OBSERVATIONAL STUDY ON THE PRE-TROPICAL CYCLONE SQUALL LINE OF 8 AUGUST 2007 OVER THE COAST OF SOUTH CHINA
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摘要: 2007年8月8日，热带风暴“帕布”移动到华南近海，在珠江三角洲至湛江以西地区出现了一次强飑线天气过程。根据多普勒天气雷达、风廓线雷达和气象探空等观测数据，分析此次台前飑线的生成、演变过程、组织结构以及环境大气条件特征，并对热带风暴“帕布”与台前飑线环境大气的关系进行初步探讨。观测和分析结果显示：(1)此次台前飑线系统是由孤立的对流单体逐渐发展而成，陆风环流的抬升作用有可能对飑线的初始生成起到重要作用；(2)台前飑线移动路径和强度受海岸附近环境条件的影响；在海岸靠近陆地一侧的强度远比内陆和海洋上强，移动路径倾向于沿海岸线平行；(3)台前飑线在发展和成熟阶段，其水平结构具有典型的尾流层云降水特征；其冷池强度和垂直结构具有典型的热带飑线特征；(4)台前飑线发生在具有深厚水汽层、对流凝结高度较低的环境大气条件中，与热带飑线的环境大气条件类似；而对流不稳定能量和低层垂直风切变强度与中纬度飑线接近；(5)热带气旋外围大风一方面使低层风切变加强，同时为环境大气提供了高层的水汽。在下沉环流区内太阳辐射使陆地明显增温，一方面使位势不稳定能量增大，另一方面也使海陆温差增大、海风环流加强，导致低层风切变进一步加强，低层水汽输送增大。下沉逆温抑制了低层弱对流的发生，为强对流的发展积累了对流不稳定能量。Abstract: A squall line in front of the tropical cyclone Pabuk occurred in the west of the Pearl River Delta to Zhanjiang on August 8th, 2007 when the storm approached South China. The development, structure and environmental conditions for this squall line were investigated in this study, with particular attention paid to the possible connection of this squall line with Pabuk. The observational data employed in this study are from soundings, Doppler weather radars and wind profile radars. The following six major conclusions are drawn by our observational analyses. (1) This squall line developed gradually from individual convective cells, and land breeze may be responsible for the onset of the squall line. (2) The path and intensity of the squall line were modulated by the environmental conditions. The squall line propagated along the coastline, and it was stronger on the landing side of the coastline compared with the surrounding in-land regions and oceanic regions. (3) The typical characteristics of tropical squall lines were seen in this squall line, including the cold-pool intensity, vertical structure and the wake flow stratiform precipitation at its developing and mature phases. (4) The environmental conditions of this squall line resemble those of tropical squall lines in terms of deep moist air and low convection condensation level. They also resemble mid-latitude squall lines in terms of the convective instable energy and vertical wind shear in the lower troposphere. (5) Two roles were played by the strong wind around Pabuk. On the one hand, it made the atmosphere more unstable via suppressed shallow convection and increased solar radiation. On the other hand, it enhanced the land-sea thermal contrast and therefore strengthened the sea breeze and the resultant water vapor transport. The sinking temperature inversion prevented the occurrence of low-layer weak convection and accumulated convection instability energy for the development of the strong convection.
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Key words:
- squall line /
- tropical cyclone /
- land breeze /
- environmental conditions
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图 6 强热带风暴“帕布”移动路径与对应的台前飑线雷达强回波位置
实线表示每隔2小时强回波带的位置；点断线和虚线分别表示8日14时—9日00时飑线和“帕布”的移动距离和方向。
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