MESOSCALE FEATURES OF A SQUALL LINE IN SPRING 2013
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摘要: 运用多种观测数据，结合WRF模式分析了2013年3月19日发生在黔湘地区一次飑线形成期的中尺度特征。（1）此次飑线发生在高空500 hPa槽前的西南气流中，地面冷锋附近。环境风为西南向，且垂直于飑线长轴的分量小于沿着飑线长轴的分量。（2）飑线东、西两段存在显著差异：东段所在环境干燥，下沉对流有效位能DCAPE大，故雷暴大风强而短时强降水弱，对流单体初始于锋区及冷空气一侧，呈碎块状分布；西段环境湿润，短时强降水和冰雹集中，对流单体出现于地面锋区附近中尺度辐合线内，辐合线持久，其上辐合中心处不断有单体新生。（3）此次过程有重力波作用，且飑线西段重力波特征更明显。急流中的波动与中尺度辐合线相交，波动上升气流叠加辐合线上升运动，引起对流发生并迅猛发展，使得对流单体趋向于沿着波动等振幅面排列成带状，进而形成飑线。（4）旧单体南、北两侧均有新对流单体发生：北侧新单体高、低层重力波反相位叠加，对流受到抑制；南侧的新单体高、低层波动同相，上升气流加强，对流得以发展；新旧单体不断迭代更新，飑线整体向东南传播。Abstract: Based on several kinds of observational data and numerical simulation, the mesoscale features of a strong squall line on March 19, 2013, in southern China were investigated. The results show that: (1) The squall line occurred in front of a 500-hPa trough and was near a cold front on the ground. The ambient wind was mainly southwestern, and its cross-line component was smaller than the along-line component. (2) There were significant differences between the east and the west of the squall line. The eastern part was dry, and the DCAPE was large; as a result, short-term heavy rainfall was weaker than that in the western part, while thunderstorm gales were stronger. Convective cells in the eastern part initially distributed in a fragmented manner at the cold side of the frontal zone. The western part was humid, with short-term heavy rainfall and concentrated hail. Convective cells appeared within the mesoscale convergence line near the surface front area, and the convergence line was persistent. There are constantly new cells growing at the convergence center above them. (3) The gravity waves affected the maintenance of the squall line, and their effect in the western part was obviously stronger than that in the eastern part. When the updraft of the jet intersected with the convergent line, the lower-level updraft at the intersection strengthened and led to convective initiation (CI). Finally, the squall line formed along the amplitude of the gravity waves. (4) New convective cells emerged in both the north and the south of older cells. The upper and lower gravity waves of those new cells in the north were out of phase, restraining the convection; however, they were in phase in the southern new cells, which positively contributed to CI. New cells continued to emerge and replaced old ones, and the squall line propagated to the southeast.
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Key words:
- baroclinic frontogenesis /
- squall line /
- convection cell /
- mesoscale /
- gravity wave
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图 2 2013年3月19日11:00—23:00累积降水量（单位：mm）分布（a）、其中6个站点逐小时降水量演变（b）及灾害性天气（短时强降水、雷暴大风、冰雹）分布（c）
图 3 2013年3月19日12:00 500 hPa（a）、700 hPa（b）、850 hPa（c）、地面（d）天气图，贵阳（e）、桂林（f）、郴州（g）探空图及桂林（红色）、郴州（黑色）站相对于飑线的风场在垂直于飑线长轴（实线）和平行长轴（长虚线）的分量廓线图（h）
其中a~d中紫色方框为对流起始区，绿色方框为飑线经过区; 粗棕线代表槽线或切变线，蓝粗线代表地面冷锋，黑色等值线为位势高度（单位：dagpm）或海平面气压，红色等值线为温度（单位：℃），蓝色风向杆（每长杠10 m/s），彩色阴影为风速大小（单位：m/s）或雷达组合反射率因子（单位：dBZ）；c中紫色数字为各站点的温度露点差。e~g中黑线为温度层结曲线，紫线为状态曲线，绿线为露点温度曲线及单站风廓线黑色风向杆。
图 4 形成期12:00—16:00地面要素场与雷达组合反射率因子的逐小时（第一行为12:00，第二行为13:00，第三行为14:00，第四行为15:00，第五行为16:00）演变
a~e中填色为雷达组合反射率因子（单位：dBZ），红色等值线为气温，箭矢为地面风场，蓝色粗实线代表地面冷锋；f~k中填色为1小时变压（单位：hPa），红色等值线为变温（单位：℃）、黑色等值线为散度场（单位：10-5 s-1）；m~q中散点为小时降水量和黑色等值线为海平面气压（单位：hPa）。
图 7 12:15（a）、12:45（b）、13:15（c）、13:45（d）、14:00（e）低空2 km高度的散度场（彩色填色：10-5 s-1）、风场（灰度箭矢）、组合反射率因子（粗红色等值线：dBZ）演变及各要素沿107.8 °E经线5 min演变（f）
图 9 图 7中O点处各要素随时间的变化
a. 2 km（黑色实线）、4 km（点线）和9 km（长短虚线）高度上的垂直速度（单位：m/s）；b. 0~12 km垂直速度（其中＞0 m/s的区域填黄色）；c. 0~12 km内云水混合比（填色，g/kg），雨水混合比（黑线，g/kg），雷达反射率因子（≥20dBz，红线，等值线间隔10 dBZ）；d. a~c中蓝色箭头所示的两次沉降、两次上升过程中垂直速度的垂直分布廓线。
图 11 沿107.8 °E各要素的垂直剖面图
a~d水平风速扰动（单位：m/s）; e~h垂直速度（填色，单位：m/s），雷达反射率因子（蓝线，间隔20dBZ），气压扰动(单位：hPa)。
表 1 贵阳、桂林、郴州站主要的探空参数
CAPE/(J/kg) CIN/(J/kg) LI/K K DCAPE(600) T-Td(850 hPa) △U(0~3 km) △U(0~6 km) 贵阳 891 150 -3 34 630 4 22.2 30.7 桂林 59 552 0.5 29 / 7 21.1 34.2 郴州 1114 208 -3 30 1380 1 20.3 34.2 
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