Synergistic Effect of the Planetary-scale Disturbance, Typhoon and Meso-β-scale Convective Vortex on the Extremely Intense Rainstorm on 20 July 2021 in Zhengzhou※

Guanshun ZHANG, Jiangyu MAO, Wei HUA, Xiaofei WU, Ruizao SUN, Ziyu YAN,Yimin LIU, and Guoxiong WU

1Guangzhou Institute of Tropical and Marine Meteorology/Guangdong Provincial Key Laboratory of Regional Numerical Weather Prediction, China Meteorological Administration, Guangzhou 510640, China

2State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG),Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China

3Plateau Atmosphere and Environment Key Laboratory of Sichuan Province, Chengdu University of Information Technology, Chengdu 610225, China

4Henan Key Laboratory of Agrometeorological Support and Applied Technique, CMA, Zhengzhou 450003, China

5Henan Meteorological Service Center, Zhengzhou 450003, China

6School of Geo-Science and Technology, Zhengzhou University, Zhengzhou 450001, China

ABSTRACT On 20 July 2021, northern Henan Province in China experienced catastrophic flooding as a result of an extremely intense rainstorm, with a record-breaking hourly rainfall of 201.9 mm during 0800—0900 UTC and daily accumulated rainfall in Zhengzhou City exceeding 600 mm (“Zhengzhou 7.20 rainstorm” for short). The multi-scale dynamical and thermodynamical mechanisms for this rainstorm are investigated based on station-observed and ERA-5 reanalysis datasets.The backward trajectory tracking shows that the warm, moist air from the northwestern Pacific was mainly transported toward Henan Province by confluent southeasterlies on the northern side of a strong typhoon In-Fa (2021), with the convergent southerlies associated with a weaker typhoon Cempaka (2021) concurrently transporting moisture northward from South China Sea, supporting the rainstorm. In the upper troposphere, two equatorward-intruding potential vorticity(PV) streamers within the planetary-scale wave train were located over northern Henan Province, forming significant divergent flow aloft to induce stronger ascending motion locally. Moreover, the converged moist air was also blocked by the mountains in western Henan Province and forced to rise so that a deep meso-β-scale convective vortex (MβCV) was induced over the west of Zhengzhou City. The PV budget analyses demonstrate that the MβCV development was attributed to the positive feedback between the rainfall-related diabatic heating and high-PV under the strong upward PV advection during the Zhengzhou 7.20 rainstorm. Importantly, the MβCV was forced by upper-level larger-scale westerlies becoming eastward-sloping, which allowed the mixtures of abundant raindrops and hydrometeors to ascend slantwise and accumulate just over Zhengzhou City, resulting in the record-breaking hourly rainfall locally.

Key words: extreme rainstorm, potential vorticity, trajectory tracking, planetary-scale disturbance, meso-β-scale convective system

1. Introduction

During 19—22 July 2021, an exceptionally heavy rainfall event happened over northern Henan Province in China, in which the unusually extreme rainstorm occurred around Zhengzhou City on 20 July (Fig. 1a), with recording-breaking daily rainfall amount exceeding 600 mm for Zhengzhou City (Fig. 1a, named as “Zhengzhou 7.20 rainstorm” for short hereafter). Remarkably, the most intense rainfall occurred in the afternoon 1600—1700 LST (0800—0900 UTC)20 July, with hourly rainfall reaching 201.9 mm to break the record of local intensity for Zhengzhou City (Figs. 1b and 1c). This sudden rainstorm resulted in severe urban flooding,affecting more than 3 million people with a death toll of 302 and 50 people missing (Li and Shi, 2021). The direct economic losses caused by the Zhengzhou 7.20 rainstorm were more than ¥114.3 billion RMB ($17.7 billion US dollars),with some historic heritages being damaged irreversibly(CGTN, 2021). Thus, it is of great significance to explore the influential factors and formative mechanism for this rainstorm, and thereby improve the forecast skill for such highimpact weather events to minimize future losses.

Fig. 1. (a) Spatial distribution of the 24-hour accumulated rainfall (color shading, mm) in Henan Province from 0000 UTC 20 July to 0000 UTC 21 July 2021. Solid circles represent the positions of the 113 rain gauges. The black rectangle denotes the Zhengzhou key-region (34.25°—35.25°N, 113.12°—114.12°E). (b) Same as (a), but for the extreme hourly rainfall in Zhengzhou from 0800 UTC to 0900 UTC 20 July 2021. (c) Time series of hourly rainfall at Zhengzhou station from 0000 UTC 18 July to 0000 UTC 23 July 2021 (bars, mm), and the area-averaged hourly rainfall from the 113 rain gauges in Henan Province (black curves, mm). (d) Pressure—time cross sections of areaaveraged PV (color shading; 1 PVU = 10-6 K m2 s-1 kg-1) and vertical velocity (contours, Pa s-1) over the Zhengzhou key-region shown in (a) and (b).

Generally, the occurrence of a heavy rain or rainstorm over central-eastern China depends on large-scale favorable circulation conditions such as a frontal or westerly-trough system involving multi-scale tropical—extratropical interactions(Tao and Chen, 1987; Lau et al., 1988; Ding, 2015). For instance, in early August 1975, typhoon Nina (1975) was found to be associated with an upper-tropospheric flow reconfiguration, causing the notorious “Henan 75·8 rainstorm”and the resultant catastrophic flood with huge loss of tens of thousands of people lives (Ding, 2015; Yang et al., 2017).As suggested by Ding (2015), the landfalling typhoon Nina(1975) was confined to migrate northwestward toward Henan Province over eastern China for several days by an amplified blocking high in high latitudes. Before and when the typhoon reached southern Henan Province, stronger southeasterlies on its northern side continuously transported abundant water vapor to converge over southern Henan Province,in conjunction with ascending motion induced by the uppertropospheric divergent flow ahead of the westerly-trough to result in the “Henan 75·8 rainstorm” (Yang et al., 2017).This indicates that the synergistic influence of the reinforced landward moisture transport guided by the typhoon and the mid-latitude upper-tropospheric flow disturbance is an important configuration to generate extreme rainstorms over Henan Province.

Similarly, prior to the Zhengzhou 7.20 rainstorm, two typhoons named as “In-Fa” (2021) and “Cempaka” (2021)were already present over the East China Sea and South China Sea respectively, which synergistically formed an elongated corridor of westward-directed flow and moisture transport into northern Henan Province (Su et al., 2021; Sun et al., 2021; also refer to Fig. 3). Such enhanced landward moisture transport was blocked by the mountains over the northwest part of Henan Province so that the moist air was lifted over the mountainous area, especially on the windward side.In the meantime, there were favorable large-scale divergent circulations in the upper troposphere, leading to the occurrence of persistent heavy rainfall (e. g. Houze, 2012; Hua et al., 2020; Yin et al., 2020, 2022; Zhang et al., 2021b). However, the influence of such favorable upper-tropospheric flow patterns has not been considered enough in existing studies for the Zhengzhou 7.20 rainstorm. As suggested by Galarneau et al. (2012) and Bosart et al. (2017), most of the mid-latitude extreme weather events occur under the reconfiguration of high-amplitude upper-level flow patterns within baroclinic Rossby wave trains. Such flow patterns can reach an irreversible configuration as individual waves within the wave train amplify and undergo Rossby wave breaking, resulting in elongated stratospheric high-value potential vorticity (PV) air that intrudes equatorward and downwards into the lower troposphere (Meier and Knippertz, 2009), sometimes referred to as “PV streamers” (Wiegand and Knippertz, 2014). Given that there were indeed distinct PV streamers over Henan Province during the Zhengzhou 7.20 rainstorm (as discussed in section 4), the important impact of the upper-tropospheric circulation reconfiguration associated with PV streamers on the ascending motion over Zhengzhou City should be examined in depth.

Furthermore, regional extreme weather events, especially for short-duration heavy rainfall occurrences, are usually related to the mesoscale convective systems (MCSs)embedded in larger-scale circulations (Yin et al., 2020; Su et al., 2021). To examine the mechanism responsible for the record-breaking hourly rainfall in the Zhengzhou 7.20 rainstorm, Wei et al. (2022) utilized radar observations and performed a convection-permitting simulation using the WRFARW model. Their results suggested that this extreme hourly rainfall was caused by a single quasi-stationary storm associated with a low-level meso-β-scale vortex(MβCV) west of Zhengzhou City. They further pointed out that the MβCV was supported and maintained by the lowlevel convergent flows from the north, south and east of the storm. Yin et al. (2022) suggested that the smaller meso-γscale MCS allowed the development of convective updrafts in a three-quarter circle around the convective system during the Zhengzhou 7.20 rainstorm, emphasizing the significant contribution of the unique dynamic structure of the MCS to the record-high hourly rainstorm. These results indicate the important role that MCSs played in the transient, localized urban rainstorm in Zhengzhou City.

In brief, the Zhengzhou 7.20 rainstorm involved a complicated multi-scale process with strong interaction of tropical and extratropical systems. Although Wei et al. (2022)pointed out the dominant role of the MβCV in producing the record-breaking hourly rainfall, they only paid attention to the low-level behavior of the MβCV and its impact from the synoptic-scale disturbances, without examining the MβCV structure in the middle- and upper-troposphere and other influential factors, let alone the effects of planetaryscale PV disturbances in the upper troposphere. In the present study, however, we not only emphasize the role of the low-level and middle-level synoptic-scale systems in generating the MβCV but also highlight the impact of the upperlevel planetary-scale PV streamers, demonstrating the coordinated impacts of the planetary-scale and synoptic-scale disturbances [including typhoon In-Fa (2021) and typhoon Cempaka (2021)] on the deep, eastward-sloping MβCV and their resultant synergistic effect on the record-breaking hourly rainfall in the Zhengzhou 7.20 rainstorm. In addition, as suggested by Xu et al. (2012) and Jiang et al. (2019), the MCS-related latent heat tends to create positive PV below the middle troposphere, promoting the development of the low-level MβCV and deeper convection through positive feedback, especially around mountains. Therefore, the objective of this study is to explore the synergistic effect of multi-scale systems including planetary-scale disturbances associated with PV streamers, typhoons and the MβCV on the Zhengzhou 7.20 rainstorm from a PV perspective by applying the water vapor tracking and PV-related vertical velocity diagnoses.

Section 2 introduces the data and methods. Section 3 describes the basic characteristics of the Zhengzhou 7.20 rainstorm. Section 4 analyzes the moisture condition and the vertical motion development in association with the planetaryscale and synoptic-scale processes for this rainstorm event.The physical mechanism of the MβCV leading to the recordbreaking hourly rainstorm under the influence of multiple factors are examined in section 5. Finally, summary and discussion are given in section 6.

2. Data and methods

2.1. Data

In this study, hourly rainfall data from 113 rain gauges in Henan Province was derived from the National Meteorological Information Center, China Meteorological Administration. To reveal the physical mechanism for the Zhengzhou 7.20 rainstorm, hourly atmospheric circulation data were extracted from the fifth-generation of European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis data (ERA5) (Hersbach et al., 2020), with 37 pressure levels from 1000 to 0.1 hPa and a horizontal resolution of 0.25° ×0.25°. Three-dimensional composite radar reflectivity with a horizontal resolution of 0.01° × 0.01° was integrated by combining nine individual radar observations over Henan Province (see Fig. 2a for their locations), to better demonstrate the distribution and evolution of multi-scale convective rainfall.

Fig. 2. Distributions of the combined radar reflectivity around 3000-m altitude (color shading, dBZ) over Henan Province, 700-hPa PV (red contours, PVU) and wind vectors (m s-1) at (a) 1200 UTC 19 July, (b)1800 UTC 19 July, (c) 0000 UTC 20 July, (d) 0300 UTC 20 July, (e) 0600 UTC 20 July, and (f) 0800 UTC 20 July 2021. The diamonds in (a) indicate the locations of the nine radar stations with the purple diamond in each panel representing the position of Zhengzhou station. Black curves represent the provincial boundaries.

2.2. Methods

2.2.1. Backward trajectory tracking

The trajectory tracking of target particles used in this study is based on the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model, which is provided by the Air Resources Laboratory of the National Oceanic and Atmospheric Administration (NOAA) (Stein et al., 2015;Rolph et al., 2017). The computational method of the HYSPLIT model is a hybrid coordinate approach using a moving frame of reference for the advection and diffusion calculations as the air particles move from their initial location. The backward trajectory tracking is based on the type of ensemble simulation, and the trajectory ensemble option will start multiple trajectories from the first selected starting location. Every starting location contains 27 particles as members. The experimental settings are described in the following paragraph.

The target destination of the simulative particles was focused on Zhengzhou City (34.7°N, 113.6°E) with a starting time of 0000 UTC 20 July, which is when the deeply convective motion was first established over Zhengzhou City (see Fig. 1d), and with another starting time of 0800 UTC 20 July, which is when the extreme hourly rainstorm occurred.The initial heights of the particles for the starting time of 0000 UTC 20 July were set at 5 heights (0.5, 1.5, 3, 6, and 10 km, respectively), while 3 initial heights (5, 5.5, and 6 km, respectively) were set for the starting time of 0800 UTC 20 July. Each height contains 27 members over Zhengzhou City at the initial time based on the ensemble simulation. The information of every single particle was output every hour, including particle identification number, relative humidity, and three-dimensional spatial position (longitude,latitude and altitude).

2.2.2. PV equation

The PV equation in the isobaric coordinate system can be expressed as follows (Ertel, 1942; Hoskins et al., 1985;Hoskins, 1997; Zhang et al., 2021a):w

here P is Ertel PV, which is the dot product of absolute vorticity vector for unit mass and potential temperature gradient, i .e.,P=αξa·▽θ (in which α is the specific volume, ξais the three-dimensional absolute vorticity, θ is the potential temperature, and ▽ is the three-dimensional gradient operator in pressure coordinates), u and v are the zonal and meridional wind components, ω is the vertical velocity, ζ is the vertical relative vorticity, g is the gravitational acceleration, θ˙ is the diabatic heating rate, and F is frictional acceleration in the momentum equation. The left-hand side of Eq. (1) refers to the local rate of change of PV (or the PV tendency), while the five underlined forcing terms (indicated by letters F1—F5)on the right-hand side refer, respectively, to the zonal, meridional and vertical PV advection, and the PV generation by the horizontally and vertically non-uniform diabatic heating.Note that the PV dissipation associated with frictional force[the sixth term on the right-hand side of Eq. (1)] is not analyz ed here because its effect is relatively small in the free atmosphere for analyzing the Zhengzhou 7.20 rainstorm. The quantitative diagnoses associated with PV using Eq. (1) will be given in section 4.

3. Basic features of the Zhengzhou 7.20 rainstorm

As introduced in section 1, northern Henan Province suffered heavy rain from 19 to 22 July 2021, with the most intense rainfall occurring in Zhengzhou City on 20 July.Figure 1a illustrates the spatial distribution of daily accumulated rainfall recorded by 113 meteorological stations in Henan Province for the period from 0000 UTC 20 to 0000 UTC 21 July. In northern Henan Province north of around 33.5°N, more than 66% of the stations where the daily rainfall was observed reached or exceeded the magnitude for severe rainstorm (≥100 mm). Of special note is that 20 of the stations closest to Zhengzhou City suffered an extraordinary rainstorm(rainfall amount ＞200 mm), with maximum rainfall of 624 mm occurring in Zhengzhou City (Fig 1a), directly causing severe urban waterlogging and casualties. This unprecedented catastrophic rainfall arose in Zhengzhou City in the afternoon from 0800 UTC to 0900 UTC 20 July (Fig. 1b), with hourly rainfall reaching 201.9 mm, creating a record-breaking intensity for the historical hourly rainfall recorded by the rain gauges of China (Zhao and Cai, 2021). However, the rainfall intensity in other surrounding stations was noted to be dramatically weaker than that in Zhengzhou City (Fig. 2b), suggesting that the formative mechanism for such extreme hourly rainfall locally in Zhengzhou City may have involved mesoscale convective activity and interactions with synopticscale circulation (as discussed in section 5).

To highlight how anomalous the rainstorm in Zhengzhou City was during the heavy rainfall event over Henan Province, the time series of hourly rainfall intensity at Zhengzhou City for the period from 0000 UTC 18 to 0000 UTC 23 July is displayed in Fig. 1c. Also shown is the area-averaged hourly rainfall over the 113 rain gauges in Henan Province for comparison. For Zhengzhou City, the intense hourly rainfall greater than 15 mm was observed to occur consecutively for hours within one day (from 0000 to 2100 UTC) on 20 July (Fig. 1c), which was much stronger than the area-averaged rainfall intensity over Henan Province of only around 5 mm per hour during the same period, indicating that the Zhengzhou 7.20 rainstorm was a persistently heavy rainfall event. For the most intense hourly rainfall of 201.9 mm in Zhengzhou City from 0800 UTC to 0900 UTC 20 July (Fig. 1c), such an intensity was 10 times stronger than that from the nearest stations(Fig. 1b), implying that spatially localized anomalous circulations might have existed around Zhengzhou City. Thus, a much more localized target domain covering Zhengzhou City was selected as a key region (34.25°—35.25°N, 113.12°—114.12°E, Fig. 1a) to examine the dynamical and thermodynamical processes responsible for such an extraordinary rainfall event over Zhengzhou City.

Rainfall occurrence depends on ascent of warm, moist air. Figure 1d shows a pressure—time cross section of areaaveraged vertical velocity as well as the PV over the Zhengzhou key-region. Notice that apparent ascending motion greater than -1.2 Pa s-1first appeared over Zhengzhou City in the upper-troposphere after 1800 UTC 19 July in association with the explosive development of the corresponding rainfall over Zhengzhou City to more than 10 mm h-1after a sightly weakening (Fig. 1c). Meanwhile,the mid-tropospheric PV at ～ 500 hPa also intensified during this period with two prominent maxima before and after 0000 UTC 20 July (Fig. 1d). Such high PV reflected the development of a newly-established MβCV (as discussed in section 5), which immediately strengthened the localized moisture convergence to further facilitate the development of ascending motion and the resultant rainfall (Figs. 1b and 1d), signifying the outbreak of the Zhengzhou 7.20 rainstorm. The ascending motion tended to develop downward afterwards to cover almost the entire troposphere (Fig. 1d), and the hourly rainfall in Zhengzhou City also experienced a significant intensification around 0000 UTC 20 July (Fig. 1c). However, the peak rainfall rate (201.9 mm h-1) for 0800—0090 UTC 20 July in Zhengzhou City (Fig. 1c) corresponded to weak ascending motion rather than the strongest velocity(Fig. 1d). Such a mismatch between the most intense hourly rainfall and weak updraft within a relatively small spatial range around Zhengzhou City also suggests that the Zhengzhou 7·20 rainstorm was not caused by the large-scale convective motion alone. Instead, the MCSs interacted with the synoptic-scale system to generate such flash heavy rainfall through complicated cloud microphysical process (as discussed in section 5).

The MCSs and large-scale convective activities can be illustrated to a great extent by the radar echoes and the PV distributions. Figure 2 shows the combined radar reflectivity at 3000 m with high spatial resolution covering Henan Province and the corresponding 700-hPa PV. When the areaaveraged rainfall over Henan Province began to intensify around 1200 UTC 19 July (Fig. 1c), apparent radar echoes were observed over central Henna Province as an elongated band across Zhengzhou City in association with coherent southeasterlies (Fig. 2a), with several intense echo cores indicating repetitive back-building. Subsequently, the radar echo grew and expanded mostly westward to cover a wider area by 1800 UTC 19 July (Fig. 2b). Note that a weak irregular high-PV system identified by the 1-PVU contour formed southwest of Zhengzhou City (Fig. 2b), accompanied by significant cyclonic wind shear over central Henan Province,in association with moderate rainfall over Zhengzhou City(Fig. 1b). This rainfall-related diabatic heating tended to increase the local PV according to Eq. (1) (as discussed in section 4.3), thus the high-PV system intensified remarkably by 0000 UTC 20 July, as evidenced by the occurrence of the 2-PVU contour and northeastward extension of the 1-PVU contour covering Zhengzhou City (Fig. 2c), corresponding to the beginning of the extreme rainstorm (Fig. 1c). This enhanced high-PV system represented a meso-β-scale cyclone with spatial scale around 200 km, as evidenced by strong radar echoes greater than 30 dBZ exhibiting a circular distribution to the northeast section of the PV center.Notably, according to previous studies (Davis and Trier, 2007; Davis and Galarneau, 2009), such a local meso-βscale cyclone accompanied by the concentrated high-value PV in lower troposphere could be identified as a typical system of MβCV (Fig. 2c) similar to that in Xu et al. (2012).

Affected by this MβCV, the heavy rainfall in Zhengzhou City thus intensified steadily (Fig. 1c). Afterwards, the MβCV tended to move northwestward, with PV around its center having intensified to 3 PVU during 0300—0600 UTC 20 July (Figs. 2d—2e). Although the radar echoes weakened immediately on the southern side of the MβCV center, the echoes became stronger on the eastern side of the MβCV center with the 1-PVU contour intruding northeastward in such a way that Zhengzhou City was just under the influence of stronger convective cloud and/or convective rainfall embedded within coherent large-scale southerlies. Note that a superior cloud cluster was present over southwestern Zhengzhou City, which was evidenced by a small area where stronger radar echoes exceeded 50 dBZ at 0600 UTC 20 July (Fig. 2e), leading to the more intense rainfall locally (Fig. 1c).

Two hours later, the superior cloud cluster with further intensified radar echoes was located over Zhengzhou City by 0800 UTC 20 July (Fig. 2f), causing more extreme heavy rainfall to occur within the hour from 0800 to 0900 UTC, corresponding to the strongest hourly rainfall in Zhengzhou City (Fig. 1c). Note that the MβCV center also migrated slightly northwestward (Fig. 2f) as compared to the location two hours earlier (Fig. 2e), which was about 100 km away from the super cloud cluster over Zhengzhou City (Fig. 2f), indicating that a dynamical connection may have existed between the MβCV and super cloud cluster for the extreme hourly rainstorm over Zhengzhou City in terms of mesoscale dynamics (as discussed in section 5). The above analyses demonstrate that the MβCV associated with the localized high-PV indeed played an important role in the Zhengzhou 7.20 rainstorm.

4. Synergistic effect of planetary-scale and synoptic-scale circulation systems on the Zhengzhou 7.20 rainstorm

4.1. Synoptic-scale typhoon systems

The occurrence and intensification of rainstorms depend indispensably on abundant moisture transport and low-level mass convergence at a larger spatial scale. Figure 3 shows the 500-hPa geopotential height field along with the vertically-integrated moisture fluxes and divergence before and during the Zhengzhou 7.20 rainstorm event. Note in Fig. 3a that almost two days before the event, two typhoons In-Fa (2021) and Cempaka (2021) as synoptic-scale systems had already existed over the western North Pacific and South China Sea, respectively, as evidenced by the closed 500-hPa geopotential contours and apparent cyclonic moisture fluxes. To the north of typhoon In-Fa (2021), the western North Pacific subtropical high (WPSH) was at an extraordinarily northward position with its center over Japan. Thus,typhoon In-Fa (2021) and the WPSH formed a meridional dipole pattern, with westward moisture fluxes between them(Fig. 3a). To the west of the WPSH, a planetary-scale longwave trough was connected with a “cutoff low” over Henan Province and an upstream high-pressure system over the Tibetan Plateau in the mid-latitudes around 35°N.

Subsequently, both typhoons tended to intensify in conjunction with the eastward propagation of the mid-latitude long-wave ridges and troughs (Figs. 3b—3c), with the geopotential height of typhoon center decreasing by at least 40 gpm for typhoon In-Fa (2021) and 20 gpm for typhoon Cempaka(2021) from 0000 to 1200 UTC 19 July. In correspondence with the development of mid-latitude long-wave disturbances by 1200 UTC 19 July (Fig. 3c), a weak ridge appeared over southern China extending from the western WPSH (Fig. 3b).Accordingly, the pressure gradient was enhanced not only between the WPSH and typhoon In-Fa (2021) but also between the mid-latitude ridge and typhoon Cempaka(2021), which accelerated the southeasterlies on the northeastern side of the typhoons, and thereby transported more moisture toward Henan Province (Fig. 3c) and contributed to the formation of the convective cloud band (Fig. 2a).

The stable WPSH was still maintained over Japan by 0000 UTC 20 July (Fig. 3d); however, the nearly stagnant typhoon In-Fa (2021) continued to intensify with its central low pressure region expanding further westward. As a result,the westward moisture transport from western North Pacific guided by typhoon In-Fa (2021) merged with the poleward moisture transport by typhoon Cempaka (2021) from the South China Sea, and in conjunction with the geopotential height ridge over southern China, forming a northwestward corridor of strong moisture transport that converged over northern Henan Province (Fig. 3d). Because water vapor was mainly concentrated in the lower troposphere, the significant moisture convergence over northern Henan Province was to a great extent associated with the blocking effect of mountains as shown in the inset of Fig. 3d. In turn, such convergent mountain-lifted moist air also facilitated the rapidly development of the MβCV (Fig. 2c) and heavy rainfall around 0000 UTC 20 July (Fig. 1c).

Fig. 3. Distributions of the vertically-integrated (300—1000 hPa) moisture flux (vectors, kg m-1 s-1),associated moisture flux divergence (color shading with respect to the bottom-left color bar, 10-3 kg m-2 s-1),500-hPa geopotential heights (contours, gpm) at (a) 0000 UTC 18 July, (b) 0000 UTC 19 July, (c) 1200 UTC 19 July, (d) 0000 UTC 20 July, (e) 0800 UTC 20 July, and (f) 0000 UTC 21 July 2021. Gray shading depicts the terrain altitude (with respect to the bottom-right gray bar, m). The boundary of Henan Province is outlined by solid purple curve. The inset in the top-left corner of (d), (e) and (f) is a magnification of Henan province to show more detailed information. Typhoon In-Fa (2021) and typhoon Cempaka (2021) are labeled in each panel with the typhoon symbol “” showing their positions.

Eight hours later, typhoon Cempaka (2021) and typhoon In-Fa (2021) both intensified locally (Fig. 3e) without any change in the positions of the two typhoon centers,forming a huge cyclonic system consisting of two intense typhoons. Note that the upstream ridge also strengthened as evidenced by the expansion of the 5860-gpm contour south of Henan Province. Thus, the northwestward moisture fluxes toward Henan Province became stronger and more coherent (Fig. 3e) at 0800 UTC 20 July. However, the moisture convergence in front of the mountains was weaker(Fig. 3e) than that eight hours ago. In fact, the moisture in the lower troposphere was mostly transported upward to higher levels by the eastward-sloping MβCV, which in turn cooperated with the upper-tropospheric westerlies to result in the localized urban rainstorm (as discussed in section 5).Afterwards, with the rapidly decay of typhoon Cempaka(2021) at 0000 UTC 21 July and with the southwestward intrusion of the WPSH (Fig. 3f), the direction of water vapor flux affected by this deformed ridge gradually veered to the north, causing the moisture to converge over the northern boundary of Henan Province (see the inset of Fig. 3f),and the heavy rainfall in Zhengzhou City subsequently ceased (Fig. 1c).

To further identify the source of water vapor causing the Zhengzhou 7.20 rainstorm, Figure 4a illustrates the trajectories of the target air particles traced backward from Zhengzhou City eight days prior to the rainstorm event. As shown in Fig. 4a, most of the particles arriving in the Zhengzhou City on 0000 UTC 20 July could be traced back to three main areas, namely the western North Pacific(Area1), the South China Sea (Area 2) and northwestern China (Area 3), respectively. The initial position of the tracked particles shows that the particles in Area 1 and Area 2 mostly came from the lower troposphere below 1500 m,while those in Area 3 originated in the middle troposphere above 3500 m, even reaching to a height of 6500 m(Fig. 4a). As a result, the initial particles from the oceanic areas carried abundant water vapor with relative humidity equal to 73.58% for Area 1 and 70.61% for Area 2, respectively, in comparison to the dry particles from Area 3 with a relative humidity of only 46.45%. Note also that the trajectory from western North Pacific to Zhengzhou City coincided well with the moisture transport corridor associated with typhoon In-Fa (2021) as discussed in Fig. 3, with another trajectory from South China Sea corresponding to the meridional moisture corridor related to typhoon Cempaka (2021). This fact reflects a thermodynamical effect of the dual typhoons in the Zhengzhou 7.20 rainstorm in terms of moisture transport. However, the descending particles from Area 3 first migrated eastward, and then veered southwards over northern China to arrive to the west of Henan Province, finally moving northward to reach Zhengzhou City (Fig. 4a). The trajectory of these particles is consistent with the anticyclonic flow behind the trough at ～115°E as shown in Fig. 3, indicating the contribution of the middle and upper tropospheric dry air from the mid-latitudes to the Zhengzhou 7.20 rainstorm.

To demonstrate more clearly the locations of the uplifted air parcels two days before the Zhengzhou 7.20 rainstorm event, Figure 4b shows the backward trajectories of the target air particles in the vicinity of Zhengzhou City from 0000 UTC 18 July to 0000 UTC 20 July. Note that at 0000 UTC 18 July, the moist air parcels were still concentrated in the lower troposphere below 1500 m (Fig. 4b), distributed mainly over the East China Sea and some provinces including Zhejiang, Jiangxi, Hunan, Hubei, and Anhui. Subsequently, some of these parcels experienced a rapid ascent over the region just 100 km south of Zhengzhou City(Fig. 4b), with a lifting value greater than 1000 m, which was obviously favorable for the condensation of water vapor. Immediately, the air parcels arriving in the middle troposphere were observed to continue rising rapidly to reach above 7500 m as they migrated northeastward towards Zhengzhou City, indicating that the converged moist air towards Zhengzhou City was uplifted rapidly into the middle-upper troposphere to condense within the eastward-sloping MβCV (as discussed below), producing extreme hourly rainfall (Fig. 1c).

4.2. Planetary-scale PV streamers

The above analysis indicates that the propagation and variation of the mid-tropospheric planetary-scale long-wave troughs and ridges in mid-latitudes had great impact on the direction and intensity of the moisture transport from the tropics in the Zhengzhou 7.20 rainstorm (Fig. 3). As suggested by Hoskins (1997, 2015), PV complies with the Lagrangian conservation principle when advective processes dominate over frictional and diabatic processes, and thus the isentropic PV and its movement are generally used to track the development of particular weather systems, especially in the upper troposphere where friction and diabatic heating are relatively small. Figure 5 shows the 350-K isentropic-surface (equivalent to about 250-hPa isobaric-surface as shown in Fig. 6)PV and winds during the Zhengzhou 7.20 rainstorm to clarify the contribution of the upper-tropospheric planetary-scale disturbances. A subtropical anticyclone corresponding to the South Asia High existed west of 100°E at 0000 UTC 18 July (Fig. 5a), with its ridge line at ～30°N. On the northern side of the anticyclone, there was a zonally oriented westerly jet stretching eastward along 45°N, with high-PV ＞ 2 PVU being accompanied within and north of the jet. Downstream were a closed cyclone accompanied by high-PV over the Yellow Sea and a dipole east of 140°E (Fig. 5a). Note that Henan Province was mostly influenced by northerlies with low-PV on the western side of the cyclone, and that the high-PV cyclone (Fig. 5a) corresponded well to the mid-tropospheric “cutoff low” (Fig. 3a), which may represent a vertically equivalent barotropic structure of Rossby waves in middle latitudes.

With the planetary-scale Rossby wave trains migrating eastward, the high-PV air was coherently advected southward into the subtropics to nearly 28°N by northerlies on the eastern side of the subtropical anticyclone, forming a meridionally elongated PV streamer west of Henan Province (Fig. 5b).In response, the ridge over Henan Province also amplified due to baroclinic energy dispersion, as suggested by Wiegand and Knippertz (2014). The downstream high-PV cyclone east of Henan Province similarly intruded southward, forming another PV streamer (Fig. 5b), while the ridge north of the typhoon In-Fa (2021) expanded noticeably northward(Fig. 5b). Such an amplified flow pattern became more distinct by 1200 UTC 19 July (Fig. 5c), with Henan Province located between two PV streamers along with the typhoon Cempaka (2021) in the south. Importantly, northern Henan Province was dominated by the divergent southwesterly flow immediately downstream of the PV streamer in the west, facilitating ascending motion locally (Fig. 1d) and convective rainfall (Fig. 2a).

Fig. 5. Distributions of the 350-K isentropic-surface PV (color shading, 2-PVU contours given by black curves) and wind (vectors, m s-1) at (a) 0000 UTC 18 July, (b) 0000 UTC 19 July, (c) 1200 UTC 19 July, (d) 0000 UTC 20 July,(e) 0800 UTC 20 July, and (f) 0000 UTC 21 July 2021. Purple curve in each panel shows the boundary of Henan Province. Typhoon In-Fa (2021) and typhoon Cempaka (2021) are labeled in each panel with the typhoon symbol “”showing their positions.

Subsequently, the ridge of the subtropical anticyclone extended northeastward to undergo anticyclonic wave breaking at 0000 UTC 20 July (Fig. 5d). Thus, the low-PV air was more favorable for being advected over Henan Province by the upper-tropospheric divergent southwesterlies ahead of the narrow southwestward-intruding PV streamer(Fig. 5d), which certainly amplified the downstream ridge and trough. As suggested by Archambault et al. (2015), advection of low-PV air by upper-tropospheric divergent outflows above a region of latent heating can amplify a ridge and intensify a jet streak by tightening the PV gradient. Note that at 1200 UTC 19 July, strong latent heating associated with convective rainfall exist over Henan Province (Fig. 2c). The ridge north of Henan Province indeed amplified at 0000 UTC 20 July (Fig. 5d) as compared to that 12 hours ago.The PV streamer east of Henan Province (Fig. 5d) as an amplified trough was observed to intrude more southward,even insofar as being able to connect with the intensified typhoon In-Fa (2021) in such a manner that its downstream ridge also amplified significantly.

With the intensification of the anticyclonic wave breaking, the PV streamer west of Henan Province further intruded southward to reach 20°N by 0800 UTC 20 July, leading to stronger divergent southwesterlies over Henan Province (Fig. 5e), which in turn triggered the sloping MβCV to produce extreme hourly rainfall (Fig. 1c, as discussed in section 5). Subsequently, both PV streamers tended to weaken: by 0000 UTC 21 July (Fig. 5f) the PV streamer west of Henan Province retreated to the north of 25°N, while another streamer decreased to ＜ 1.5 PVU south of 30°N. Therefore, the rainfall intensity in Zhengzhou City also decreased steadily (Fig. 1c).

4.3. PV diagnoses for development of the ascending motion

Given that ascending motion is a necessary dynamical condition to produce heavy rainfall, the vertical motion in relation to the upper tropospheric PV streamers before and during the Zhengzhou 7.20 rainstorm is shown in Fig. 6. In addition, because the mid-tropospheric PV was also consistent with the development of the vertical motion, with the high-PV intensifying mostly during the extreme rainstorm in Zhengzhou City (Fig. 1d), the dynamical mechanism for ascending motion associated with PV forcing was further examined through PV budget diagnosis and based on Eq.(1). Figure 7a displays the variation of the mid-tropospheric PV tendency as well as the forcing terms due to the PV advection and diabatic heating processes averaged over the Zhengzhou key-region. For a more detailed comparison,moments when the mid-tropospheric PV over Zhengzhou City changed sharply were selected at 1200 UTC 19 July,0000 UTC and 0800 UTC 20 July to determine the dominant factor for the PV variation (Figs. 7b—7d).

At 0000 UTC 19 July (Fig. 6a), high-PV (＞1 PVU) was observed mostly in the upper troposphere above 200 hPa,with a zone of concentrated higher-PV in the form of a tropopause fold intruding downward to 350 hPa and equatorward to around 39°N, which corresponded to the high-PV in mid-high latitudes north of Henan Province (Fig. 5b). Dynamically, such downward high-PV intrusion could cause a variation in the vertical gradient of isentropic surfaces as suggested by Hoskins et al. (1985), who pointed out that tropospheric high-PV forcing can induce warm (cold) temperature anomalies above (below) the PV forcing center due to the thermal wind relationship (see their Fig. 8). Therefore, the isentropic surfaces became concentrated and dense around the upper tropospheric high-PV, thus affecting the static stability in lower latitudes (Fig. 6a). On the other hand, because of the upper-level divergent flow filed caused by the PV streamer west of Henan Province (Fig. 5b) as well as the decreased static stability (Fig. 6a), moderate ascending motion was thus induced in the upper troposphere over the Zhengzhou key-region around 34°N. Similar ascending motion was also present in the lower troposphere over and south of Zhengzhou City at this time (Fig. 6a), which was due mainly to the topographic lifting effect as inferred from the intense westward moisture fluxes in front of the mountainous area (Fig. 3b).

More importantly, note in Fig. 6a that an isolated high-PV system ＞ 1 PVU was present in the mid-troposphere to the north of Zhengzhou City, with weak ascending motion above it, corresponding to the low pressure north of Henan Province sandwiched between the WPSH and the upstream ridge (Fig. 3b). This high-PV system subsequently divided into two parts with one part present at 37°N and another over Zhengzhou City at 1200 UTC 19 July (Fig. 6b). Such a PV redistribution can be understood through the PV budget according to Eq. (1). As shown in Fig. 7a, positive PV advection at 500 hPa reached 7×10-5PVU s-1over the Zhengzhou key-region at 1200 UTC 19 July, while negative PV generation due to diabatic heating was much less than the former in magnitude. As a result, the net positive PV tendency was about 2×10-5PVU s-1, and such a local rate of change could cause PV to increase about 1.7 PVU within only a day. This increased PV agreed well with observed high PV over Zhengzhou City shown in Fig. 6b, suggesting that the high PV resulted mostly from the horizontal PV advection. Further decomposition for PV forcing terms as in Eq. (1) demonstrated that the meridional PV advection was the dominant factor leading to the mid-tropospheric high PV over Zhengzhou City by 1200 UTC 19 July (Fig. 7b).Thermodynamically, the reinforced high PV tended to further enhance the convergence in middle layers, in conjunction with the divergent flows aloft, inducing ascending motion to occur in the lower troposphere, thus forming a deep updraft near Zhengzhou City (Fig. 6b). Consequently, moderate rainfall appeared at this time (Fig. 1c) in association with sufficient moisture supply (Fig. 3b). This also corresponded well to the localized strong radar reflectivity over central Henan Province at 34°N (Fig. 2a).

Fig. 6. Pressure—latitude cross sections (along113.6°E across Zhengzhou station) of PV (color shading, the 1-PVU contours are highlighted by black curves), potential temperature (green dashed contours, K) and meridional-vertical circulation (vectors; meridional wind in m s-1 and vertical motion (multiplied by a factor of -50) in Pa s-1, for vertical motion stronger than -0.01 Pa s-1) at (a) 0000 UTC 19 July, (b) 1200 UTC 19 July, (c) 0000 UTC 20 July, and 0800 UTC 20 July 2021. The gray shading shows the terrain altitude. The purple triangle on the axis indicates the position of Zhengzhou City.

Fig. 7. (a) Time series of the 500-hPa PV tendency (gray shading, 10-5 PVU s-1) and forcing terms resulting from the PV advection effect [green shading, 10-5 PVU s-1, equal to (F1+F2+F3) in Eq. (1)] and diabatic heating effect [red shading,10-5 PVU s-1, equal to (F4+F5) in Eq. (1)] averaged over the Zhengzhou key area (34.25°—35.25°N, 113.12°—114.12°E) from 0800 UTC to 0000 UTC 23 July 2021. The vertical dashed lines denote the critical moments used in (b) to (d). (b)Histograms of the PV tendency (bar, 10-5 PVU s-1) averaged over the Zhengzhou key area and its individual forcing terms(bars, 10-5 PVU s-1, indicated by letters F1—F5) in Eq. (1) at 1200 UTC 19 July when the rainfall around Zhengzhou City started. (c) As in (b), except for 0000 UTC 20 July when the rainfall around Zhengzhou City was intensifying. (d) As in (b)except for 0800 UTC 20 July when the strongest rainfall occurred in Zhengzhou City.

As the two PV streamers stretched further southward to induce much stronger rising motions with the upper-level divergence being more evident by 0000 UTC 20 July over Henan Province (Fig. 5d), the high PV over Zhengzhou City rapidly developed with more coherent and intense ascending motion in the entire troposphere (Fig. 6c), producing a deeply intense convective system with a funnel-shaped structure in the vertical direction (Fig. 6c). As shown in Fig. 2c,this convective system corresponded to the well-developed MβCV. It should be noted that during the 12 hours after 1200 UTC 19 July, the mid-tropospheric PV related to MβCV experienced two similar strengthening processes in terms of the variation in the PV tendency, one tendency peaking with 8×10-5PVU s-1at around 1800 UTC 19 July and another peaking at around 0000 UTC 20 July (Fig. 7a), and these two peaks also corresponded to the two centers of midtropospheric high-PV before and after 0000 UTC 20 July,respectively, over the Zhengzhou key-region (Fig. 1d). For the first strengthening process, the positive PV tendency was dominated by the diabatic heating (Fig. 7a). This was because the enhanced rainfall led to considerable release of latent heat after 1200 UTC 19 July (Fig. 1c), in turn manufacturing high PV immediately below the heating center to cause the reinforcement of PV at 1800 UTC 19 July (Fig. 7a)according to the PV equation [forcing term F5in Eq. (1)].Nevertheless, such enhanced high PV quickly dissipated afterwards by the rapidly intensified horizontal diabatic heating at 0000 UTC 20 July (Fig. 7c) due to increased vertical wind shear and horizontal heating gradient [forcing term F4in Eq. (1)]. However, the drastic ascending motion continually advected the newly manufactured high PV upward to further reinforce the PV of the MβCV in the second strengthening process (Fig. 7c). Finally, the middle and lower tropospheric convergent flows caused by such intense PV induced more rainfall over Zhengzhou City (Fig. 1c).

By 0800 UTC 20 July, however, the extreme ascending motion and mid-tropospheric PV had weakened (Fig. 6d),with the upper-tropospheric ascent dividing into two isolated centers and the mid-tropospheric PV propagating northward to 35°N. Indeed, the PV tendency in mid-troposphere turned to negative after 0500 UTC 20 July over Zhengzhou City(Fig. 7a). As shown in Fig. 7d, this is mainly attributed to the negative contribution of the gradually enhanced horizontal PV advection, which was actually affected by the deformation and retracement of the upper-level PV streamers (Fig. 5e).However, such weakened vertical motion could hardly explain the localized hourly extreme rainstorm in Zhengzhou City, thus another convective system, the MβCV, is explored from a multi-scale perspective in the following section.

5. Physical mechanism of the MβCV leading to hourly extreme rainstorm

As mentioned above, the formative mechanism for the Zhengzhou extreme hourly (0800—0900 UTC 20 July) rainfall was related to the MβCV and its interactions with planetaryand synoptic-scale circulations, which was necessarily different from that for the province-wide rainstorm in Henan Province. The synoptic-scale circulation structures including MβCV at 0000 UTC 20 July when the large-range rainstorm occurred in Henan Province (Fig. 8a) are compared with those at 0800 UTC 20 July when the localized urban rainstorm hit Zhengzhou City (Fig. 8b) from a PV perspective. Also shown are the fraction of the cloud cover and the associated specific cloud liquid water content (Figs. 8c and 8d),because their distributions in the atmospheric column are directly related to the total rainfall amount over Zhengzhou City. Since the size and density of the raindrop particles involved in the cloud microphysical processes are essential for a localized rainstorm event, the cross sections of the radar reflectivity are illustrated correspondingly in Figs. 8e and 8f to portray the spatial distribution and density of the hydrometeors, with the diabatic heating and the potential temperature being superimposed to capture the energy release in relation to the phase change of the hydrometeors. Finally,the accumulated hourly rainfall recorded by the rain gauges are shown along the adjacent longitudes of the Zhengzhou City (Figs. 8e and 8f).

It is clearly evident in Fig. 8a that at 0000 UTC 20 July,coherent strong ascending motion occurred in the troposphere nears 113°E, accompanied by high PV between 400 and 700 hPa, with abundant moisture given that the relative humidity exceeded 98% within the total atmospheric column. This combination of factors generated the strong MβCV-related cloud clusters, as evidenced by a high fraction of cloud cover ≥80% (Fig. 8c) within the high PV core and radar reflectivity ≥40 dBZ (Fig. 8e). The specific cloud liquid water content from low to high clouds reached 0.14 g kg-1between 112 to 114°E (Fig. 8c), leading to subsequent hourly rainfall ranging from 6.6 to 23 mm at several stations closest to Zhengzhou City (Fig. 8g).

Note also that the moist air with relative humidity greater than 90% within the convective column was mainly transported by strong low-level easterlies below 700 hPa,especially near the surface layer (Fig. 8a). To further examine such moisture transport ahead of the mountainous region,Figure 9 displays the corresponding distribution of the 975-hPa divergence and specific humidity. Note in Fig. 9a that sufficient moisture transport was persistently maintained by strong southeasterlies that resulted from the influence of the double-typhoon system as analyzed in section 4, which immediately formed a prominent convergent zone of moisture flux slightly southwest of Zhengzhou City. Dynamically,such convergent wind fields around Zhengzhou City were caused by the blocking effect of the upstream mountains together with the pumping effect of the upper-tropospheric divergent southwesterlies (Fig. 5d). As a result, the converged moisture in the lower troposphere was inevitably lifted into the upper troposphere (Fig. 8a) to support MβCV activity(Figs. 2c and 6c). According to Eq. (1), on the other hand,the rainfall-related diabatic heating in the mid-troposphere caused by the latent heat release due to condensation concurrently resulted in redistribution of the isentropic surfaces(Fig. 8e) and increased the PV below the level of the heating center. In turn, this increased high PV strengthened the convergence of the lower-tropospheric circulation, thus forming a positive feedback between the rainfall and high PV. Such a developmental mechanism for the convective updraft is similar to the eastward propagating Tibetan Plateau vortex causing the underlying Southwest China vortex to intensify and merge vertically, as studied by Zhang et al. (2021a),who found that the vertically nonuniform diabatic heating process is mainly responsible for the development of lowlevel Southwest China vortex forced by the overlying high PV of the Tibetan Plateau vortex. Therefore, both the lowlevel PV and ascending motion forced by the low-level convergence tended to strengthen.

Indeed, distinct high-PV was generated 8 hours later in the lower troposphere below 700 hPa west of 112.5°E(Fig. 8b) when the extreme hourly rainstorm happened in Zhengzhou City (Fig. 8h). Note that this low-level high PV was located more to the west than at 0000 UTC 20 July, a result of the more intense easterlies in the lower troposphere(Fig. 9b). The westward migration of the high PV over Zhengzhou key-region is also evidenced by the negative PV tendency associated with the zonal PV advection (Fig. 7d).Note in Fig. 9b that more intense moisture transport by the enhanced low-level easterlies converged towards Henan Province with higher values of specific humidity (18 g kg-1)reaching Zhengzhou City at this time, while the convergent circulation was still present ahead of the mountains and even to the lee of the mountainous area. In the meantime,strong westerlies began to reestablish in the upper troposphere due to the deformation of the PV streamer (Fig. 5e), in conjunction with the convective system and the low-level easterlies, resulting in an eastward-sloping MβCV as indicated by the eastward-tilting high PV core (Fig. 8b). As a result, a corridor of moist air was thus lifted within the MβCV in the form of an “L” pattern in the vertical as represented by the high values of specific humidity (＞ 98%) (Fig. 8b). The convective clouds indeed exhibited similar eastward-sloping structure, as implied by high fraction of cloud cover ＞ 50%(Fig. 8d). To the east of these convective clouds, compensatory descent was induced, which was the source for the“cold pool” outflows east of Zhengzhou City (Fig. 8b).These facts indicate the important effect of the large-scale background vertical wind shear on the inner structure of the MβCV and the resultant slantwise convective clouds.

Fig. 8. (a) Pressure—longitude cross sections (along 34.7°N across Zhengzhou station) of PV (color shading, PVU), relative humidity (transparent blue shading with blue contours, %), zonal-vertical circulation [vectors; zonal wind in m s-1 and vertical motion (multiplied by a factor of -50) in Pa s-1] at (a) 0000 UTC 20 July and (b) 0800 UTC 20 July 2021. (c) and (d)As in (a) and (b), except for the fraction of cloud cover (color shading, %) and the specific cloud liquid water content(contours, g kg-1). (e) and (f) As in (a) and (b), except for the combined radar reflectivity (color shading, dBZ), potential temperature (red dashed contours, K) and diabatic heating (black dashed contours, 10-4 K s-1). (g) Accumulated hourly rainfall (blue bar, mm) from 0000 UTC 20 July to 0100 UTC 20 July for the 11 rain gauges close to the latitude of Zhengzhou station in adjacent longitudes (each number along abscissa corresponds to the longitude of each station). (h) As in (g), except for the hourly rainfall from 0800 UTC 20 July to 0900 UTC 20 July during which the record-breaking hourly rainfall of 201.9 mm occurred in Zhengzhou City. The purple triangle on the axis in each panel indicate the position of the Zhengzhou City.

It is interesting that strong updrafts occurred to the west(around 112°E) of Zhengzhou (Fig. 8b), yet rainfall intensity was very weak (Fig. 8g) as shown by the weak radar reflectivity below 700 hPa (Fig. 8f). Instead, stronger diabatic heating existed in the middle and lower troposphere due to release of the latent heat of condensation, with maximum heating located at ～700 hPa and another branch of diabatic heating on the order of 4 × 10-4K s-1extending eastward to the midtroposphere over Zhengzhou City (Fig. 8f). The implication is that the local convective clouds were comprised of mixtures of water vapor and raindrops as well as other hydrometeors due to the phase change of water. In the meantime, these mixtures aloft were immediately lifted by the eastward-sloping MβCV to higher levels to further condense in such a manner that raindrops and ice crystal hydrometeors rapidly grew through complicated cloud microphysical processes (Chen et al., 2022). This deduction can be further verified by the distribution of the specific cloud liquid water content, with its maximum of 0.14 g kg-1concentrated between 400 and 700 hPa over Zhengzhou City and another center at 700 hPa west of 111.5°E (Fig. 8d), indicating that the liquid water content in clouds rapidly increased and accumulated within the atmospheric column over Zhengzhou City at this time. Such a phenomenon is supported by Chen et al. (2022), who demonstrated that the number concentration and the size of the raindrops suddenly increased at 1600 LST 20 July (equivalently 0800 UTC 20 July) over the Zhengzhou station from data provided by a dense network of disdrometers (see their Fig. 4c). Consequently, these enlarged and aggregated raindrops along with hydrometeors fell with great intensity, resulting in record-breaking hourly rainfall in Zhengzhou City(Fig. 8h), as evidenced by the strongest radar reflectivity(Fig. 8f).

To further substantiate the physical processes by which the moisture transport and phase transition caused the increase of the raindrops and hydrometeors over Zhengzhou City, another trajectory analysis was conducted (Fig. 10) to examine the trajectories and humidity variations of air parcels traced backward from Zhengzhou City at 0800 UTC 20 July. Note in Fig. 10a that except for a part of the parcels guided by the westerlies propagating eastward to Zhengzhou City in upper troposphere, there were also some air parcels in the lower troposphere (mostly below 1.5 km)from Yellow and East China Seas that migrated westward towards the west of Zhengzhou City, and were then abruptly lifted to above 4.5 km and veered to the east to arrive in the middle-upper troposphere over Zhengzhou City.This indicates that the lifted moist air to the west of the eastward-sloping MβCV was rapidly transported eastward and upward over Zhengzhou City (see Fig. 8b). Such features can be seen more clearly from the height-time cross section of the moving particles (Fig. 10b). Some moist particles with relative humidity ＞ 80% were observed to exist below 1 km before 12 hours, while these air parcels ascended rapidly to 6-km altitude within 12 hours (Fig. 10b) and became saturated or nearly so in the eastward-sloping MβCV until they finally approached the middle troposphere over Zhengzhou City. This corresponded well to the MβCVrelated circulation structure (Fig. 8b) as well as the distributions of relative humidity and specific cloud liquid water content (Figs. 8b and 8d), indicating the crucial role of the eastward-sloping MβCV and its interaction with larger-scale circulation in triggering the extremely hourly rainstorm over Zhengzhou City through complicated microphysical processes.

Fig. 10. (a) Same as Fig. 4, except for the trajectories of the target air parcels from 0800 UTC 17 July to 0800 UTC 20 July and the newly-selected initial heights (5, 5.5, and 6 km,respectively). The inset is the magnified Henan province to highlight the trajectory information around Zhengzhou City. (b) Height-time cross section of the 81 (3 heights × 27 members) air parcels. Color shading along each trajectory denotes the relative humidity of the moving parcels (%). Black solid circles indicate the initial location of the trajectories.

6. Summary and discussion

During the summer of 2021, an unprecedented heavy rainfall event took place over northern Henan Province, in which Zhengzhou City suffered a devastating flooding on 20 July (Zhengzhou 7.20 rainstorm) with a 24-hour accumulated amount reaching 627 mm and a record-breaking hourly rainfall of 201.9 mm occurring during 1600—1700 LST (0800—0900 UTC). The evolution of atmospheric circulations along with the high-resolution radar reflectivity demonstrates that the Zhengzhou 7.20 rainstorm resulted from a synergistic effect between multi-scale systems including planetary-scale disturbances related to PV streamers, synopticscale typhoons and cutoff lows. and mesoscale MβCV associated with mountain lifting and PV forcing. Therefore, the purpose of the present study was to investigate the dynamical and thermodynamical factors causing the Zhengzhou 7.20 rainstorm, with special attention being paid to how the MβCV interacted with larger-scale circulations to result in the record-breaking hourly rainfall during 0800—0900 UTC 20 July in Zhengzhou City (Fig. 11). The major findings are summarized as follows:

Fig. 11. Schematic diagram in the form of pressure—longitude cross section (along 113.6°E across Zhengzhou station) showing how the eastward-sloping MβCV under large-scale circulations gave rise to the extreme hourly rainfall during Zhengzhou 7.20 rainstorm. The black contours indicate the distribution of PV (PVU), while the broad blue-shaded bold arrows represent transport paths of the moisture. Pink arrows show the distribution of the actual winds including strong updraft (relative weak downdraft) to the west (east) of the sloping MβCV, the large-scale low-level easterlies associated with moisture transport and upper-level westerlies showing vertical shear. Diabatic heating associated with latent heat release is represented by the red shading. The regions with the fraction of cloud cover greater than 50% are highlighted with gray shading, as presented in Fig. 8d. Hydrometeors are indicated by white ice-crystals and snowflakes, and raindrops are shown in blue color. Dark brown shading along the abscissa denotes the mountain terrain, while the purple triangle indicates the position of the Zhengzhou City. See section 6 for details.

Two days before the Zhengzhou 7.20 rainstorm event,typhoon In-Fa (2021) and typhoon Cempaka (2021) had already formed over the western North Pacific Ocean and South China Sea respectively, with the WPSH being located unusually northward. Henan Province was under the influence of a cutoff trough in the middle troposphere between the WPSH and an upstream ridge over the Tibetan Plateau due to an eastward propagating planetary-scale wave train.Subsequently, both typhoons intensified by 0000 UTC 20 July in conjunction with the eastward propagation of the mid-latitude ridge and trough behind the stable WPSH, accelerating the moisture transport from the ocean toward Henan Province due to the enhanced pressure gradient between the mid-latitude ridges and the two typhoons. Such moisture transport was further demonstrated by the use of backward trajectory analyses, in which the moist air parcels from the northwestern Pacific was mainly transported toward Henan Province by confluent southeasterlies on the northern side of stronger typhoon In-Fa (2021), with the convergent southerlies associated with typhoon Cempaka (2021) concurrently transporting water vapor northward from South China Sea,supporting the moisture supply for the Zhengzhou 7.20 rainstorm.

On the other hand, the trajectory tracking also identified some air parcels arrived from the middle troposphere at middle latitudes. Evolution of the 350K PV isosurface showed that two PV streamers representative of planetary-scale disturbances in the upper troposphere were manifested as elongated high PV phenomena that extended equatorward from the higher latitudes. Henan Province was located between these two PV streamers, which synergistically formed stronger divergent flows aloft over Zhengzhou City to induce intense ascending motion, leading to the full development of a localized MβCV with mid-tropospheric high PV at 0000 UTC 20 July. This phenomenon was also substantiated by the coexistence of locally strong radar reflectivity with high PV at low levels.

A PV budget analysis for the 500-hPa PV tendency over the Zhengzhou key-region demonstrated that the formation of the deep MβCV was firstly attributed to the horizontal PV advection, which significantly enhanced the localized moisture convergence in the presence of sufficient water vapor supply to produce considerable rainfall. In turn, the mid-tropospheric latent heat released by such rainfall processes immediately created high PV below the heating center. The low-level high PV was continually advected upward by the strong ascending motion within the MβCV to further reinforce the mid-tropospheric convergent circulation, thus forming a positive feedback between the rainfall and high-PV, and ultimately leading to the outbreak of the Zhengzhou 7.20 rainstorm. Subsequently, due to the enhanced low-level easterlies as well as the upper-tropospheric westerlies caused by the deformation of the PV streamers, the MβCV exhibited an eastward sloping structure eight hours later and its center at low-levels was further enhanced with the stronger high-PV moving to the mountainous area west of Zhengzhou City (Fig. 11). This led to more latent heat release within the MβCV below 500 hPa (Fig. 11),thus generating abundant raindrops due to moisture phase change locally. Importantly, the mixture of the raindrops with other hydrometeors was then lifted within the eastwardsloping MβCV to further increase and accumulate immediately over Zhengzhou City (Fig. 11), forming eastward-sloping convective clouds and inducing the extreme hourly rainstorm during 0800—0900 UTC 20 July through microphysical processes. This feature was further manifested by backward trajectory analysis of parcels from Zhengzhou at 0800 UTC 20 July, which showed some parcels were vigorously lifted into the middle-upper troposphere to become saturated or nearly so and migrated simultaneously eastward to arrive over Zhengzhou City. This emphazied the crucial role of the eastward-sloping MβCV and its interaction with planetaryand synoptic-scale circulations in triggering the record-breaking hourly rainfall in the Zhengzhou 7.20 rainstorm.

It should be noted that although the present study attributes the direct factor causing the extreme hourly rainfall to the eastward-sloping MβCV, smaller-scale convective systems such as meso-γ-scale convective systems may have been involved (Yin et al., 2022), but current ERA reanalysis data can only capture the convective systems whose spatiotemporal scales are greater than the meso-β-scale due to relatively coarse spatial and temporal resolutions. Note that the formation and variation of the eastward-sloping convective clouds shown in Figs. 8d and 11 were dependent not only on the multi-scale dynamical processes but also on the complex cloud microphysical processes. For instance, how did the raindrops and hydrometeors increase and grow so rapidly in clouds immediately before and during the record-breaking hourly rainfall? This issue can be only partially understood based on Chen et al. (2022), in which the cloud microphysical processes deserve further investigations using both intensive atmospheric microphysical observations and numerical modeling. Also, although PV streamers exerted great impact on the Zhengzhou 7.20 rainstorm, how and to what extent did the equatorward intrusion of the PV streamers induce the initiation and development of the MβCV and even meso-γ-scale convective system? These questions need to be addressed in future research, thereby improving understanding of the physical mechanism for the eastward-sloping MβCV leading to the record-breaking hourly rainfall in Zhengzhou City.

Acknowledgements.This research was supported by the National Natural Science Foundation of China (Grant Nos.42288101, and 42175076), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB40000000)and the Open Research Fund Program of Plateau Atmosphere and Environment Key Laboratory of Sichuan Province (Project PAEKL-2022-K02).