An Extreme Monsoonal Heavy Rainfall Event over Inland South China in June 2022: A Synoptic Causes Analysis

The monsoon region is the region with the largest variability of precipitation in the world, and the issue of monsoonal heavy precipitation has always been the major scientific and operational concern for the meteorological community (Luo et al. ^[1]). The World Weather Research Program (WWRP) of the World Meteorological Organization (WMO) has been actively promoting the research and communication on monsoonal heavy precipitation in recent years, and four thematic monsoonal precipitation workshops were held in 2011, 2012, 2015 and 2019 to promote the communication among scientists and weather forecasters around the world in conjunction with the development of modern observation and numerical forecasting techniques. Despite meaningful progress has been achieved in monitoring, modeling, and forecasting monsoonal precipitation, many scientific questions remain to be revealed due to the complexity of the mechanism of heavy monsoonal precipitation and the lack of forecasting capability remains an operational challenge (Luo et al. ^[1]; Kumar et al. ^[2]; Wang et al. ^[3]).

Due to the influence of summer monsoon, south China, the southernmost part of the Chinese mainland, is the region with the earliest and most frequent heavy precipitation during the rainy season in China (Tao and Chen ^[4]; Ding ^[5]). The major synoptic scale forcing systems of heavy rains during the early summer in south China are frontal systems and monsoonal flows (Tao and Chen ^[4]; Chen et al. ^[6]). The summer monsoon, together with the topography of south China and local sea-land contrasts, form the warm-sector heavy rainfall in the pre-summer rainy season of south China under the background of southwestern flows (Huang et al. ^[7]; Wu et al. ^[8]). In the context of global warming, the combination of strong summer winds with mid-latitude synoptic systems, tropical cyclones and local topography in recent years has caused multiple extreme heavy rainfall events in south China and triggered severe landslides and floods (Meng and Wang ^[9]; Cai et al. ^[10]; Wu et al. ^[11]).

To deepen the understanding of heavy rainfall events in the south China monsoon region, a Southern China Monsoon Rainfall Experiment (SCMREX) (Luo et al. ^[12]) was held by the Chinese Academy of Meteorological Sciences in 2013 that focused on scientific observations and studies of heavy precipitation events in the pre-monsoon period in south China, which gained a lot of knowledge on the characteristics of the boundary layer and its microphysics and performed assimilation tests (Liu et al. ^[13]; Wang et al. ^[14]; Zhang et al. ^[15]; Zhang ^[16]). However, due to the lack of maritime observations and insufficient understanding of the scientific mechanisms of different types of heavy monsoonal precipitation, the forecasting skill of heavy precipitation in the south China monsoon region, especially in the warm region, is still very low compared with that of frontal heavy rainfall forced by mid-latitude synoptic-scale systems (Sun et al. ^[17]; Wu et al. ^[18]; Min and Wu ^[19]).

Statistical data showed that the heavy rainfalls in the pre-summer rainy season in south China, which are related to the triggering of monsoonal airflow, usually occur in the coastal areas, last for about 1-2 days, and have significant nighttime characteristics (Wu et al. ^[8]). In mid-June 2022, a rare extreme monsoonal heavy precipitation occurred in south China, mostly in the interior of its northern part, lasted for nine days, triggering a once-in-a-century major flooding event in the Beijiang River basin and affecting more than 1.6 million people. Regional hydrometeorological observations and ERA5 reanalysis data were applied to diagnose the characteristics of this extreme monsoonal rainfall. Focus was paid on the causes for the recurrence and long duration of the heavy precipitation in the northern mountainous areas. The answers will improve our understanding of the mechanism of the inland-type extreme monsoonal rainfall. The remainder of this paper is organized as follows: The data and methodology are given in Section 2. Section 3 reveals the characteristics of the extreme precipitation event. Section 4 analyzes the synoptic-scale dynamic system and the characteristics of monsoonal moisture transport. Section 5 compares the difference of the physical mechanisms of heavy precipitation between two stages. Finally, section 6 provides a summary and discussion.

The precipitation data used in this paper include national and regional station data covering the five provinces of south China (Hunan, Jiangxi, Guangxi, Guangdong, and Fujian), which can be downloaded from the MUSIC interface at http://172.22.1.175/di/index. ERA5 reanalysis data was used to characterize the circulation, which is obtained from the following link: https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era5.

The 2022 South China Sea (SCS) summer monsoon set off on May 12, transporting abundant amount of warm and humid water vapor to China's mainland. With the onset of the monsoon, the precipitation in south China began to increase significantly, especially in the middle of June, heavy rainfall occurred in south China and persisted for a long time. Fig. 1 shows the distribution of accumulated rainfall from June 12 to 21. Large values of accumulated precipitation occurred to the south of Nanling Mountains and in northern south China, while the precipitation in the coastal areas of south China was relatively small. The area most seriously affected by the flooding was located in the northern part of Guangdong where 328 towns and streets recorded cumulative rainfall of more than 250 mm, including 990.5 mm in Yingde City, Qingyuan and 979.4 mm in Weng Yuan County, Shaoguan.

Figure 1.  Cumulative rainfall in south China from June 13-21, 2022 (units: mm).

For further understanding of the evolution of the precipitation process, Fig. 2 presents the distribution of daily rainfall in Guangdong. The heavy precipitation lasted for nine days and had large amounts in regional areas. In the process, more than 100 mm of heavy daily rainfall occurred every day. Except for June 15-16 when the precipitation was relatively weakened, more than 250 mm of rainfall was recorded per day in the remaining six days. At the same time, the falling area of this persistent heavy rain was relatively stable, which was repeatedly seen in the mountainous areas of northern Guangdong, triggering a once-in-a-century flooding event in the Beijiang River Basin.

Figure 2.  Daily rainfall in Guangdong from June 13 to 21, 2022 (units: mm).

Figure 3 shows the basic synoptic systems and their temporal evolution during June 13-21, 2022. At the upper geopotential level of 200 hPa (Fig. 3a), the South Asian high moved northward to the vicinity of the Qinghai-Tibet Plateau, stable and maintained there. Located to the east side of this high, south China was provided with favorable upper-level divergence conditions for continuous heavy rainfall (Fig. 3b). At 500 hPa, a western Pacific subtropical high to the southeast of south China was stronger than normal (figure omitted), covered the western Pacific and the eastern SCS, while a continental high over the region from north China to Lake Baikal was unusually strong (Fig. 3c). Between the two highs, there were continuous westerly wind bands fluctuating eastward and southward, affecting the central-north region of south China (Fig. 3d). These frequent westerly wind disturbances, which affect south China, originated from the middle and high latitudes as well as from the southern branch fluctuations on the southern side of the Qinghai-Tibet Plateau. On the one hand they promoted the development of low troughs in the lower layers, and on the other hand formed a strong pressure gradient with the subtropical high, favoring the strengthening of southwesterly airflow, providing favorable dynamic instability conditions for the occurrence of this persistent heavy rainfall.

Figure 3.  Time mean circulation and its temporal evolution from June 13-21, 2022: (a) 200 hPa geopotential height (units: dagpm); (b) 200 hPa dispersion in northern south China (23-25° N, 112-116° E, units: 10^-7 s^-1); (c) 500 hPa geopotential height (units: dagpm); and (d) latitude-time evolution of 500 hPa geopotential heights along 113°E (units: dagpm).

In order to find out the water vapor conditions for this sustained heavy rainfall, the temporal mean of 850 hPa winds and water vapor flux during this process and its climatological mean counterpart are shown in Fig. 4. The climate-averaged map (Fig. 4a) shows that after the summer monsoon outbreak in the SCS in mid-May, the subtropical high retreated eastward from the SCS by mid-June, and the southwest monsoon transported water vapor from the Bay of Bengal and the SCS region to the southern part of China, while the strongest transport of water vapor was still going on in the Bay of Bengal and the Indochina Peninsula. However, during the process in 2022, the monsoonal moisture transport was significantly stronger than the climatological mean (Fig. 4b). Since the subtropical high dominated the central-eastern part of the SCS, which made the monsoonal moisture transport channel more northward than normal, and the strongest center of moisture transport moved north to south China. The southwest monsoon transported and converged the water vapor from the Bay of Bengal to south China continuously, providing abundant amount of water vapor for the occurrence of the extremely heavy precipitation in the interior of south China.

Figure 4.  (a) Climatological mean (June 12-22), (b) temporal mean (June 12-22, 2022), and (c) anomaly (deviation from climatology) of 850 hPa winds (arrow, units: m s^-1) and water vapor flux (shades, units: 10^-3 g hPa^-1 cm^-1 s^-1).

From the daily rainfall in Fig. 2, we can see that this sustained heavy rainfall event can be divided into two stages. The first stage is from June 13 to 15, and the second stage from June 16 to 21. Latitude-time diagram of 850 hPa low-level wind along the center of heavy rainfall (113° E) is given in Fig. 5a. Although the two stages of heavy rainfall were in the same context of the SCS summer monsoon outbreak, they were influenced and triggered by different synoptic systems. At the first stage, the frontal (shear) heavy rainfall was triggered by the southward movement of cold air (northerly winds) brought by an upper-level trough; at the second stage, the monsoonal (warm-sector) heavy rainfall was caused by the strengthening of monsoonal jets over the sea penetrating northward.

Figure 5.  (a) Latitude-time distribution of 850 hPa horizontal wind along 113°E (blue dashed lines denote northerly winds, units: m s^-1, shaded areas denote 500 hPa upward motion, units: m s^-1) and (b-c) 850 hPa horizontal wind (units: m s^-1) during the two stages. (Shaded area represents wind speed > 8 m s^-1)

Figure 5b-5c shows the distribution of the 850 hPa wind during the two stages. In the first stage (June 13-15), although the region from the SCS to the Indochina Peninsula was controlled by a consistent southwest monsoonal airflow, a low-level shear appeared and stayed in south China from 13 June and gradually moved southward. The heavy precipitation was induced by the shear and occurred mostly in the nearby area. Due to the weak intensity of seasonal cold air, the front moved south slowly. From the evening of June 12 to June 15, the shear shifted back and forth between northern Guangdong and the northern Pearl River Delta for more than 48 hours, making the center of heavy precipitation locate inland in northern Guangdong. During the second stage (June 16-21), the southwest monsoon intensified significantly, with its wind speed more than 12 m s^-1 reaching the standard of a jet stream. On the morning of June 18 and June 19, the winds in south China reached their strongest, with the intensity of the jet center around Guangxi exceeding 20 m s^-1 and the southwest monsoon over Guangdong reaching more than 16 m s^-1. The low-level jets near the 925 hPa level (figure omitted) also strengthened significantly at this stage. The SCS and south China are uniformly dominated by the monsoonal airflow, with its center located over the south China region. The center of the jets extended from the northern part of the SCS to inland Guangdong, which was just located in the exit area of the low-level jets. The extremely strong convergence of wind caused the heavy rain to occur mostly in the unstable area near the southwest low-level jets and the convergence area in front of the exit area of the low-level jets.

Figure 6 presents the 500 hPa geopotential height. Although both were influenced by the upper-level westerly trough, the rainfalls of the two stages were significantly different. In stage 1, the East Asian trough was strong and deep, carrying cold air from the north to the south to affect south China. The subtropical high dominated the western Pacific and SCS, causing the low-level cold and warm air to converge in south China, forming a strong frontal-shear heavy rainfall. In stage 2, the streamlines were flatter in the middle and high latitudes of Asia, only a short-wave trough of the Qinghai-Tibet Plateau moved eastward to affect south China. At low latitudes, the main body of the subtropical high retreated eastward and its westernmost ridge point was located in the eastern part of SCS, which was conducive to establishing and strengthening the transport of monsoonal moisture from the SCS to south China.

Figure 6.  Distribution of 500 hPa geopotential height (contours, units: dagpm) and anomaly (shades, units: dagpm) for two different precipitation stages of (a) June 12-15 and (b) June 16-21.

Many kinds of research have shown that the low-level jet plays a critical role in the occurrence of frontal and warm-sector heavy rainfall in the pre-summer rainy period of south China. There are differences in the influence of synoptic-system-related low-level jet (SLLJ) and boundary layer jet (BLJ) on the two types of heavy rainfall (Huang et al. ^[7]; Du and Chen ^[20]. Liu et al. ^[21]), and the structure, intensity, and daily variation of the boundary layer low-level jet play a key role in the development of warm-sector heavy rainfall in coastal areas (Wu et al. ^[8]).

To understand the influence of low-level jets on different types of heavy rainfall in this two-stage event in the context of a monsoon outbreak, the vertical profiles of the winds during the two stages of rainfall along the 113°E are given in Fig. 7. In the first stage, the center of the low-level jet was located to the south of a shear between 850 and 700 hPa, which belonged to the SLLJ associated with the upper-level trough and the subtropical high, and the heavy rainfall mainly occurred in the area near the shear in the front of the SLLJ jet center. In the second stage, south China and the SCS from 18°N to 28°N were dominated by strong southerly winds, resulting in double low-level jets characterized by a northward slope (SLLJ at 850-550 hPa and BLJ at 975-900 hPa). Unlike the previous coastal-type warm-sector heavy rains, the low-level jets shifted northward during this stage, and the warm-sector heavy rainfall center was located to the north of the BLJ under the influence of the northward BLJ trigger.

Figure 7.  Vertical structure of horizontal wind (units: m s^-1) and daily rainfall (units: mm) along 113°E at the two stages (shaded area represents wind speeds, with north wind < 0, south wind > 0) for (a) June 13 and (b) June 19.

To further understand the influence of the low-level monsoonal jets on the diurnal variation of precipitation, the hour-by-hour evolution of the 850 hPa/925 hPa low-level winds and rainfall in the central area for the two stages are given in Fig. 8. During the first stage (June 13-14), the low-level winds were stronger in the daytime than in the nighttime, and the corresponding heavy rainfall mainly occurred during the daytime hours on both days. During the second stage of the warm-sector heavy rainfall (June 17-21), the jets on 850 hPa and 925 hPa were significantly stronger than in the first stage, sharing similar trends. The five nighttime intensifying processes of the low-level jet were closely coordinated with the rainfall intensification in the northern Guangdong region. Therefore, the enhancement of the nighttime monsoonal low-level jets was not only conducive to the occurrence of precipitation in coastal south China, but also played a key role in the nighttime rainfall of the heavy rainfall in the warm sector of northern Guangdong.

Figure 8.  Hourly evolution of rainfall (units: mm) and low-level winds (units: m s^-1) averaged over the central area of precipitation during the two stages of (a) June 13-14 and (b) June 17-21.

For a comprehensive understanding of the mechanism of this sustained warm-sector heavy rainfall in inland south China, a conceptual synoptic diagram of the heavy rains for the two stages is given separately in Fig. 9 for comparison. The main differences are mainly focused on the following aspects: during the first stage (frontal heavy rainfall), the East Asian trough of 500 hPa was deeper, which guided the cold air to move southward, resulting in the formation of frontal (shear) heavy rainfall in northern south China. However, during the second stage (warm-sector heavy rainfall), only a shallower plateau trough moved eastward to affect south China, bringing high-level dynamic instability conditions. The specific triggers and falling areas of heavy rainfall were primarily influenced by low-level circulation. The first stage of the heavy rainfall was primarily triggered by the frontal shear, and the rainfall area mostly occurred near the shear. The heavy rainfall in the second stage occurred in the southward flow of the middle and lower levels, primarily triggered by the boundary layer low-level jet. The falling area of the rain occurred steadily in the convergence area of BLJ, with a feature of nighttime enhancement in both the heavy precipitation and the BLJ.

Figure 9.  Integrated conceptual synoptic model for heavy rainfall during the two stages of (a) June 13-15 frontal heavy rainfall and (b) June 16-21 warm-sector heavy rainfall.

Previous studies have shown that coastal-type warm-sector heavy rainfall events are primarily triggered by the combined effect of the BLJ, local sealand winds and coastal topography (Wu et al. ^{[8, 11]}). The heavy precipitation usually occurred in a small range and lasted briefly. Liu et al.^[21] pointed out that the non-geotropic southerly wind (like BLJ) in the northern SCS contributes little to the inland frontal precipitation, but has a significant impact on the coastal warm-sector heavy rainfall. In addition to the BLJ, the dispersion conditions caused by the low-level SLLJ play an important role in the development of convection in the low-level warm sector (Du and Chen ^[20]). During this process of inland-type warm-sector rainfall, it was BLJ that was enhanced in the northern part of the SCS and played the decisive role in forming the heavy rainfall, though the effect of coastal areas was relatively small. The low-level convergence mostly came from the convergence between the BLJ and the windward slope of large terrain (the Nanling Mountains in this case), while the high-level dispersion conditions mostly came from stable synoptic-scale systems in the middle and upper levels. The effects of these key factors are similar to those of coastal-type warm-sector storms, and the main difference between them is the strength of the low-level SLLJ and the BLJ, especially the difference of BLJ. Previous studies (Wu et al. ^[8]) and operational experience show that the occurrence of heavy rainfall in the warm coastal zone is favored when the BLJ reaches 10 m s^-1 or more with the center lying in the northern part of the SCS. However, the intensity of BLJ of inland-type warm rainstorms often reaches 12 m s^-1 or more, and its center often extends from the northern part of the SCS to the inland area. In this process, the BLJ intensity from northern SCS to the Pearl River Delta area was more than 12 m s^-1. On June 18 and 19 when the precipitation reached its maximum, the BLJ intensity in the northern part of SCS exceeded 16 m s^-1. Therefore, this inland warm-sector heavy rainfall was larger on the spatial scale and longer (5 days) on the temporal scale, with the falling area stably maintained in the inland area of northern Guangdong.

Heavy monsoonal rainfall usually causes severe disasters. At present, there is still insufficient scientific understanding and forecasting about it. In this paper, we analyze and diagnose the characteristics and synoptic causes of a persistent and extreme monsoonal heavy rainfall event that occured in the northern interior part of south China in June 2022 and summarize its conceptual diagram. The major findings can be summarized as follows:

(1) From June 12 to 21, a rare monsoonal precipitation occurred in northern south China, which lasted for nine days, with daily heavy rainfall of more than 100 mm, and rain falling areas repeatedly appeared in the interior northern south China.

(2) Synoptically, the South Asian high was stable on the southern side of the Qinghai-Tibet Plateau, while the western Pacific subtropical high retreated eastward to dominate the eastern part of the SCS. The stable situation at mid - and upper-levels provided favorable conditions for upper-level dispersion over south China and was also conducive to the westerly trough that kept moving eastward and southward to affect northern south China. Meanwhile, the low-level southwest monsoonal moisture transport was stronger and more northward than normal, bringing abundant amount of warm and humid water vapor to south China.

(3) The persistent monsoonal heavy precipitation can be divided into two stages. The first stage (June 13-15) was frontal rainfall affected by the cold air brought by the East Asian trough. The second stage (June 16-21) was the warm-sector heavy rainfall triggered by low-level jets penetrating north, and the high level was mostly affected by the eastward shift of the short-wave trough from the Qinghai-Tibet Plateau.

(4) In the first stage, the center of SLLJ was located near 850-700 hPa. The heavy rainfall mostly occurred in the area near the shear in the front of the SLLJ center, and both the jet and precipitation were the strongest during daytime hours. In the second stage, the center of BLJ was more inland. The heavy rainfall mostly occurred on the windward slope of the Nanling Mountains and to the north of BLJ. The five nighttime enhancements of the BLJ corresponded well to the enhancement of the inland rainfall center so that the rainfall exhibited a significant nighttime feature.

Based on the above results, a conceptual synoptic diagram of inland-type warm-sector heavy rainfall in the south China monsoon region is summarized. The mechanism of extremely heavy precipitation in the monsoon region is complex. This paper only conducts a diagnostic study on the rare extreme precipitation in June 2022 from the perspective of synoptic scale. In the future, it is necessary to enhance our understanding of this extreme monsoonal precipitation by combining high-resolution numerical simulations and microphysical analysis. More individual cases of different types of extreme monsoonal precipitation need to be classified and compared.