Water Vapor Transportation Features and Key Synoptic-scale Systems of the “7.20” Rainstorm in Henan Province in 2021
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摘要: 本文重点分析了2021年“7.20”河南暴雨水汽输送特征、水汽来源以及关键天气尺度系统。双台风“烟花”和“查帕卡”以及西太平洋副热带高压共同为“7.20”河南暴雨提供了充足的水汽条件。然而，就暴雨的水汽供应而言，仅以台风和西太平洋副热带高压的作用难以解释2021年7月20日发生的日降水量663.9 mm和1小时最大降水量201.9 mm的极端暴雨事实。水汽通量分析和LAGRANTO模式轨迹分析结果表明，20日在河南南侧形成了一个很强的经向水汽通量带（850 hPa以上），它与台风和西太平洋副热带高压引起的低层水汽通量带在河南附近汇合，为暴雨提供了最为充沛的水汽条件。我们强调，20日在河南以西地区上空发生了对流层顶反气旋式波破碎事件，它与台风协同作用，引发了河南南侧的强经向水汽通量，从而导致此次极端暴雨事件。Abstract: This study analyzes the water vapor transportation features, water vapor sources, and key synoptic-scale systems of the “7.20” rainstorm in Henan in 2021. Double typhoons “In-fa” and “Cempaka” and the western Pacific subtropical high jointly provided sufficient water vapor conditions for the “7.20” rainstorm in Henan. However, the extreme rainstorm event, which has daily precipitation of 663.9 mm and 1-hour maximum precipitation of 201.9 mm on July 20, can hardly be explained only by the roles of typhoons and western Pacific subtropical high. Results of the water vapor flux analysis and trajectory analysis based on the LAGRANTO model show that a strong northward water vapor flux zone (above 850 hPa) was formed on the southern side of Henan on July 20, 2021. It then converged with the low-level water vapor flux zone facilitated by the typhoon and western Pacific subtropical high near Henan, thus providing the most abundant water vapor conditions for the rainstorm. This work emphasizes that the anticyclonic wave-breaking event that occurred at the tropopause to the west of Henan on July 20 triggered a strong meridional water vapor flux on the southern side of Henan and worked synergistically with typhoons, resulting in this extreme rainstorm.
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Key words:
- Henan rainstorm /
- Water vapor flux /
- Lagrangian trajectory model /
- Typhoon /
- Wave breaking
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图 1 2021年（a）7月17日08:00至22日08:00（北京时）河南省累计降水量（单位：mm）分布和（b）7月17～22日的平均比湿沿34°N的经度—气压垂直剖面（单位：g kg−1）
Figure 1. (a) Distribution of accumulated precipitation (units: mm) in Henan from 0800 BT (Beijing time) 17 to 0800 BT 22 July 2021 and (b) pressure–longitude cross section of the specific humidity (units: g kg−1) along 34°N averaged from 0800 BT 17 to 0800 BT 22 July 2021
图 2 2021年7月18日08:00至21日08:00（北京时）的平均比湿（填色，单位：g kg−1）和等压面平均水汽通量（箭头，单位：m s−1 g kg−1）：（a）850 hPa；（b）700 hPa；（c）500 hPa；（d）400 hPa。河南省界以红色线标出
Figure 2. Specific humidity (shaded, units: g kg−1) and water vapor flux (vectors, units: m s−1 g kg−1) averaged from 0800 BT 18 to 0800 BT 21 July 2021: (a) 850 hPa; (b) 700 hPa; (c) 500 hPa; (d) 400 hPa. The border of Henan Province is marked with a red line
图 3 2021年7月（a–f）17～22日1000～300 hPa日平均整层水汽通量（箭头，单位：kg m−1 s−1）和水汽通量散度（填色，单位10−4 kg s−1）。右下角标度尺为1000 kg m−1 s−1，其中红色（蓝色）箭头表示大于或等于（小于）250 kg m−1 s−1。河南省界用黑色线标出
Figure 3. Daily evolutions of the vertically integrated water vapor flux (vectors, units: kg m−1 s−1) between 1000 and 300 hPa and its divergence (shaded, units: 10−4 kg s−1) from July (a–f) 17 to 22, 2021. The scale bar in the lower right corner is 1000 kg m−1 s−1. The red (blue) arrow indicates that it is greater than or equal to (less than) 250 kg m−1 s−1. The border of Henan Province is marked with a black line
图 4 2021年7月19日（左列）和20日（右列）各层日平均水汽通量（箭头，单位：kg m−1 s−1）及其散度（填色，单位：10−4 kg s−1）分布：（a、b）1000～850 hPa层；（c、d）850～700 hPa层；（e、f）700～300 hPa层。其中红色（蓝色）箭头表示大于或等于（小于）100 kg m−1 s−1。河南省界用黑色线标出
Figure 4. Daily evolutions of the vertically integrated water vapor flux (vectors, units: kg m−1 s−1) and its divergence (shaded, units: 10−4 kg s−1) in different layers on July 19 (left column) and July 20 (right column), 2021: (a, b) 1000–850 hPa; (c, d) 850–700 hPa; (e, f) 700–300 hPa. Red (blue) arrows indicate values greater than or equal to (less than) 100 kg m−1 s−1. The boundary of Henan Province is marked with a black line
图 5 由河南区域（a）南边界和（b）东边界流入等压面的水汽通量（单位：105 m2 s−1）随时间和高度的变化。横坐标为时间，从2021年7月18～21日，每日四次
Figure 5. Pressure–time cross section of the water vapor flux (units: 105 m2 s−1) averaged along (a) the southern boundary and (b) the eastern boundary of the Henan region. The horizontal axis represents the time, four times per day, from July 18 to 21, 2021
图 6 2021年7月20日00:00、06:00、12:00、18:00四个时次1000～300 hPa整层水汽通量（箭头，单位：kg m−1 s−1）及其散度（填色，单位：10−4 kg s−1）。右下角标度尺为1000 kg m−1 s−1，其中红色（灰色）箭头表示大于或等于（小于）250 kg m−1 s−1。河南省界用黑色线标出，郑州站用绿色圆点标出
Figure 6. Daily evolutions of the vertically integrated water vapor flux (arrows, units: kg m−1 s−1) between 1000 and 300 hPa and its divergence (shaded, units: 10−4 kg s−1) at 0000 UTC, 0600 UTC, 1200 UTC, and 1800 UTC of July 20, 2021. The scale bar in the lower right corner is 1000 kg m−1 s−1. The red (blue) arrow indicates that it is greater than or equal to (less than) 250 kg m−1 s−1. The border of Henan Province is marked with a black line. Zhengzhou station is marked with a green dot
图 7 采用后向持续追踪3天的方案情景下初始场为2021年7月（a、e）18日、（b、f）19日、（c、g）20日和（d、h）21日的日平均轨迹密度（左列）以及轨线平均比湿Q大于12 g kg−1的轨迹密度（右列）。蓝色方框为LAGRANTO轨迹模式的初始场
Figure 7. Daily averaged trajectory densities (left column) and the track densities with the track average specific humidity Q greater than 12 g kg−1 (right column) with the initial field on (a, e) 18, (b, f) 19, (c, g) 20, and (d, h) 21 July 2021 in the scheme with backward tracking for 3 days, respectively. The blue box indicates the initial field domain for the LAGRANTO model
图 8 2021年7月16～21日东亚/西太平洋区域700 hPa流场，其中蓝色框为LAGRANTO模式初始场选定范围
Figure 8. 700-hPa streamlines over East Asia/western Pacific region from July 16 to 21, 2021. The blue box indicates the initial field domain for the LAGRANTO model
图 10 初始场为2021年7月19日06:00（左列）和20日06:00（右列）的轨线分布：（a、b）起始点位于900～500 hPa层的所有轨线；（c、d）起始点位于900～850 hPa的轨线；（e、f）起始点位于850～700 hPa的轨线；（g、h）起始点位于700～500 hPa的轨线。其中每条轨线必须满足平均比湿大于12 g kg−1的条件，线条的颜色代表对应的气压值（hPa），黑色方框与图7蓝色方框一致
Figure 10. Track distribution with the initial field at 0600 UTC on 19 July (left column) and 20 July (right column) 2021: (a, b) All tracks with starting points within 900–500 hPa layer; (c, d) tracks with starting points within 900–850 hPa; (e, f) tracks with starting points within 850–700 hPa; (g, h) tracks with starting points within 700–500 hPa. The mean specific humidity of each track must be greater than 12 g kg−1. The color bar marks the corresponding air pressure (hPa). The black box has the same meaning as the blue box in Fig. 7
图 11 2021年7月19日06:00（左列）和20日06:00（右列）位势高度场（等值线，单位：gpm）和风场（箭头，单位：m s−1）分布：（a、b）250 hPa；（c、d）500 hPa；（e、f）700 hPa。等值线间隔在（a、b）中为20 gpm，而在（c–f）中为10 gpm
Figure 11. Geopotential height fields (contours, units: gpm) and the corresponding wind fields (arrows, units: m s−1) respectively at (a, b) 250 hPa, (c, d) 500 hPa, and (e, f) at 700 hPa at 0600 UTC on 19 July (left column) and 20 July (right column) 2021. Contour intervals in (a) and (b) are 20 gpm and in (c–f) are 10 gpm
图 12 2021年7月19和20日两个位涡单位面（2 PVU面，PVU=10−6 K kg−1 m2 s−1）的位温（单位：K）分布，其中绿色加粗实线为360 K等位温线。河南省界用紫色线标出
Figure 12. Potential temperature distribution (units: K) on the 2 PVU (1 PVU=10−6 K kg−1 m2 s−1) surface on July 19 and 20, 2021. The green thick solid line is for the potential temperature of 360 K. The boundary of Henan Province is marked with a purple line.
表 1 2021年7月19和20日区域（30°～40°N, 110°～117°E）水汽通量最大值以及向西和向北水汽通量最大值
Table 1. The maximum water vapor flux in the region (30°–40°N, 110°–117°E) and the maximum westward and northward water vapor fluxes on 19 and 20 July 2021
日期 水汽通量最大值/kg m−1 s−1 区域最大值 向西最大值 向北最大值 整层 7月19日 534 505 352 7月20日 594 425 536 1000～850 hPa 7月19日 265 264 121 7月20日 309 306 149 850～700 hPa 7月19日 194 180 112 7月20日 196 186 187 700～300 hPa 7月19日 124 62 110 7月20日 193 86 190 
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表 2 2021年7月19和20日从不同边界进入河南区域的轨线数量
Table 2. Number of trajectories entering Henan area from different borders on 19 and 20 July 2021
起始点位置 轨线数量 总轨线数 从东（北）边界进入 从南边界进入 19日 所有起始点 127 94（3） 30 900～850 hPa 40 29（3） 8 850～700 hPa 62 50 12 700～300 hPa 25 15 10 20日 所有起始点 102 42（3） 57 900～850 hPa 33 16（3） 14 850～700 hPa 42 18 24 700～300 hPa 27 8 19
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