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The Role of Boundary Layer Convergence Line in Initiation of Severe Weather Events
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摘要: 应用天津CINRAD/SA雷达探测到的边界层辐合线信息，对2008—2009年6—9月发生在天津地区的边界层辐合线进行了统计分析，与2002—2007年海风锋触发形成的局地强对流天气分析相结合，研究结果表明：天津地区边界层辐合线的雷达低仰角基本反射率因子产品的回波特征一般表现为弱的窄带回波，回波强度仅为15~25 dBZ，宽度一般为3~10 km。在合适的层结状态和水汽条件下，这些边界层辐合线的演变和碰撞与强对流天气的发生、发展密切相关。单一边界层辐合线一般不能形成大范围的雷暴天气；两条以上边界层辐合线之间碰撞，一般在碰撞交叉处能够形成强对流天气。若已存在强对流天气，则强对流天气将加强。Abstract: CINRAD/SA Doppler Weather Radar can always detect weak and narrow echoes. Generally, those echoes can be closely associated with boundary layer convergence line, the feature and evolution of which may indicate the initiation of severe weather events. The data detected by Doppler radar from June to September of 2008 and 2009 in Tianjin areas, and the data on the events of sea breeze fronts from 2002 to 2007 are statistically analyzed. It shows that radar echo of boundary layer convergence line is also weak and thin, the intensity of echoes is 15—25 dBZ, and the width of echoes is 3—10 km. Boundary layer convergence line may be divided into three types, including sea breeze front, gust front and thin line due to special weather conditions. Single boundary layer convergence line may form isolated thunderstorm with favorable situation with moderate impact. The collision between boundary layer convergence lines can trigger thunderstorms or enhance the existing thunderstorms to be more severe.56 and 44 sea breeze front cases are observed by radar in 2008 and 2009 respectively. The total number of 25 events of the initiation of severe weather events is closely associated with sea breeze front, including 17 events in 2008 and 8 events in 2009. The formation times of sea breeze front are different, the earliest time is 09:30, while the latest is 16:00. The sea breeze front's maintenance time is also of different length, the longest maintenance time is 6.5 hours, and the shortest is only 1 hour. The general distance that sea breeze front extends to the interior is 70—80 km, and the farthest distance reaches 120 km, with the height of 1.5 km in general. Those provide important information on the initiation of thunderstorms for nowcasts.11 and 9 gust front cases are observed by radar during the observation period in 2008 and 2009 respectively. The maintenance time of gust fronts are inhomogeneous, with the longest maintenance time of 4.0 hours. Collision between sea breeze front and gust front can trigger thunderstorms. The boundary layer convergence lines due to local conditions will be discussed in future research. Analysis on the AWS data in Tianjin area combined with the data of Beijing sounding above 850 hPa indicates that thunderstorms are more likely to occur in the region with large temperature gradient, the temperature difference between 850 hPa and 500 hPa may reach 28℃.
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图 1 2009年6月 (a)、7月 (b)、8月 (c) 和9月 (d) 渤海湾海风锋与强对流天气形成统计特征
Fig. 1 Statistics of interrelation between characteristics of sea breeze fronts and thunderstorms in Bohai Bay from Jun to Sep in 2009 (a) Jun, (b) Jul, (c) Aug, (d) Sep
图 2 2010年7月4日14:00(a) 和16:30(b) 海风锋0.5°仰角基本反射率因子(相邻圈距离为50 km)
Fig. 2 0.5° PPI base reflectivity image on 4 Jul 2010 (a)14:00, (b)16:30(the interval between adjacent circles is 50 km)
图 3 2008年 (a)、2009(b) 年天津多普勒天气雷达探测阵风锋天气个例统计
Fig. 3 The total number of gust front detected by Doppler radar in Tianjin in 2008(a) and 2009(b)
图 4 2010年7月31日12:00(相邻圈距离为50 km)(a)、12:40(b) 和13:00(c)0.5°仰角的雷达基本反射率因子产品
Fig. 4 0.5° PPI base reflectivity evolution on 31 Jul 2010(the interval between adjancent circles is 50 km)(a)12:00, (b)12:40, (c)13:00
表 1 2002—2007年渤海湾海风锋与其他边界层辐合线碰撞触发形成强对流天气统计
Table 1 Statistics of the initiation of thunderstorms triggered by the collision between sea breeze fronts and boundary layer convergence lines from 2002 to 2007
海风锋个例编号 碰撞时间 有无强对流天气 碰撞地点 边界层辐合线路径 1 15:00 雷暴形成 天津宁河 东北 2 17:00 雷暴加强 天津宝坻 东北 3 13:30 雷暴形成 天津宁河 东北 4 17:30 雷暴形成 天津蓟县 西南 5 17:30 雷暴加强 天津蓟县 西北 6 19:00 雷暴减弱 天津市区 西北 7 17:36 无 天津武清 西北 8 16:00 无 天津北辰 东南 9 16:00 无 市区天津 东南 10 14:49 无 天津武清 准静止 11 13:30 无 天津北辰 西北 12 15:00 无 天津东丽 东南 13 20:00 雷暴加强 天津宝坻 西北 14 16:00 雷暴加强 天津宝坻 西北 
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