Synoptic-Scale Analysis on Development and Maintenance of the 19–21 July 2021 Extreme Heavy Rainfall in Henan, Central China

Dorina CHYI; Xiuming WANG; Xiaoding YU; Junhui ZHANG

doi:10.1007/s13351-023-2914-z

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Synoptic-Scale Analysis on Development and Maintenance of the 19–21 July 2021 Extreme Heavy Rainfall in Henan, Central China

2021年7月19–21日河南极端暴雨发展与维持的天气尺度分析

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Corresponding author: Xiuming WANG, wangxm@cma.gov.cn;

Funds:

Supported by the National Natural Science Foundation of China (41875058 and 42275013), Weather Nowcasting Project for Teaching and Research Teams of China Meteorological Administration, Research Project for Young Talents of China Meteorological Administration Training Centre (2022CMATCQN03), and Innovation and Development Program of China Meteorological Administration

DOI: 10.1007/s13351-023-2914-z

Abstract

Abstract

In this paper, synoptic-scale analyses of frontogenesis, moisture budget, and tropospheric diabatic heating are performed to reveal the development and maintenance mechanisms for the extreme heavy rainfall in Henan Province of central China from 19 to 21 July 2021, based on station observations and the ECMWF Reanalysis version 5 (ERA5) data. The results demonstrate that owing to the blocking effect of local topography, low-level wind convergence in Henan appeared underneath high-level divergence, conducive to development and maintenance of a midtropospheric low-pressure system saddled by the Asian continental high and the western Pacific subtropical high (WPSH), during the extreme heavy rainfall. In the lower troposphere, frontogenesis occurred in the ${\theta }_{{\rm{se}}}$ intensive region, as a result of the divergence and horizontal deformation (which play equally important roles), generating frontal secondary circulation with strong vertical motion favorable to heavy rainfall. Moisture budget analysis reveals that 1) with the continuous strengthening of the easterly wind from the north side of Typhoon In-Fa (2106), strong wind shear and orogra-phic uplift led to abnormally strong convergence of water vapor flux in the boundary layer in Henan; 2) there occurred extremely strong net inflow of moisture in the boundary layer from the east. Horizontally, both the apparent heat source <Q₁> and the moisture sink <Q₂> coincided with the area of heavy rainfall; vertically, however, Q₁ exhibited a single peak with the heating center in the middle and upper troposphere, while large Q₂ values evenly resided over 850–400 hPa; and Q₁ (Q₂) was dominated by vertical (horizontal) transport of potential temperature (moisture). These indicate that the latent heat release from condensation of initial heavy rainfall provided a positive feedback, leading to increasingly heavy precipitation. All these synoptic settings sustained the extreme rainfall process.
- extreme heavy rainfall,
- frontogenesis,
- apparent heat source,
- moisture sink,
- water vapor budget
摘要

为了揭示2021年7月19–21日河南极端暴雨的发展和维持机制，本文利用台站观测和ECMWF再分析第5版(ERA5)资料，对锋生函数、水汽收支和对流层非绝热加热进行了天气尺度分析。结果表明：（1）本次极端暴雨过程中，由于局地地形的阻挡作用，在河南上空出现低层风辐合和高层辐散，有利于位于亚洲大陆高压和西太平洋副热带高压(WPSH)之间鞍型场上的低压系统的发展和维持。（2）在对流层低层，由于水平辐散和水平变形项的作用(两者同等重要)，在θ_se高值区出现锋生和锋面次级环流，垂直运动强烈，有利于强降雨。（3）水汽收支分析表明，随着台风In-Fa(2106)北侧东风的持续增强，强烈的风切变和地形抬升导致河南边界层水汽通量异常强辐合，边界层出现了极强的东部水汽净流入，对极端暴雨的维持和加强十分关键。（4）视热源Q₁和视水汽汇Q₂大值区在水平方向上与暴雨落区吻合；垂直方向Q₁只有一个加热中心位于对流层中上层，而Q₂大值均匀分布在850–400 hPa；Q₁ (Q₂)主要来源于位温(湿度)的垂直(水平)输送；这表明初始强降水的凝结潜热释放提供了正反馈，导致了强降水增加。在以上这些天气条件下，河南极端暴雨得以发展和维持。
- 极端暴雨,
- 锋生,
- 视热源,
- 视水汽汇,
- 水汽收支

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Fig. 5. Distributions of the horizontal convergence term of frontogenesis function (shadings; 10⁻⁹ K m⁻¹ s⁻¹), wind (barbs), and divergence (contours; 10⁻⁵ s⁻¹) at 925 hPa at (a) 0800 BT 20 July, (b) 2000 BT 20 July, (c) 0800 BT 21 July, and (d) 2000 BT 21 July. The black dot denotes Zhengzhou meteorological station.

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Fig. 1. Distributions of (a) the surface weather stations with record-breaking daily accumulative rainfall (black dots), superimposed with terrain height of the area (shadings; m); (b) daily accumulative rainfall (mm) from 0800 BT 20 to 0800 BT 21 July 2021; and (c) daily accumulative rainfall (mm) from 0800 BT 21 to 0800 BT 22 July 2021. In (b) and (c), the black dot denotes Zhengzhou meteorological station and the black box indicates the area of heavy rainfall (33.5°–36°N, 112.5°–115°E).

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Fig. 2. Synoptic weather maps at 0800 BT 20 July 2021 of (a) geopotential height (black contours; dagpm) and wind (barbs) at 200 hPa, (b) geopotential height (black contours; dagpm), temperature (red solid lines; °C), and wind (barbs) at 500 hPa, (c) specific humidity (green solid lines; g kg⁻¹) and wind (barbs) at 850 hPa. The black dot denotes Zhengzhou meteorological station where maximum hourly rainfall was observed.

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Fig. 3. Distributions of frontogenesis function (shadings; 10⁻⁹ K m⁻¹ s⁻¹) and ${\theta }_{\rm se}$ (contours; K) at 925 hPa at (a) 0800 BT 20 July, (b) 2000 BT 20 July, (c) 0800 BT 21 July, and (d) 2000 BT 21 July. The black dot denotes Zhengzhou meteorological station.

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Fig. 4. Latitude–height sections of frontogenesis function (shadings; 10⁻⁹ K m⁻¹ s⁻¹) and θ_se (contours; K) along the longitude of Zhengzhou meteorological station at (a) 0800 BT 20 July, (b) 2000 BT 20 July, (c) 0800 BT 21 July, and (d) 2000 BT 21 July.

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Fig. 6. Distributions of horizontal deformation term (shadings; 10⁻⁹ K m⁻¹ s⁻¹) and shear deformation term (contours; 10⁻⁹ K m⁻¹ s⁻¹) at 925 hPa at (a) 0800 BT 20 July, (b) 2000 BT 20 July, (c) 0800 BT 21 July, and (d) 2000 BT 21 July. The black dot denotes Zhengzhou meteorological station.

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Fig. 7. Vertical profiles of Q₁ (K h⁻¹), Q₂ (K h⁻¹), and vertical velocity (Pa s⁻¹) averaged over the heavy rainfall region (33.5°–36.5°N, 112.5°–115°E) on (a) 20 July and (b) 21 July 2021.

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Fig. 8. Distributions of (a, b) <Q₁> (W m⁻²) and (c, d) <Q₂> (W m⁻²) on (a, c) 20 July and (b, d) 21 July 2021.

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Fig. 9. Vertical profiles of area-averaged Q₁ (K h⁻¹) in the heavy rainfall region (33.5°–36.5°N, 112.5°–115°E) on (a) 20 July and (b) 21 July 2021. (c, d) As in (a, b), but for Q₂ (K h⁻¹).

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Fig. 10. Distributions of (a) H_s (K h⁻¹) corresponding to averaged rainfall from 0800 to 2000 BT 20 July and (b) H_s corresponding to averaged rainfall from 0800 to 2000 BT 21 July 2021. (c, d) As in (a, b), but for H_c (K h⁻¹). The black circle denotes Zhengzhou meteorological station.

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Fig. 11. Distributions of vertically integrated (1000–700 hPa) water vapor flux (arrows; kg m⁻¹ s⁻¹) and the corresponding flux divergence (shadings; 10⁻⁵ kg m⁻² s⁻¹) at (a) 0800 BT 20 July, (b) 2000 BT 20 July, (c) 0800 BT 21 July, and (d) 2000 BT 21 July. The black dot denotes Zhengzhou meteorological station and the black box indicates the region of water vapor flux convergence (33°–38°N, 112°–116°E).

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Fig. 12. Vertical profiles of the water vapor flux budgets (10⁷ kg s⁻¹) along the east–west direction (solid line), along the north–south direction (dashed line), and averaged over the entire domain of the water vapor flux convergence region (32°–38°N, 111°–116°E) (dotted line) at (a) 0800 BT 20 July, (b) 2000 BT 20 July, (c) 0800 BT 21 July, and (d) 2000 BT 21 July.

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Chyi, D., X. M. Wang, X. D. Yu, et al., 2023: Synoptic-scale analysis on development and maintenance of the 19–21 July 2021 extreme heavy rainfall in Henan, central China. J. Meteor. Res., 37(2), 174–191, doi: 10.1007/s13351-023-2914-z.

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Manuscript History

Received: 16 January 2023

Revised: 22 February 2023

Accepted: 02 March 2023

Available online: 12 March 2023

Final form: 12 March 2023

Typeset Proofs: 13 March 2023

Issue in Progress: 13 March 2023

Published online: 25 April 2023

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Synoptic-Scale Analysis on Development and Maintenance of the 19–21 July 2021 Extreme Heavy Rainfall in Henan, Central China

2021年7月19–21日河南极端暴雨发展与维持的天气尺度分析

Abstract