Radar Echoes Characteristics of the Sudden Convective Rainstorm over Beijing Area on July 10, 2004

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.