圈间隔为50 km) (a) 16:29, (b) 17:35, (c) 17:42, (d) 18:06
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