Investigation of Severe Precipitation Event Caused by an Eastward-Propagating MCS Originating from the Tibetan Plateau and a Downstream Southwest Vortex
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摘要: 基于加密自动站降水、葵花8卫星和ECMWF ERA5再分析等多种资料，本文对2018年6月17日08时至18日22时（协调世界时，下同）一次青藏高原（简称高原）中尺度对流系统（Mesoscale Convective System，简称MCS）东移与下游西南低涡作用并引起四川盆地强降水的典型事件进行了研究（四川盆地附近最大6小时降水量高达88.5 mm）。研究表明，本次事件四川盆地的强降水主要由高原东移MCS与西南低涡作用引起，高原MCS与西南低涡的耦合期是本次降水的强盛时段，暴雨区主要集中在高原东移MCS的冷云区。高原东移MCS整个生命史长达33 h，在其生命史中，它经历了强度起伏变化的数个阶段，总体而言，移出高原前后，高原MCS对流的重心显著降低，但对流强度大大增强。在高原MCS的演变过程中，四川盆地有西南低涡发展，该涡旋生命史约为21h，所在层次比较浅薄，主要位于对流层低层。西南低涡与高原MCS存在显著的作用，在高原MCS与西南低涡耦合阶段，两者的上升运动区相叠加直接造成了强降水。此后，由于高原MCS系统东移而西南低涡维持准静止，高原MCS与西南低涡解耦，西南低涡由此减弱消亡，东移高原MCS所伴随的降水也随之减弱。涡度收支表明，散度项是西南低涡发展和维持的最主导因子，此外，倾斜项是800 hPa以下正涡度制造的第二贡献项，而垂直输送项则是西南低涡800hPa以上正涡度增长的另一个主导项，这两项分别有利于西南低涡向下和向上的伸展。相关分析表明，在西南低涡发展期间，高原MCS中冷云面积（相当黑体亮度温度TBB≤−52°C）可以有效地指示西南低涡强度（涡度）的变化，超前两小时的相关最显著，相关系数可达0.83。Abstract: Based on precipitation data obtained by an automatic observation station, Himawari-8 satellite black-body temperature data, and European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis data, we investigated a severe precipitation event wherein an eastward-propagating mesoscale convective system (MCS) originating from the Tibetan Plateau (TP) influenced a downstream Southwest vortex (SWV), causing heavy precipitation over the Sichuan Basin (maximum of 6 hours of precipitation around the Sichuan Basin totaled 88.5 mm). The main results of our analysis are as follows: this heavy precipitation event was mainly induced by the effect of an eastward-propagating MCS and a downstream SWV, with strong rainfall appearing during the coupling of the MCS and SWV within the cold cloud area of the MCS. The eastward-propagating MCS lasted for a total of 33 hours, during which its intensity obviously changed. Overall, compared with the stage prior to leaving the TP, after leaving the TP the centroid convection of the eastward-propagating MCS decreased in height but significantly increased in its convection intensity. During the lifespan of the eastward-propagating MCS, the SWV exhibited quasi-stationary behavior around the Sichuan Basin. This vortex lasted for about 21 hours and persisted in a shallow layer that was mainly located in the lower troposphere. The eastward-propagating MCS significantly affected the SWV. During the coupling of the MCS and SWV, the superposition of the ascending motions associated with the two systems directly induced the heavy precipitation. Subsequently, the MCS moved eastward, whereas the location of the SWV changed very little, which resulted in the decoupling of the eastward-propagating MCS from the SWV. This reduced the intensity of the SWV and decreased the precipitation associated with the eastward-propagating MCS. The vorticity budget indicates that the convergence effect dominated the development and maintenance of the SWV. In addition, the tilting effect was the second greatest contribution to the production of positive vorticity below 800 hPa. The vertical transport was another dominant factor in the positive vorticity enhancement associated with the SWV above 800 hPa. Overall, these two effects promoted the downward and upward extensions of the SWV, respectively. The results of our correlation analysis reveal that during the development of the SWV, the cold cloud area of the eastward-propagating MCS (using −52°C as the boundary) could effectively reflect variation in the SWV intensity (vorticity), with the largest correlation (up to 0.83) appearing two hours in advance.
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图 1 2018年6月17～18日自动站6 h累积降水量（单位：mm）：（a）17日12～18时（协调世界时，下同）；（b）17日18时至18日00时；（c）18日00～06时；（d）18日06～12时。红色线表示青藏高原（简称高原）边界，为3000 m等高线
Figure 1. 6-h accumulated precipitation observed by the automatic station from 17 to 18 Jun 2018 (units: mm): (a) 1200 UTC 17 to 1800 UTC 17 Jun, (b) 1800 UTC 17 to 0000 UTC 18 Jun, (c) 0000 UTC 18 to 0600 UTC 18 Jun, and (d) 0600 UTC 18 to 1200 UTC 18 Jun. The red line indicates the Tibetan Plateau boundary, which has a terrain height of 3000 m
图 2 2018年6月17日08时至18日18时逐2 h的MCS分布。阴影：黑体亮温（TBB，单位：°C），黑色等值线：500 hPa位势高度（单位：gpm，褐色线为槽线），黑色风羽：500 hPa风场（风速≥12 m s−1，一根长羽为4 m s−1），红点：自动站站点（1小时降水量≥5 mm h−1），红圈为关注的MCS，红色竖线指103°E
Figure 2. Mesoscale convective system (MCS) distribution at 2-h intervals from 0800 UTC 17 to 1800 UTC 18 Jun 2018.Shaded area: temperature of black body (units: °C); black contours: 500-hPa geopotential height(units: gpm, thick brown lines are trough lines); blackwindbarb:500-hPa wind(speed ≥12 m s−1, and a full bar indicates 4 m s−1); red dots: automatic stations (hourly precipitation ≥ 5 mm h−1); red circles and red vertical lines indicate the MCS and 103°E, respectively
图 3 2018年6月17日12时至18日09时的（a）700 hPa时间平均流场[彩色阴影：涡度（单位：10−5 s−1），蓝色风羽：低空急流（风速≥8 m s−1，一根长羽为4 m s−1），紫色框线：西南低涡形成发展关键区，灰色阴影：地形（≥3000 m）] 及（b）涡度（阴影，单位：10−5 s−1）、散度（黑色线，单位：10−5 s−1）、垂直速度（红色线，单位：m s−1）区域平均高度—时间演变（紫色和蓝色虚线分别表示西南低涡最高层和最低层高度）
Figure 3. (a) 700-hPa temporal average streamline field[color shaded area: vorticity (units: 10−5 s−1), blue wind barb: lower-level jets (speed≥8 m s−1, and a full bar indicates 4 m s−1), purple rectangular box: the key region for the formation and development of the Southwest vortex (SWV), gray shading: terrain≥3000 m] and (b) vortex-averaged vorticity (shaded area, units: 10−5 s−1), divergence (black lines, units: 10−5 s−1), and vertical motions (red lines, units: m s−1) (the dotted purple and blue lines indicate the top and bottom levels of the vortex, respectively)from 1200 UTC 17 to 0900 UTC 18 Jun 2018
图 4 2018年6月（a）17日12时、（b）17日18时、（c）17日22时和（d）18日09时的700 hPa流场。彩色阴影：涡度（单位：10−5 s−1），蓝色风羽：低空急流（风速≥8 m s−1，一根长羽为4 m s−1），灰色阴影：地形（≥3000 m）
Figure 4. 700-hPa streamline field at (a) 1200 UTC 17, (b) 1800 UTC 17, (c) 2200 UTC 17, and (d) 0900 UTC 18 Jun 2018. Color shaded area: vorticity (units: 10−5 s−1), blue wind barb: lower-level jets (speed≥8 m s−1, a full bar indicates 4 m s−1), the gray shaded area with terrain above 3000 m in height
图 5 2018年6月（a–c）17日22时和（d–f）18日09时沿32°N的纬向剖面。（a, d）散度（彩色阴影，单位：10−5 s−1）、涡度（等值线，单位：10−5 s−1）和矢量 [（u，−100ω），黑色箭矢，垂直速度ω单位为Pa s−1，水平速度u单位为m s−1]; (b, e）位涡（彩色阴影，单位：1PVU=10−6 K m2 kg−1 s−1）、相对湿度（等值线，单位：%）；（c）, f）温度离差（彩色阴影，单位：°C）、全风速（等值线，单位：m s−1）。灰色阴影代表地形
Figure 5. Zonal section along 32°N at (a–c) 2200 UTC 17 and (d–f) 0900 UTC 18 Jun 2018.(a, d) Divergence (color shaded area, units: 10−5 s−1), vorticity (contours, units: 10−5 s−1), and vector [(u, −100ω), black vector, the unit of vertical velocity is Pa s−1, the unit of horizontal wind is m s−1]; (b, e) potential vorticity (color shaded area, units: 1PVU=10−6 K m2 kg−1 s−1) and relative humidity (contours, units: %); (c, f) temperature deviation (color shaded area, units: °C) and full wind speed (contours, units: m s−1). The gray shaded area indicates terrain
图 6 2018年6月17日12时至18日09时西南低涡区域平均涡度收支各项（单位：10−10 s−2）的高度—时间剖面：（a）散度项；（b）倾斜项；（c）水平平流项；（d）垂直输送项
Figure 6. Vorticity budget averaged over the SWV region from 1200 UTC 17 to 0900 UTC 18 Jun 2018 (shaded area, units: 10−10 s−2): (a) Divergence term; (b) tilting term; (c) horizontal advection term; (d) vertical transport effect term
图 7 2018年6月17日13时～18日09时高原MCS随时间的演变图：（a）涡度（单位：10−5 s−1）；（b）散度（单位：10−5 s−1）；（c）垂直速度（单位：m s−1）；（d）位涡（单位：1 PVU = 10−6 K m2 kg−1 s−1）。黑色虚线指17日15时，红色虚线是17日20时，地形（灰色阴影）表示目标MCS的中心对应的参考高度
Figure 7. Variation of the moving-propagating MCS originating from the Tibetan Plateau with time from 1300 UTC 17 to 0900 UTC 18 Jun 2018: (a) Vorticity (units: 10−5 s−1); (b) divergence (units: 10−5 s−1); (c) vertical motion (units: m s−1); (d) potential vorticity (units: 1 PVU = 10−6 K m2 kg−1 s−1). The black and red dashed lines indicate 1500 UTC 17 and 2000 UTC 17, respectively, and the gray shaded area shows the central terrain height of the MCS
图 8 2018年6月17日08～18时（a）理塘风廓线雷达风以及甘孜微波辐射计（b）温度和（c）液态水含量随时间的演变
Figure 8. (a) Wind observed by the Litang wind profile radar, (b) temperature and (c) liquid water content observed by the Ganzi microwave radiometer with time from 0800 to 1800 UTC 17 Jun 2018
图 9 程序客观追踪得到的2018年6月17日13时至18日22时（a）MCS移动路径（GZ和LT分别表示表示甘孜、理塘测站位置，阴影表示地形）和（b）MCS面积和最小TBB的时间演变
Figure 9. (a) Track of MCS obtained by computer program objectively (the blue GZ and LT indicate the station positions of Ganzi and Litang, respectively, and the shaded area means terrain), and (b) area and minimum TBB of the MCS varies with time from 1300 UTC 17 to 2200 UTC 18 Jun 2018
表 1 2018年6月17日13～17时青藏高原MCS云团参数与17日15～19时西南低涡关键区700 hPa物理量的超前2小时的相关结果
Table 1. Correlation results two hours in advance of the MCS originating from the Tibetan Plateau from 1300 UTC 17 to 1700 UTC and physical variables in the key region for the SWV from 1500 UTC 17 to 1900 UTC
与MCS云团参数的相关系数 涡度 垂直速度 散度 高度 MCS面积 0.83（0.08） 0.99（0.0007） −0.80（0.10） −0.86（0.06） 最小TBB 0.73（0.16） 0.51（0.38） −0.80（0.10） −0.79（0.11） TBB梯度 −0.45（0.44） −0.60（0.29） 0.52（0.37） 0.62（0.27） 最大直径 0.69（0.20） 0.93（0.02） −0.65（0.24） −0.70（0.20） 平均温度 0.62（0.27） 0.66（0.23） −0.73（0.16） −0.69（0.20） 注：括号内数值为检验相关系数是否显著的返回统计值，加粗数值表示超过90%的置信度水平 
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