Characteristics of newly formed mesoscale convective systems during the abnormal precipitation over the Yangtze River basin from June to July, 2020

Abstract

A 62‐day “extremely violent Meiyu” occurred in the Yangtze River basin from June to July in 2020, and its abnormal precipitation caused serious losses to the social economy. In this paper, the characteristics of newly formed mesoscale convective systems (MCSs) and the environmental conditions affecting the MCS formation during this abnormal precipitation event are investigated based on hourly datasets, including the temperature of brightness blackbody (T_BB) data derived from the FY‐2F geostationary meteorological satellite, and the three‐source merged precipitation products from China Meteorological Administration. The results show that there were 31,778 times of short‐duration heavy precipitation (hourly precipitation exceeding 20 mm) occurred in the Yangtze River basin from June to July in 2020. The 69.3% of short‐duration heavy precipitation (hourly precipitation exceeding 20 mm) are closely related to the MCSs. The increase of newly formed MCSs leaded to more short‐duration heavy precipitation. The abnormal precipitation in the Yangtze River basin from June to July is mainly caused by elongated MCSs with an average lifetime of 6 h. The number of the newly formed MCSs presents obvious diurnal variations, with the most frequent at 20:00 (Beijing Time, the same below). The newly formed MCSs are prone to appear near the Yangtze–Huaihe shear line, and the magnitude of southerly wind at 500 hPa determines the distance between the MCSs and the shear line. The enhancement of vorticity advection at 500 hPa and water vapor convergence are conducive to the MCS formation. The weakening of low‐level jet corresponds to the increase of newborn MCS.

INTRODUCTION

The abnormal climate in 2020 caused an early starting and late ending in the Meiyu season in China, and thus the rainy season lasted for as long as 62 days. Ten heavy rainfall processes occurred successively in the middle and lower reaches of the Yangtze River, and the total precipitation amount during the Meiyu period is the largest in 2020 among all Meiyu periods since 1961. Therefore, the Yangtze–Huaihe River basin suffered the strongest attack from continuous Meiyu in this century (Liu & Ding, 2020). The ultra-strong Meiyu in 2020 presented significant characteristics of quasi-biweekly oscillation (Y. Liu, Wang et al., 2021; Y. Liu, Yao, et al., 2021). Studies have pointed out that the occurrence of continuous heavy rainfalls during the Meiyu season is closely related to the repeated reconstruction of unstable atmospheric stratification (H. Zhang et al., 2018), while the regions with significant increase of extreme precipitation in China is concentrated in the middle and lower reaches of the Yangtze River (Qian & Lin, 2005).

The mesoscale convective system (MCS) is an important weather system causing not only rainstorms (Schumacher & Johnson, 2005). Moreover, it can brought severe weather events and floods and also affect the atmospheric circulations (Houze, 2018). After objective detection of propagating MCSs were present for 72% of the heavy rain days in Bangladesh and surrounding area (Ahsan Habib & Sato, 2019). The precipitation related to eastward-propagating MCSs accounts for more than 35% of the total warm-season precipitation over the second-step terrain in the Yangtze River Valley (R. Yang et al., 2019). The activity characteristics, movements, and organization forms of MCSs can directly affect the precipitation intensity (Ai et al., 2016; J. Sun et al., 2010; Wang & Cui, 2011). The slow-moving MCSs result in more local precipitation, which is more conducive to extreme rainfall and flash floods (S. J. Chen et al., 1998).

The distributions and activities of MCSs are closely related to the precipitation during the Meiyu seasons. The MCS in the zonal convection near the Meiyu front contributes significantly to the precipitation in eastern China (Luo et al., 2018). The vicinity of the vortex along the Meiyu front is often accompanied by developing MCSs with a long life time (B. Liu et al., 2020). In central and eastern China, the contribution rate of MCSs to heavy precipitation events is 45% on average (He et al., 2016). The precipitation during the Meiyu season in the Yangtze River basin is closely related to the MCS formed and developed along the Meiyu front (T. J. Chen & Yu, 1988; Ding & Chan, 2005). An analysis on the impact of MCSs on the precipitation during Meiyu seasons in the Yangtze River basin from 2014 to 2018 revealed that the precipitation related to MCSs accounts for 30%–90% of the total precipitation during Meiyu seasons (Cui et al., 2020). Particularly in the northeast of the Yangtze River basin, the precipitation caused by MCSs contributes up to 90% of the total. The heavy rainfall during the flood in the Yangtze River basin in 1998 was mostly caused by MCSs (Bei & Zhao, 2002; X. Zhang et al., 2002). The extraordinary rainstorm occurred in the Huai River basin from July 4 to 5, 2003 was caused by meso-α-scale MCSs (Sun et al., 2006). The occurrence of “extremely violent Meiyu” in the Yangtze River basin from June to July in 2020 is mainly attributed to the heavy rainfall (F. Zhang et al., 2020) and thus is influenced by the MCSs.

The organizational form of MCS varies greatly with different environmental conditions and regions (Carbone et al., 2002). The MCSs in China mainly occur in the middle and lower reaches of the Yangtze River basin, the Yellow River basin and the Southwest China (Ma et al., 1997; Z. Tao et al., 1998; X. Yang et al., 2015). As the most typical form of MCS, mesoscale convective complex (MCC) mainly occurs at night and is mainly in the downwind direction of large terrains (Laing & Fritsch, 2000). So far, there is little research on the formation of MCSs. This study focuses on the characteristics and causes of newly formed MCSs that lead to abnormal precipitation in the Yangtze River basin from June to July in 2020, which greatly helps predict the intensity and area of rainstorm during the Meiyu season.

The remainder of this paper is organized as follows. Section 2 introduces the data and methods. Section 3 investigates the characteristics of newly formed MCSs leading to abnormal precipitation in the Yangtze River basin from June to July in 2020. Section 4 discusses the reasons affecting the MCS formation. Finally, Section 5 provides the summary and discussion.

DATA AND METHODS Data

This study adopts three datasets as follows. The hourly T_BB of FY-2F is derived from the National Satellite Meteorological Center of China Meteorological Administration (CMA), with the spatiotemporal resolution of 0.1° × 0.1° and 1 h, the product range of 60°S–60°N and 52°E–172°E and the data length from June to July in 2020. The hourly European Centre for Medium-Range Weather Forecasts reanalysis 5 (ERA5) and the merging precipitation products with the spatial resolution of 0.25° × 0.25° are applied. The hourly precipitation data includes the hourly precipitation data from the Meteorological Information Combine Analysis and Process System (MICAPS) and the three-source merging precipitation products provided by the China Meteorological Administration with the spatial resolution of 0.1° × 0.1° and the data range of 70°E–140°E and 15°N–60°N.

T_BB-based threshold determination for MCS identification

MCS refers to a convective system composed of several convective cells or isolated convective systems and their derived stratiform cloud systems (Chen et al., 2020). Based on satellite data, the threshold method is the most commonly used to identify convective systems (Shou et al., 2016). There are different criteria of thresholds based on the cloud top T_BB at the long-wave infrared channel. Early studies mainly adopt the T_BB of < −32°C to identify convective systems (Laing & Fritsch, 1993a, 1993b; Maddox, 1980; Mapes & Houze, 1993; Miller & Fritsch, 1990; X. Zhang & Sun, 2018), which is also applied in convective system monitoring operations by the National Meteorological Center of CMA (Y.-G. Zheng et al., 2013). The T_BB within the range of −33°C to −23°C is another standard for identifying convective systems (Machado et al., 1998). Some study revealed that the strong convection with infrared brightness temperature lower than −52°C along the Meiyu frontal cloud system is an important weather system causing the heavy precipitation in the Huai River basin (Y. Zheng et al., 2007). Therefore, the T_BB of −52°C is selected as the threshold for identifying the MCSs during the Meiyu period in this study.

Standards for MCS classification

Based on the existing research studies (Anderson & Arritt, 1998; Augustine & Howard, 1988; Jirak et al., 2003; Y. Liu, Wang et al., 2021; Y. Liu, Yao, et al., 2021), MCSs are firstly classified into two types according to their shapes (quasi-circular, elongated), and further divided into three types according to the scales (meso-α, meso-β, and small meso-β scales).

As a result, the MCSs in this study can be divided into six types, including the meso-α quasi-circular convective complex (MCC), the meso-α elongated convective system (MECS), the meso-β circular convective system (M_βCC), the meso-β elongated convective system (M_βEC), the small meso-β circular convective system (SM_βCC), and the small meso-β elongated convective system (SM_βEC). The specific classification standards are shown in Table 1.

TABLE 1 MCS classification and standard

Category	Area (km2) (TBB ≤ −52°C)	Duration (h)	Eccentricity (at maximum area)
MECS	≥50,000	≥6	[0.2, 0.7)
MCC	Same as above	≥6	[0.7, 1]
MβEC	≥30,000 and the largest area ≥50,000 km2	≥3	[0.2, 0.7)
MβCC	Same as above	≥3	[0.7, 1]
SMβEC	≥30,000 km2 and the largest area <50,000 km2	≥3	[0.2, 0.7)
SMβCC	Same as above	≥3	[0.7, 1]

MCS identification

Among all identification methods for MCSs, the automatic identification has a strong advantage in long-term investigation on MCSs (Carvalho & Jones, 2001; Garcia-Herrera et al., 2005; Li et al., 2012), but there are some errors in identifying the development and variation of MCSs (Carvalho, 2002; Durkee & Mote, 2010).

The methods of determining newly formed MCSs in this paper were as follows. First, convective cloud was selected over the Yangtze River basin (24°N–35°N, 100°E–123°E) according to the area and eccentricity of cloud in Table 1. Then, the new cloud acts as the movement of the original cloud away from the center of the convective cloud 20–50 km. A newly formed MCSs were identified when it lasted more than 3 h. Finally, the newly formed MCSs are manually revised by using T_BB diagram.

In this paper, the formation moment of MCS refers to the time when the cold cloud area with the T_BB ≤ −52°C just reaches 30,000 km². The dissipation moment refers to the time when the cold cloud area with the T_BB ≤ −52°C is just less than 30,000 km². The lifetime of an MCS refers to the period from its formation to dissipation.

CHARACTERISTICS OF NEWLY FORMED MCSs CAUSING ABNORMAL PRECIPITATION IN THE YANGTZE RIVER BASIN FROM JUNE TO JULY IN 2020 The relationship between MCSs and short-duration heavy precipitation

Based on the hourly precipitation data from MICAPS, the T_BB values before and after 31,778 times of short-duration heavy precipitation (the hourly precipitation >20 mm h⁻¹) in the Yangtze River basin during June and July in 2020 are calculated (Figure 1). It is found that the T_BB value increases after the occurrence of short-duration heavy precipitation in different magnitudes, and the T_BB value is the smallest before the occurrence of 50–80 mm h⁻¹ short-duration heavy precipitation. When the precipitation intensity exceeds 80 mm h⁻¹, the T_BB value is 6–10°C higher than those of other magnitudes, with the median value of −26.6°C. For short-duration heavy precipitation in other magnitudes, the median T_BB values are −34.1°C (20–30 mm h⁻¹), −35.2°C (30–50 mm h⁻¹), and − 37.2°C (50–80 mm h⁻¹), respectively.

FIGURE 1. (a) TBB distribution before and after the occurrence of short-duration heavy precipitation in the Yangtze River basin during June–July 2020; (b) distance between short-duration heavy precipitation station and MCS (box lines are marked with 25%, 50%, and 75% quantiles, respectively)

For the rainfall event in the Yangtze River basin from June to July in 2020, satellite images demonstrate that the occurrence of short-duration heavy precipitation in areas where the minimum distance (from the stations to the area of T_BB ≤ −52°C of the MCSs) between the MCSs and the stations (minDist) with hourly precipitation intensity over 20 mm h⁻¹ is less than 100 km is closely related to MCSs, while the short-duration heavy precipitation in areas with the minDist more than 150 km has little relationship with the MCSs. The stations with short-duration heavy precipitation within the range of MCSs (with the minDist less than 10 km) account for 24.8% of the total stations, while the stations with short-duration heavy precipitation at the edge of MCSs (with the minDist between 10 and 100 km) account for 44.5% of the total stations. Therefore, 69.3% of short-duration heavy precipitation is closely related to the MCSs over the Yangtze River basin from June to July in 2020. Table 2 reveals that for minDist of less than 150 km, the closer the stations are to the MCSs, the larger the average short-duration heavy precipitation is 77.6% of the short-duration heavy precipitation is attributed to the MCSs in the Yangtze River basin from June to July in 2020. Moreover, the closer the distance to the MCSs is, the larger the average short-duration heavy precipitation is. The following section will focus on the characteristics of the newly formed MCSs causing large-scale short-duration heavy precipitation.

TABLE 2 Proportion of short-duration heavy precipitation stations from MCSs at different distances in all short-duration heavy precipitation stations

Distance (km)	≤10	10–50	50–100	100–150	150+
Proportion of short-duration heavy precipitation stations (%)	24.8	26.1	18.4	8.3	22.4

Relationships between newly formed MCSs and short-duration heavy precipitation

By comparing the daily number of stations with short-duration heavy precipitation and the daily frequency of newly formed MCSs over the Yangtze River basin from June to July in 2020 (Figure 2), it is found that the six concentration periods (June 12–17, June 20–23, June 26–30, July 5–8, July 15–18, and July 24–26) when the MCSs are newly formed have a good correspondence with the six heavy precipitation processes during the Meiyu season in 2020, except the period with short-duration heavy precipitation around July 2. As the number of newly formed MCSs increases, the average number of stations with short-duration heavy precipitation also increases (Table 3).

FIGURE 2. Daily frequency of newly formed MCSs (histogram) and station number (red line) of short-duration heavy precipitation events (20 mm h−1) in the Yangtze River basin during June–July 2020 (unit: station)

TABLE 3 Daily frequency of newly formed MCS and the daily average number stations of short-duration heavy precipitation

Number of daily newly formed MCS (each)	0	1	2	3	4
The daily average number stations of short-duration heavy precipitation (each)	22	50	65	72	99

Frequency, spatial distribution, and diurnal variation of newly formed MCSs

In the Yangtze River basin from June to July in 2020, there are 86 MCSs (as listed in Table 4) that meet the standards of Table 1, with an average lifetime of 6 h and most of the MCSs lasting 3–7 h. There are 20 MCSs with a lifetime of 3 h, which account for the most. The longest life time is 19 h. In terms of the scale, there are 46 meso-β-scale MCSs (53.5%), 24 small meso-β-scale MCSs (28%), and 16 meso-α-scale MCSs (less than 20%). In terms of the shape, there are 59 elongated MCSs (MECS, M_βEC, SM_βEC), accounting for about 70% of the total, while there are 27 quasi-circular MCSs (MCC, M_βCC, SM_βCC), which account for less than 1/3 of the total MCSs.

TABLE 4 The frequency of newly formed MCS in the Yangtze River basin during June–July 2020

[Table omitted. See PDF]

Spatial distributions elucidate that the newly formed MCSs over the Yangtze River basin from June to July in 2020 are mainly located in Sichuan, Guizhou and the middle and lower reaches of the Yangtze River (24°N–35°N, 110°E–123°E), with large-value centers of precipitation in their surroundings (Figure 3a).

FIGURE 3. (a) Total precipitation (shadow, unit: mm) and newly formed MCS location in the Yangtze River basin; (b) daily variation distribution of newly formed MCS frequency in the Yangtze River basin (colors indicate different types of MCS) (unit: Number) during June–July 2020

The frequencies of newly formed MCSs over the Yangtze River basin from June to July in 2020 present obvious diurnal variations (Figure 3b). The MCSs are mainly formed from the early morning to the morning (00:00–09:00) and from the afternoon to the first half of the night (13:00–22:00). About 70% of the total MCSs are formed during from 13:00 to 22:00. Particularly, MCSs are mostly formed at 20:00, and the formation of MCSs at all scales can be found at that time. Moreover, 1/3 of the MECSs are formed at 20:00, 60% of the newly formed M_βECs are mostly concentrated at 14:00–20:00, and 2/3 of the SM_βECs are formed during 18:00–22:00.

ENVIRONMENTAL CONDITIONS AFFECTING MCS FORMATION Influences of circulation situations on MCS formation

Considering the environmental conditions for the formation of MCSs (Figure 4), the Meiyu front provided favorable conditions for the convergence and uplift. As the 850 hPa jet stream intensified during the Meiyu season, the convergence and uplift also enhanced. In front of the 500 hPa trough and the southern part of 850 hPa shear line, that is, the areas where the cold and warm air flows converged were prone to produce MCSs. The distance between the MCSs and 850 hPa shear line depended on the magnitude of 500 hPa southerly wind. It is proved that the background field plays an important role in the formation of sub-synoptic scale weather systems (J. Sun et al., 2018).

FIGURE 4. (a) 500 hPa height field (blue isolines), 850 hPa wind field, 850 hPa shear line brown solid line, and the newly formed MCS (left) on June 19–30; (b) 500 hPa meridional wind (shadow), 850 hPa wind field, 850 hPa shear line, and the newly formed MCS (right) on July 5–8 (wind field: wind-direction shaft, shear line: brown solid line, newly formed MCS: magenta dots)

Influences of water vapor and low-level jet on MCS formation

The persistent heavy rainfall largely depends on the water vapor transport (J. Sun et al., 2016). The divergence of the whole-layer integrated water vapor flux (between the surface and 100 hPa), respectively, averaged in the two periods are shown in Figure 5. According to statistics, there was a great difference in the number of newly formed MCSs at 02:00 and 20:00 in the west of 107°E. It was found that the newly formed MCSs were mainly located at the position where the vorticity advection on 500 hPa and the whole-layer integrated water vapor flux were significantly stronger at 20:00 than at 02:00 both on June 19–30 and July 5–8 in the west of 107°E.

FIGURE 5. Divergence of the vertically integrated water vapor flux (unit: 10−4 kg m−2 s−1, shadow), 500 hPa vorticity advection (unit: 10−9 s−2, isolines), and newly formed MCSs (blue dots)  June 19–30 (a) July 5–8 (b) 02:00 (above); 20:00 (below)

Figure 6 reveals that there was a good correspondence in the diurnal variations between the frequencies of newly formed MCSs and the intensities of 850 hPa southwest jet. The weakest time of 850 hPa southwest jet was 17:00–20:00 in a day, with the most newly formed MCSs at 18:00–22:00. With the enhancement of 850 hPa southwest jet, the number of newly formed MCSs decreased. The jet reached the strongest at 08:00 in the morning, without newly formed MCSs at 10:00–11:00. Therefore, the weakening of low-level jet was more conducive to the regeneration of MCSs. There were no newly formed MCSs in 2–3 h after the strongest low-level jet.

FIGURE 6. June 19–30 (a), July 5–8 (b) at 02:00, 11:00, 20:00, 850 hPa wind field (wind shaft) and wind speed (shadow) (from top to bottom) (unit: m s−1)

SUMMARY AND DISCUSSION

The spatiotemporal distributions and evolution characteristics of the newly formed MCSs resulting in the abnormal precipitation in the Yangtze River basin from June to July in 2020 were analyzed. The main conclusions are as follows.

The increase of newly formed MCSs leaded to more short-duration heavy precipitation. There are the 69.3% of short-duration heavy precipitation closely related to the MCSs.

The average lifetime of MCSs over the Yangtze River basin from June to July 2020 is 6 h. The MCSs are mostly elongated shaped (68.6%), while the meso-β-scale MCSs account for the most (53.5%). The newly formed MCSs have obvious diurnal variations, with the least from 10:00 to 11:00 and the most at 20:00. The weakening of low-level jet corresponded to more newborn MCSs. And there were more newly formed MCSs at the significantly stronger vorticity advection on 500 hPa and the whole-layer integrated water vapor flux in the west of 107°E.

The newly formed MCSs over the middle and lower reaches of the Yangtze River are mainly located in the south of 850 hPa shear line, and their distances with the shear line depend on the magnitude of 500 hPa southerly wind.

AUTHOR CONTRIBUTIONS

Xiuping Yao: Conceptualization (equal); methodology (equal); resources (equal); writing—review and editing (equal). Honghua Zhang: Formal analysis (equal); writing—original draft (equal). Jiali Ma: Investigation. Dawei Shi: Data curation; validation. Weijian Wang: Visualization. Guichen Wang: Investigation; validation.

ACKNOWLEDGEMENTS

This work was Supported by the Open Grants of the State Key Laboratory of Severe Weather (2021LASW-B17), Shanghai Typhoon Research Foundation (TFJJ202006), the Key Program of the National Science Foundation of China (42030611).