AN OBSERVATIONAL ANALYSIS OF A TORRENTIAL RAINSTORM IN THE WARM SECTOR OF SOUTH CHINA COASTAL AREAS
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摘要: 2007年5月20日粤西茂名与阳江海岸带地区发生一场小时降水量达到115 mm的短时大暴雨。采用NCEP/NCAR FNL 1 °×1 °再分析资料，结合多普勒天气雷达、高空探空、自动站资料和风廓线雷达观测资料，分析此次暴雨过程的天气尺度系统与中尺度系统特征，探讨不同尺度系统对这次暴雨天气过程的作用。分析结果表明：（1）此次暴雨过程是由准线状中尺度对流系统导致的，对流系统的移动速度缓慢是导致出现暴雨的主要原因；（2）在暴雨发生前，整层大气水汽充沛，处于近饱和稳定状态；暴雨发生期间局地大气处于条件不稳定状态；中层弱冷槽过境及“上干下湿”的温湿结构增加了环境大气的不稳定性；（3）北部湾低压（槽）发展导致的强盛低层西南气流为暴雨发生提供了充沛的水汽；高空槽后干冷空气南下形成的深厚干层，有助于对流凝结潜热的释放；高层辐散、低层辐合环流为低层暖湿空气提供了垂直上升动力；（4）中尺度对流系统在地势相对平缓的沿海地区发展和加强，地形的动力抬升和辐合作用不大。对流活动诱发的低层密度流在对流带前缘不断激发出新的对流单体，对对流系统的维持和发展起关键作用；（5）对流单体的风暴传播效应使对流系统具有逆风传播的特征，移速缓慢；层云降水的蒸发冷却有可能改变其低层的温度梯度，使环境大气的不稳定性加强。Abstract: On May 20th 2007, a brief but severe downpour rainstorm occurred in the coastal areas of Maoming and Yangjiang areas with rainfall of 115 mm per hour. Data from NCEP/NCAR reanalysis with 1 °×1 ° resolution, Doppler weather radar, conventional surface observations, high-altitude radiosonde and wind profiler radar were used to analyze characteristics and contributions of synoptic scale and mesoscale systems during this torrential rainstorm. The results showed that: (1) the storm was caused by a quasi-linear mesoscale convective system (MCS) and the slow-movement of this system was the primary reason triggering torrential downpourrainstorm; (2) water vapor was abundant, nearly saturated and in steady state throughout the atmosphere before the storm; intrusion of the weak dry and cold air in the middle level and a striking " wet on " knot had increased the atmospheric instability; (3) low-level southwesterly airflow from a low pressure (trough) at Gulf of Tonkin provided abundant water vapors in the onset of thefor the rainstorm; a deep dry layer was formed by dry and cold air behind the high-level trough, which facilitated latent heat release; upper-level divergence and low-level convergence circulations also provided vertical uplift for warm and moist air at lower level; (4) Topography only played a minor role as the MCS developed and strengthened over relatively flat coastal terrain. Low level density flow induced by convection triggered new convective cell generation at the leading edge of the convective system, thereby playing a key role in the sustainaation might change temperature gradient at lower layers, and results in strengthening atmospheric instability.
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图 7 19日20时（a）、20日02时（b）、20日08时（c）200 hPa与700 hPa等压面上的等风速线与急流轴分布
细实线和粗实线分别代表 200 hPa上的等风速线和急流轴; 细虚线和粗虚线分别代表 700 hPa上的等风速线和急流轴.
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