An Intensification of the Warm and Moist Conveyor Belt of the Asian Summer Monsoon in the “21.7” Henan Rainstorm and Its Key Circulation from the Quasi-geostrophic Potential Vorticity Perspective
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摘要: 本文使用站点降水资料和欧洲中期天气预报中心第五代再分析数据，利用准地转位涡分部反演，重点分析了2021年7月18～21日（简称“21.7”）河南极端暴雨中暖湿季风输送带加强的机理及其关键环流特征。结果表明：副热带高压持续西伸至中国东部地区，其西南部宽广的东南风将暖湿气流和河南地区高位涡输送至西北地区；同时，西北地区阿拉善高原热低压受感热加热而加强，在近地面层及其东侧的河套地区对流层中低层（750～650 hPa）产生正位涡异常，与河南地区低压环流形成大范围高位涡异常，从而与副热带高压形成较大范围的对峙。准地转位涡反演结果表明，对流层中低层这一天气流型导致河南南部南风的加强，有效地将暖湿输送带中高温高湿的气块输送至河南地区，成为7月20日极端暴雨发生的关键因子之一。对于7月20日河南地区的南风，主要来自于副热带高压的贡献，其次是河套地区对流层中低层高位涡异常，而河南局地低压环流的贡献略小。Abstract: This study uses rain-gauge observation data, the fifth reanalysis dataset of the European Center for Medium-Range Weather Forecasts, and the piecewise quasi-geostrophic potential vorticity (QGPV) inversion to mainly investigate the intensification of the warm and moist conveyor belt of the Asian summer monsoon of the Henan extreme rainstorm and its key circulation during July 18–21, 2021. The result shows that the continual westward extension of the subtropical high covered eastern China, on whose southwestern flank, broad southwesterlies transported not only a warm and moist air mass but also a high QGPV over Henan to northwestern China. Meanwhile, in northwestern China, the sensible heating of the Alxa Plateau maintained and deepened the local thermal low, which generated high QGPV anomalies in the near-surface layer of the low-pressure center and the middle and lower tropospheres (750–650 hPa) over the Hetao region. Thus, this high QGPV formed an extensive, high QGPV with low-pressure circulation over Henan, which yielded an extensive confrontation with the subtropical high. The QGPV inversion results show that this meteorological circulation pattern in the middle and lower tropospheres intensified the southerly wind over Henan. The amplified southerly effectively transported a hot and humid air mass of the warm and moist conveyor belt to the Henan region, which was a key factor for the extreme downpour on July 20. The southerly over Henan on July 20 is primarily contributed by the subtropical high, with secondary contributions from the positive QGPV anomaly at the middle and lower tropospheres over Hetao, while the contribution of the local low-pressure circulation over Henan is slightly smaller.
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图 1 2021年7月18～21日逐日08时（北京时，下同）至次日08时累积降水量（单位：mm d−1）：（a）18日；（b）19日；（c）20日；（d）21日
Figure 1. Daily accumulated precipitation (units: mm d−1) from 0800 BJT (Beijing time) to 0800 BJT of the following day for July 18–21, 2021: (a) 18; (b)19; (c) 20; (d) 21
图 2 2021年7月（a）18日、（b）19日、（c）20日和（d）21日的800 hPa日平均比湿（填色，单位：kg kg−1）和水平风场（箭头，单位：m s−1）。红色曲线为河南省界，黑色曲线分别为长江和黄河，灰色为3000 m以上的青藏高原
Figure 2. Daily mean 800-hPa specific humidity (shaded, units: kg kg−1) and horizontal wind (arrows, units: m s−1) on July (a) 18, (b) 19, (c) 20, and (d) 21 2021. The red and two black curved lines mark the boundary of Henan Province and the Changjiang and Huanhe Rivers, respectively. The gray shaded area designates the Tibetan Plateau above 3000 m
图 3 2021年7月18～22日河南地区（31°～36°N，110°～116°E）区域平均的（a）经向风及其（b）异常和（c）纬向风及其（d）异常的气压—时间剖面（单位：m s−1）。填色和等值线分别为原始风场和地转风，等值线间隔为1 m s−1
Figure 3. Pressure–time cross section (units: m s−1) of the areal mean (a) meridional wind and (b) its anomaly and (c) zonal wind and (d) its anomaly over the Henan region (31°–36°N, 110°–116°E) during July 18–22, 2021. The shading and contours are the wind and geostrophic wind, respectively. The contour interval is 1 m s−1
图 4 2021年7月（a）18日、（b）19日、（c）20日和（d）21日的750 hPa地转风异常场（箭头，单位：m s−1）和地转经向风偏差（填色），蓝色曲线为河南省界，黑色曲线分别为长江和黄河，灰色为3000 m以上的青藏高原。（e–f）同（a−d），但为准地转位涡反演得到的地转异常风和地转经向风偏差（填色）
Figure 4. 750-hPa geostrophic wind anomaly (arrows, units: m s−1) and a geostrophic meridional wind (shading) on July (a) 18, (b) 19, (c) 20, and (d) 21 2021. The blue and two black curved lines mark the boundary of Henan Province and the Changjiang and Huanhe Rivers, respectively. The gray shaded area designates the Tibet Plateau above 3000 m. (e–h) same as (a–d), but for the geostrophic wind anomaly inverted from the quasi-geostrophic potential vorticity (QGPV) and the bias of the meridional geostrophic wind to the original meridional geostrophic wind (shading)
图 5 2021年7月（a）18日、（b）19日、（c）20日和（d）21日逐层气压面上的准地转位涡异常反演得到的河南地区（31°～36°N，110°～116°E）区域平均750 hPa和250 hPa经向风异常（单位：m s−1）
Figure 5. 750-hPa and 250-hPa geostrophic meridional wind anomalies (units: m s−1) averaged over the Henan region (31°–36°N, 110°–116°E) inverted from the QGPV anomaly on each pressure level on July (a) 18, (b) 19, (c) 20, and (d) 21 2021
图 6 2021年7月（a）18日、（b）19日、（c）20日和（d）21日650 hPa位势高度（等值线，单位：gpm）及其异常（填色），蓝色曲线为河南省界，黑色曲线分别为长江和黄河，灰色为3000 m以上的青藏高原，等值线间隔为10 gpm，其中虚线为负值。（e–h）同（a–d），但为650 hPa准地转位涡异常（填色，单位：s−1）及其反演得到的750 hPa位势高度异常（等值线）和地转风异常（箭头，单位：m s−1）
Figure 6. 650-hPa geopotential height (contours, units: gpm) and its anomalies (shading) on July (a) 18, (b) 19, (c) 20, and (d) 21 2021. The blue and two black curved lines mark the boundary of Henan Province and the Changjiang and Huanhe Rivers, respectively. The gray shaded area designates the Tibet Plateau above 3000 m. A contour is drawn every 10 gpm and the dashed line indicates the negative value. (e–h) same as (a–d), but for the 650-hPa QGPV anomaly (shading, units: s−1) and the 750-hPa geopotential height anomaly (contours) as well as the geostrophic wind anomaly (arrows, units: m s−1) inverted from the 650-hPa QGPV anomaly
图 8 2021年7月（a）18日、（b）19日、（c）20日和（d）21日沿图6g蓝粗线准地转位涡异常（填色，单位：s−1）及其热力部分（等值线）和风矢量（水平风单位：m s−1；垂直风单位：10−2 Pa s−1）的剖面。等值线间隔为0.2×10−4 s−1，虚线为负值
Figure 8. Crosssections of the QGPV anomaly (shading; units: s−1) as well as its contribution by static stability (contours) and wind (units of horizontal wind: m s−1; units of vertical velocity: 10−2 Pa s−1) along the blue line in Figure 6g on July (a) 18, (b) 19, (c) 20, and (d) 21 2021. The vertical velocity is scaled by 100, The contour interval is 0.2×10−4 s−1 and the dashed line indicates the negative value
图 9 2021年7月（a）18日、（b）19日、（c）20日和（d）21日沿40°N位温（等值线，单位：K）、风矢量（箭头，纬向风单位：m s−1；垂直风单位：10−2 Pa s−1）和热力部分的位涡异常（填色，单位： s−1）的剖面
Figure 9. Cross sections of the potential temperature (contours, units: K), wind (arrows, units of horizontal wind: m s−1; units of vertical velocity: 10−2 Pa s−1), and the QGPV anomaly of static stability (shading; units: s−1) along 40°N on July (a) 18, (b) 19, (c) 20, and (d) 21 20210
图 10 沿40°N的地表平均感热通量（单位：W m-2）的Hovmöller图，向上通量为正
Figure 10. Hovmöller diagram of the mean surface sensible heat flux (units: W m−2). Upward fluxes are positive
图 12 2021年7月（a）18日、（b）19日、（c）20日和（d）21日由非绝热加热造成的850 hPa位势高度倾向（填色，单位：gpm d−1）和地转风倾向（箭头，单位：m s−1 d−1），蓝色曲线为河南省界，黑色曲线分别为长江和黄河，灰色为3000 m以上的青藏高原。（e–h）和（i–l）同（a–d），但分别为750 hPa和250 hPa
Figure 12. 850-hPa geopotential height tendency (contours, units: gpm d−1) and geostrophic wind tendency (arrows, units: m s−1 d−1) due to diabatic heating on July (a) 18, (b) 19, (c) 20, and (d) 21 2021. The blue and two black curved lines mark the boundary of Henan Province and the Changjiang and Huanhe Rivers, respectively. The gray shaded area designates the Tibet Plateau above 3000 m. (e–h) and (i–l) same as (a–d), but for 750 and 250 hPa, respectively
图 11 （a）2021年7月20日750～600 hPa平均准地转位涡异常（填色，单位：s−1）及其反演的750 hPa地转风异常（箭头，单位：m s−1），（c）、（e）和（f）分别为副热带高压、河套和河南位涡异常及其反演的地转风异常，（b）为（c）、（e）和（f）之和，（d）为（e）和（f）之和。蓝色和黑色曲线分别为河南省界和黄河，灰色为3000 m以上的青藏高原
Figure 11. (a) 750–600 hPa QGPV anomaly (shading, units: s−1) and 750 hPa geostrophic wind anomaly (arrows, units: m s−1) inverted from the QGPV anomaly on July 20 2021. (c), (e), and (f) are geostrophic wind anomalies inverted from QGPV anomalies associated with subtropical highs and lows over Hetao and Henan. (b) is the sum of (c), (e), and (f), while (d) is the sum of (e) and (f). The blue and black curved lines mark the boundary of Henan Province and the Huanghe River, respectively. The gray shaded area designates the Tibet Plateau above 3000 m
表 1 不同地区准地转位涡异常反演的750 hPa河南地区（31°～36°N，110°～116°E）平均经向风异常（单位：m s−1）
Table 1. The 750 hPa area-mean meridional wind anomalies (units: m s−1) over the Henan region (31°–36°N, 110°–116°E) inverted from the QGPV anomalies over different regions
河南地区平均经向风异常 气压层 全球 副热带
高压—河套河南—
河套副热带
高压河套 河南 750–600 hPa 6.19 4.80 1.90 2.90 1.00 0.90 650 hPa 2.22 1.42 0.47 0.95 0.31 0.16 
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