Comparison between the Roles of Low-Level Jets in Two Heavy Rainfall Events over South China

Xinyu ZHOU, Zhengquan CHENG, Haowen LI, and Dongming HU

1 Guangzhou Meteorological Observatory, Guangzhou 511430

2 Guangdong Meteorological Observatory, Guangzhou 510080

ABSTRACT Two heavy rainfall events occurred over the Pearl River Delta during 20–22 May 2020: the first was a warm-sector event and the second a frontal event. Based on ERA5 reanalysis data and observations from wind profilers and Doppler weather radars, the structures and roles of low-level jets (LLJs) during these two heavy rainfall events were analyzed. The results show that: (1) South China was affected by a low-level vortex and a low-level shear line during the two processes. The two heavy rainfall events were both associated with a synoptic-system-related low-level jet(SLLJ) and a boundary layer jet (BLJ). The coupling of the convergence at the exit of the BLJ and the divergence at the entrance of the SLLJ produced strong lifting for the warm-sector heavy rainfall, and the strong convergence between the LLJs and northerly winds as the cold front moved southwards was the main lifting reason for the frontal heavy rainfall. (2) The BLJ was the main transport of water vapor during the two processes. The coupling of the BLJ and SLLJ caused the water vapor convergence to be concentrated in the boundary layer during the first process,whereas the strong convergence between the LLJs and northerly winds led to the lower and middle troposphere having strong water vapor convergence during the second process. (3) During the period of these two heavy rainfall events, the lower and middle troposphere remained unstable. Further analysis show that the differences in the intensity, location, and direction between the BLJ and SLLJ resulted in the pseudo-equivalent potential temperature advection in the boundary layer being significantly larger than in the lower and middle troposphere, which compensated for the energy loss caused by heavy rainfall and maintained the convective instability. These findings add to our knowledge on the roles of LLJs in the pre-summer rainfall over South China.

Key words: pre-summer heavy rainfall, South China, low-level jets, water vapor, convective instability

1. Introduction

During the pre-summer rainy season (April–June) over South China, the southward-moving cold front weakens,and the southerly monsoon strengthens, heavy rainfall often occurs near the cold front (hereafter referred to as frontal heavy rainfall) or in the warm sector 200–300 km ahead of the front (hereafter referred to as warm-sector heavy rainfall) (Huang, 1986; Ding, 1994; Luo et al.,2017). To improve the level of scientific understanding and operational forecasting of rainfall during the presummer rainy season over South China, four field campaigns were conducted in China: the first was during 1977–1981 (Huang, 1986), the second was the 1998“Heavy Rainfall Experiment in South China” (Zhou et al., 2003), the third was the 2008–2009 “South China Heavy Rainfall Experiment” (Zhang et al., 2011; Ni et al., 2013), and the fourth was the “South China Monsoon Rainfall Experiment” (Luo et al., 2017; Luo et al.,2022). These campaigns yielded fruitful results on the interaction mechanisms of the multi-scale systems of rainstorms (e.g., mesoscale systems and their interaction with synoptic-scale systems, ocean–land contrast, and mesoscale cyclones/shear lines), the development of new detection techniques and data applications, and high-resolution numerical simulation techniques.

Previous studies have suggested that pre-summer heavy rainfall over South China is closely related to the low-level jet (LLJ). On the one hand, it transports warm and moist air to South China, providing favorable water vapor and thermodynamic conditions for heavy rainfall(Zhu, 1975). On the other hand, the convergence and vertical shear formed by the LLJ provide favorable dynamical lifting conditions for heavy rainfall. The LLJs over South China can be classified into two types (Chen et al.,1994; Du et al., 2012; Liu et al., 2014; Zhang et al.,2019). The first type is the synoptic-system-related LLJ(SLLJ), for which the maximum wind speed occurs in the lower and middle troposphere (900–600 hPa) in association with synoptic-scale systems (Uccellini et al.,1979; Zhao, 2012). The second type is the boundary layer jet (BLJ), which occurs in the boundary layer and has obvious vertical shear and a diurnal cycle. The BLJ is influenced by both boundary layer processes and large-scale circulation or weather systems. Inertial oscillation and thermal contrast near the terrain or coastal areas are two main popular theories to explain its occurrence (Blackadar, 1957; Holton, 1967; Shapiro et al., 2016).

Previous studies have documented that both SLLJs and BLJs are closely related to pre-summer rainfall over South China. The transport of warm and moist air by low-level southwesterly jets to South China is a key factor influencing pre-summer rainfall (Huang et al.,2018), and BLJs exist over coast areas of South China or the northern South China Sea (SCS) in more than 80% of warm-sector heavy rainfall events (Wu et al., 2020).SLLJs and BLJs make different contributions to determining the generation of inland and coastal rain belts (Liu et al., 2020). Warm-sector coastal rainfall is closely related to the deceleration of the southerly boundary layer flow over the northern SCS and the associated convergence of boundary layer air with high pseudo-equivalent potential temperature (θse) near the coast (Li et al., 2020).Meanwhile, the convergence in the boundary layer and the divergence in the lower and middle troposphere caused by the coupling of the SLLJ and BLJ is an important trigger mechanism for coastal warm-sector heavy rainfall over South China (Du and Chen, 2018, 2019).However, there have still been relatively few comparative studies on the role of double LLJs (i.e., when both the SLLJ and BLJ exist) in warm-sector heavy rainfall and frontal heavy rainfall during the pre-summer rainy season over South China. Besides, the interaction between the SLLJ and BLJ and the difference between their effects on these two types of heavy rainfall are not well understood.

Two heavy rainfall events occurred over the Pearl River Delta (PRD) of Guangdong on 20–22 May 2020.The roles of the SLLJ and BLJ during these two heavy rainfall events are discussed in this study, with a view to providing some meaningful references for improving the level of scientific understanding and operational forecasting of pre-summer rainfall over South China.

2. Data and methods

In this study, ERA5 reanalysis data (spatial resolution:0.25° × 0.25°; temporal resolution: 1 h), wind profiler observations at Nansha and Conghua (sounding altitude:6 km, temporal resolution: 5 min, and spatial resolution:60 m), and Doppler weather radar observations at Guangdong are utilized.

Referring to previous studies (Whiteman et al., 1997;Du et al., 2012), the following criteria are used for identifying the LLJs in this study: (1) the maximum wind speed is more than 12 m s-1; and (2) the wind speed must decrease by at least 3 m s-1from the height of the maximum wind to the minimum wind below 600 hPa. In this study, the BLJs are identified as the LLJs occurring below 900 hPa, and the SLLJs are synoptic-system-related LLJs occurring between 900 and 600 hPa.

3. Analysis of precipitation characteristics and atmospheric conditions

3.1 Precipitation characteristics

The first heavy rainfall event occurred from 2000 BT(Beijing Time) 20 to 0300 BT 21 May 2020, with maximum hourly precipitation of 99.5 mm and maximum daily accumulated precipitation of 260.4 mm (Longgui station; Fig. 1a). During this period, the rain band affected the PRD from north to south, and the intensity and extent of the heavy rainfall increased significantly at 0000 BT 21 May (Figs. 2a–d). The second heavy rainfall event occurred from 2200 BT 21 to 0500 BT 22 May,with maximum hourly precipitation of 167.8 mm(Huangpu Bridge station) and maximum daily accumulated precipitation of 402 mm (Gaobu station; Fig. 1b),and the rainfall increased significantly at 2300 BT 21 May (Figs. 2e–j). Guangzhou was the center of the two heavy rainfall events, during which the average precipitation of the city’s 362 rainfall stations was 58.4 and 118.4 mm, respectively. Rainfall observation stations with daily accumulated precipitation exceeding 50 mm accounted for 58% and 83%, respectively (Fig. 1c).

Fig. 1. Accumulated precipitation (a) from 0800 BT 20 to 0800 BT 21 May 2020 and (b) from 0800 BT 21 to 0800 BT 22 May 2020. The black dotted curve indicates Guangzhou, yellow stars indicate Longgui and Gaobu stations, green square indicates Huangpu Bridge station, and the red square represents Qingyuan sounding station in Fig. 4. The black and orange circles represent Conghua and Nansha wind profiler stations used in Fig. 8. The orange box indicates the PRD and the black box is used to define the heavy precipitation area in Fig. 9. (c) Hourly average precipitation (mm) of Guangzhou (black bars) and hourly precipitation of Huangpu Bridge station (red solid line) from 0800 BT 20 to 0800 BT 22 May 2020.

3.2 Analysis of atmospheric conditions

The two heavy rainfall events coincided with the onset of the SCS summer monsoon (the monsoon onset was on 16–20 May), and South China was affected by a lowlevel vortex and a low-level shear line. During the first heavy rainfall event, the low-level vortex at 850 hPa was located near the border between Guangxi and Guizhou provinces, and the shear line with northeast–southwest orientation was about 400 km away from the heavy rainfall center (Fig. 3a). The PRD was controlled by the southwesterly monsoon located south of the shear line,and the LLJ transported abundant warm and moist air to the PRD continuously. From surface analysis (Fig. 3c),South China was affected by a surface low, a synopticscale cold front located north of 25°N, which was more than 300 km away from the area of heavy precipitation.After that, the low-level vortex and shear line moved gradually southeastwards. During the nighttime of 21 May when the second heavy rainfall event occurred (Fig.3b), the low-level vortex and shear line moved to the north of central Guangdong, and the northeasterly and southwesterly winds on both sides of the shear line formed a convergence zone over the PRD. Meanwhile,the cold front moved southeastwards to the north of central Guangdong, which provided favorable conditions for the heavy rainfall (Fig. 3d).

Fig. 2. Radar composite reflectivity (dBZ) derived from Doppler weather radars in Guangdong Province at (a) 2200 BT 20, (b) 2300 BT 20, (c)0000 BT 21, (d) 0100 BT 21, (e) 2200 BT 21, (f) 2300 BT 21, (g) 0000 BT 22, (h) 0100 BT 22, (i) 0200 BT 22, and (j) 0300 BT 22 May 2020.

Fig. 3. (a, b) Distributions of wind (barbed arrows; m s-1; red above 12 m s-1), pseudo-equivalent potential temperature (shaded; K) at 850 hPa,and geopotential height (contours; gpm) at 500 hPa. (c, d) Distributions of wind (barbed arrows; m s-1), pressure (contours; hPa), and variation of 24-h SLP (shaded; hPa) at sea level, and (e, f) distributions of wind (barbed arrows; m s-1), geopotential height (contours; gpm), and divergence(shaded; 10-5 s-1) at 100 hPa. The green solid line is the wind shear line at 850 hPa, the red solid line is the trough line at 500 hPa, the blue line is the cold front, the black “L” indicates low pressure, the blue “H” indicates high pressure, the blue contour is the 16700-gpm line at 100 hPa, and Guangzhou is indicated by the green solid curve.

During the two rainfall processes, shortwave troughs at 500 hPa continued to affect South China (Figs. 3a, b),and the positive vorticity advection in front of the troughs provided favorable quasi-geostrophic lifting for the heavy precipitation. The 100-hPa South Asian high was located over the Indochina Peninsula, and the divergence zone on its east side was favorable for heavy precipitation over the PRD (Figs. 3e, f).

As can be seen from the skewT–logpdiagram at Qingyuan sounding station (Fig. 4), which is close to the center of the two events (red square in Fig. 1b), the atmosphere was very moist and nearly saturated before the onset of the two heavy precipitation events. The height of the lifting condensation level and the level of free convection were low (about 1000 hPa), which was favorable for the generation of convection even without strong dynamic lifting. The convective available potential energy(CAPE) andK-index were 3018 J kg-1, 38°C and 2371 J kg-1, 44°C, respectively, with no convective inhibition energy, indicating that the PRD already had favorable thermodynamic conditions before the heavy rainfall events. Compared with the conditions in early May 2020 before the monsoon onset (not shown), the thermodynamic conditions were much more favorable, suggesting that the favorable conditions mainly resulted from the onset of the SCS summer monsoon.

Based on the analysis above, the first rainfall event was a warm-sector heavy rainfall event that occurred south of the shear line, whereas the second was a frontal heavy rainfall event caused by the cold front and shear line. Before the two rainfall events occurred, favorable water vapor and thermodynamic conditions were already in place over the PRD.

4. Horizontal and vertical structures of LLJs

4.1 Horizontal structures of LLJs

Figure 5 shows the evolution of the horizontal distribution of winds at different levels during the two rainfall events. There were two strong wind zones at 950 hPa—one located near the Beibu Gulf (around 21°N,109°E) and the other over the northern SCS. The latter strong wind zone was characterized by a strong southwesterly wind, with speed increasing before the two events. At 1400 BT 20 May 2020 (Fig. 5a), the wind core was located south of 20°N with a maximum speed of more than 12 m s-1, while the wind speed over the coastal region of the Pearl River Estuary was around 4 m s-1. After that, the strong wind zone strengthened and extended to the northeast, forming a marine BLJ to the land. Around 0000 BT 21 May 2020 when the first heavy rainfall occurred (Fig. 5b), the BLJ exceeded 16 m s-1and the wind speed over the PRD and coastal region also exceeded 12 m s-1. Meanwhile, the shear line at 950 hPa was located in the area from northern Guangxi to Hunan and Jiangxi. Thereafter, the shear line continued to move southwards and the BLJ core moved eastwards to the northeast SCS, and the wind speed over the PRD and coastal region decreased to below 8 m s-1(Fig. 5c).Around 2300 BT 21 May 2020 when the second heavy rainfall occurred, the shear line moved southwards to the north of central Guangdong, the wind speed over the northern SCS increased again, and the wind speed over the Pearl River Estuary exceeded 12 m s-1(Fig. 5d).

At the 850-hPa level, the strong southwesterly winds were situated to the south of the northeast–southwest-oriented low-level shear line, and moved southwards along with the southward-moving cold front. During the daytime of 20 May (Fig. 5e), the wind core was located around Guangxi, and the wind speed over the PRD and coastal region was below 10 m s-1. Thereafter, the wind strengthened and extended eastwards, forming two strong wind zones at 0000 BT 21 May 2020 (Fig. 5f)—one located from Guangxi to the Pearl River Estuary, and the other from northern Guangdong to southern Jiangxi. The wind speed of both strong wind cores exceeded 14 m s-1,and the heavy precipitation area was located between the two wind cores. With the southward-moving cold front,the strong wind area moved eastwards to the northeast SCS (Fig. 5g). Similar to that at 950 hPa, the 850-hPa shear line moved southwards to north–central Guangdong at around 2300 BT 21 May 2020 when the second heavy rainfall occurred (Fig. 5h).

Fig. 4. Skew T–logp diagrams at Qingyuan sounding station at (a) 2000 BT 20 May 2020 and (b) 2000 BT 21 May 2020. The values of CAPE(convective available potential energy), K (K-index), Q850 (specific humidity at 850 hPa), LCL (lifting condensation level), and LFC (level of free convection) are given.

Fig. 5. Distributions of wind speed (shaded; m s-1) and wind vectors at (a–d) 950 hPa, (e–h) 850 hPa, and (i–l) 700 hPa at (a, e, i) 1400 BT 20,(b, f, j) 0000 BT 21, (c, g, k) 1400 BT 21, and (d, h, l) 2300 BT 21 May 2020. The red solid line indicates the wind shear line at 950–850 hPa.The blue solid line indicates the trough line at 700 hPa.

A shortwave trough at 700 hPa moved southeast to affect South China during the two rainfall processes, with strong southwesterly wind in front of the trough. When the first heavy rainfall event occurred, the strong wind area was located east of the heavy precipitation area, with the wind speed exceeding 16 m s-1(Fig. 5j). During the daytime of 21 May, the strong wind area moved southwards to the northeast SCS, and the wind speed over the PRD weakened significantly (Fig. 5k). As the shear line moved southwards to the area of Hunan–Jiangxi, the westerly wind located south of the cold front strengthened again, with the maximum wind speed reaching up to 14 m s-1at 2300 BT 21 May 2020 (Fig.5l). The analysis above implies that the strong southwesterly wind at 850–700 hPa was related to the cold front,which belonged to a kind of SLLJ.

Usually, the entrance and exit of the LLJs are related to the horizontal convergence and divergence, respectively. In order to better illustrate the entrance and exit of the LLJs at different levels in this case, Fig. 6 shows the evolution of the horizontal wind and divergence/convergence at different levels, where the black contours represent the wind speed zones of 12 and 14 m s-1, and the red star indicates the center of heavy precipitation. It can be seen that the center of heavy precipitation during the first rainfall event was located in the exit of the BLJ at 950 hPa (Fig. 6a), where there was a convergence zone.Meanwhile, there was mesoscale divergence at 850–700 hPa at the entrance of the SLLJs (Figs. 6b, c). During the second rainfall event, the southward-moving shear line produced a strong convergence zone at 950–700 hPa over the heavy precipitation area (Figs. 6d–f), which benefited deep lifting for the second heavy precipitation.

Fig. 6. Distributions of wind (barbed arrows; m s-1) and divergence (shaded; 10-5 s-1) at (a, d) 950 hPa, (b, e) 850 hPa, and (c, f) 700 hPa, at(a–c) 0000 and (d–f) 2300 BT 21 May 2020. The red contours indicate the wind speeds of 12 and 14 m s-1. The red star indicates the center of the heavy rainfall at the corresponding time.

Based on the analysis above, the coupling of the convergence at the exit of the BLJ and the divergence at the entrance of the SLLJ may have provided favorable dynamic conditions for the first heavy rainfall event, whereas the strong convergence in the lower and middle troposphere related to the southward-moving shear line was the main lifting reason during the second heavy rainfall event.

4.2 Vertical structures of LLJs

To further examine the vertical structures of the BLJ and SLLJ and their relationship with convection, Fig. 7 presents vertical cross-sections of the meridional wind speed, divergence/convergence, and radar reflectivity through the heavy rainfall center (red star in Fig. 6) at 0000 and 2300 BT 21 May 2020, when the rainfall enhanced significantly. When the first heavy rainfall occurred, the cold air was mainly located north of 26.5°N and below 950 hPa, with southerly wind to the south of 26°N below 500 hPa (Fig. 7a). The southerly wind showed two strong wind zones at different levels in the south and north, respectively. The southern one extended from the ocean to near 24°N as the BLJ strengthened,the wind core over the ocean (south of 21°N) was below 900 hPa and rose to 900–850 hPa near the coast (around 21°N). It formed two convergence zones over Guangdong—one was over the Pearl River Estuary near 22°N,with the convergence center at 850–700 hPa and a weak divergence center at 650 hPa; and the other was at the exit of the BLJ over the PRD, with the convergence center at 900 hPa. The northern strong wind zone was mainly located north of 24°N at 900–700 hPa, which was associated with the SLLJ. At this time, the BLJ’s exit was well collocated with the SLLJ’s entrance over the PRD, and there was strong divergence at 850–700 hPa. It can be seen from the radar reflectivity that obvious radar echoes appeared near the convergence/divergence areas above. The collocation of the convergence in the boundary layer and divergence at 850–700 hPa led to the radar echoes being much stronger over the PRD, with the 30-dBZ contour extending from 950 to 500 hPa and the strong echo center being located near 700 hPa with intensity exceeding 50 dBZ.

At around 2300 BT 21 May 2020 when the second heavy rainfall occurred, the cold front moved southwards to near 24°N, forming strong convergence below 700 hPa over the PRD, with the convergence center located at 850–800 hPa and the intensity exceeding -20 ×10-5s-1(Fig. 7b). The strong convergence between southerly and northerly winds provided a clear boundary for the heavy precipitation, and the strong radar echoes were concentrated near the area of heavy precipitation and more widespread from north to south compared to the first heavy rainfall event. Echoes exceeding 30 dBZ extended from 900 hPa to above 500 hPa and the strong echo center was located near 700 hPa with intensity exceeding 50 dBZ.

The Nansha wind profiler (22.71°N, 113.55°E) to the south side of the heavy precipitation area, and the Conghua wind profiler (23.57°N, 113.6°E) to the north side of the heavy precipitation area, were chosen to further analyze the vertical structures of the wind. To reduce the influence of precipitation on the quality of wind profiler observations, the method of Zhou and Liao(2015) was used to quality-control the wind profiler observations. According to the Qingyuan soundings at 2000 BT 20 and 2000 BT 21 May 2020 (Fig. 4), the 850- and 700-hPa levels during the two processes were around 1.5 km and 3.1 km, respectively.

Fig. 7. Vertical cross-sections of meridional wind speed (shaded; m s-1), divergence (black dotted line; 10-5 s-1), radar reflectivity (blue solid line; dBZ), and pseudo-equivalent potential temperature (green solid line; K) along a north–south cross section through the red star in Fig. 6 at (a)0000 BT and (b) 2300 BT 21 May 2020 (red star indicates the center of heavy rainfall).

Figure 8a shows that before the two heavy rainfall events occurred, the southwesterly/southerly winds below 4 km at Nansha station both strengthened and were both shortwave troughs at around 3 km. The wind speed below 1 km weakened significantly and the wind direction above 1 km turned westerly during the daytime of 21 May. As shown in Fig. 8b, the southwesterly/southerly winds below 3 km at Conghua station also increased significantly before the first rainfall event occurred and the wind speed below 1 km also weakened during the daytime of 21 May, which was similar to the situation at Nansha station. While the second heavy rainfall occurred during the nighttime of 21 May, the wind below 1 km continued to decrease and gradually turned to a northeasterly wind, indicating that Conghua station had been affected by cold air.

As can be seen from Fig. 8c, Nansha and Conghua stations both recorded southerly winds below 4 km when the first heavy rainfall event occurred. There was an LLJ core at Nansha station at 0.8–1.5 km, while the meridional wind speed of Conghua station gradually increased with height below 3 km, and the wind core was at the height of 2.5–3.5 km. The meridional wind speed at Nansha station was larger than that at Conghua station below 1.5 km, indicating meridional wind convergence in the boundary layer. Also, the meridional wind speed at Conghua station was larger than that at Nansha station above 1.5 km, indicating meridional wind divergence. When the second heavy rainfall occurred (Fig.8d), there was still southerly wind below 4 km at Nansha station, while the wind direction below 1.5 km turned northerly at Conghua station. This indicates that the cold air had moved to the south of Conghua station and formed a strong convergence zone with the BLJ over the PRD. As can be seen, the wind profiler observations were in good agreement with the ERA5 reanalysis data(Figs. 5, 7).

Fig. 8. Time–height cross-sections at (a) Nansha and (b) Conghua wind profiler station from 0800 BT 20 to 0800 BT 22 May 2020. Comparisons of meridional wind speed (m s-1) at Nansha (black line) and Conghua (red line) wind profiler station at (c) 0000 BT 21 and (d) 2300 BT 21 May 2020.

The analysis above shows that the first rainfall event was a warm-sector heavy rainfall event that occurred to the south of the shear line, and the coupling of the convergence at the exit of the BLJ and the divergence at the entrance of the SLLJ provided favorable dynamic conditions for this first event. The second rainfall event, meanwhile, was a frontal heavy rainfall, and the southwardmoving shear line formed a strong convergence zone between the LLJs and the cold air, which provided favorable lifting conditions for heavy precipitation.

5. Analysis of the relationship between LLJs and both water vapor and convective instability conditions

5.1 LLJs and water vapor conditions

As shown in Figs. 9a, b, the strong water vapor flux during the two heavy rainfall events was concentrated below 600 hPa. The core of the water vapor flux was located near 850-hPa level, and the water vapor flux below 700 hPa in both cases strengthened before the rainfall significantly enhanced. The convergence zone of water vapor was also mainly concentrated below 700 hPa during both processes, and the convergence also strengthened before the rainfall significantly enhanced. It can be seen from the comparison of the two processes that the convergence center of water vapor during the first rainfall event was located in the boundary layer below 900 hPa, while the convergence intensity and extent both increased during the second rainfall. It is worth noting that the water vapor flux and water vapor flux divergence near 850 hPa also strengthened during the daytime of 21 May, but no significant precipitation occurred over the PRD. This suggests that the favorable water vapor conditions for the two events might have come from other levels.

To further analyze the characteristics of water vapor transport and convergence during the two processes,Figs. 9c and 9d present vertical cross-sections of the water vapor flux and water vapor flux divergence through the heavy rainfall center (red star in Fig. 6) at 0000 and 2300 BT 21 May 2020. Combined with Fig. 5, it suggests that the strong water vapor during both processes was transported from the ocean to the PRD by the BLJ.During the first heavy rainfall, the wind convergence at the exit of the BLJ led to the vapor convergence zone being mainly concentrated in the boundary layer below 900 hPa, which provided favorable water vapor conditions for heavy precipitation. Also, the wind divergence at the entrance of the SLLJ resulted in the water vapor divergence at the 850–700-hPa levels. During the second heavy rainfall event, the southward-moving dry and cold air caused the water vapor transported in the boundary layer to converge strongly over the PRD, with the convergence zone extending to around 650 hPa.

Fig. 9. Time–height cross-section of area-mean (22.5°–24.5°N, 112.5°–114.5°E; black box in Fig. 1b). (a) Water vapor flux (g cm-1 hPa-1 s-1)and (b) water vapor flux divergence (10-6 g cm-2 hPa-1 s-1); and vertical cross sections of water vapor flux (shaded; g cm-1 hPa-1 s-1) and water vapor flux divergence (black dotted line; 10-6 g cm-2 hPa-1 s-1) along a north–south cross-section through the red star in Fig. 6 at (c) 0000 BT 21 and (d) 2300 BT 21 May 2020. The red star indicates the center of heavy rainfall.

The analysis above implies that the transport and convergence of water vapor were closely related to the LLJs.The strengthening of the BLJ transported abundant water vapor for the two rainfall events. The coupling of the BLJ’s exit and the SLLJ’s entrance resulted in water vapor convergence being mainly concentrated in the boundary layer during the first process, whereas the strong convergence between the LLJs and northerly winds led to the lower and middle troposphere over the PRD having strong vapor convergence during the second process.

5.2 LLJs and convective instability conditions

An unstable atmosphere is an important condition for heavy rainfall. The variableθse, which characterizes the temperature, humidity, and air pressure features, can indicate the stability of the atmosphere. When variableθsedecreases with height, the atmosphere is unstable. Figure 10a shows that the atmosphere was unstable below 600 hPa over the PRD before the first heavy rainfall occurred, which provided favorable thermodynamic conditions for heavy precipitation. At around 0200 BT 21 May 2020 when the first rainfall had its strongest precipitation (Fig. 10b), the instability in the boundary layer to the south of 22°N increased significantly as the BLJ strengthened. Meanwhile, the atmosphere above 900 hPa over the PRD maintained unstable, while the convective instability in the boundary layer decreased. At around 0200 BT 22 May 2020 when the second rainfall had its strongest precipitation (Fig. 10c), the BLJ’s restrengthening resulted in sustained high convective instability within the boundary layer to the south of 22°N. The atmosphere above (below) 900 hPa was still unstable (stable)over the PRD, which was similar to the case in the first heavy rainfall.

Fig. 10. Vertical cros-sections of the area-mean (112.5°–114.5°E) lapse rate of θse ( 10-4 K Pa-1) at (a) 1200 BT 20, (b) 0200 BT 21, and (c)0200 BT 22 May 2020. The red star indicates the center of heavy rainfall.

Usually, when heavy precipitation occurs, the atmosphere will gradually become stable. This is because the temperature in the boundary layer will decrease due to evaporative cooling of raindrops, and the temperature of the middle and upper atmosphere will increase due to the release of latent heat from water vapor condensation.During these two processes, the convective instability in the boundary layer decreased over the PRD due to the evaporative cooling effect of the heavy precipitation, but the atmosphere above 900 hPa was still unstable, so there must have been some mechanism that maintained the convective instability of the atmosphere. As can be seen from Figs. 11a, b, the BLJ transported strong positive advection ofθseto the heavy precipitation area in the boundary layer during the two rainfall events. In contrast,the intensity, location, and direction of the SLLJ at the 700-hPa level and above resulted in weaker advection ofθse(not shown). The difference inθseadvection between the upper and lower atmosphere resulted in theθsedecreasing with height over the PRD, which may have been the reason for the maintenance of convective instability in the lower and middle troposphere above 900 hPa.

To better illustrate that the difference inθseadvection between the upper and lower atmosphere was closely related to the maintenance of convective instability over the heavy rainfall area, the local variation equation of convective instability (Zhu et al., 2000) is analyzed:

Here, the left-hand side (A) of Eq. (1) shows the local variation term of convective instability; the first term (B)on the right-hand side is theθseadvection variation with height (decreases inθseadvection with height benefit the enhancement of convective instability); the second term(C) on the right is the effect of divergence on convective instability (when the atmosphere is convectively unstable, the convergence will weaken the convective instability and vice versa); the third term (D) on the right is the vertical transportation of convective instability (when the convective instability at the lower level is higher than that at the upper level, lifting of air will transport the instability upwards and lead the convective instability layer to be thickened, otherwise, the atmosphere will tend to be stable), and the fourth term on the right is the diabatic heating term, which causes instability to decrease when precipitation occurs.

Figure 11c shows the temporal evolution of the first four terms of Eq. (1) at 900 hPa over the heavy rainfall area. As can be seen, although heavy precipitation occurred during the two processes, the local variation term of convective instability (black solid line) remained steady around the value of zero. Comparing the first three terms on the right-hand side of Eq. (1) suggests that the term forθseadvection variation with height (red solid line) during the two processes in both cases increased significantly and much more so than the other two terms.Its temporal evolution was also consistent with the local variation term of convective instability. Combined with Fig. 5, this implies that the difference in intensity, location, and direction between the BLJ and SLLJ resulted in the positiveθseadvection in the boundary layer being significantly larger than that in the lower and middle troposphere, which was an important reason for the maintenance of convective instability over the heavy rainfall area. In addition, the vertical transportation of convective instability (blue solid line) showed significant decreases during both heavy rainfall events. This is because the evaporative cooling of heavy precipitation increased the stability of the boundary layer, and the lifting of air transported the stability upwards to make the atmosphere stable. Since this paper focuses on the maintenance mechanism of the convective instability, and the fourth term on the right-hand side of Eq. (1) will cause instability to decrease when precipitation occurs, this term is not discussed in depth.

Fig. 11. Distributions of wind (barbed arrows; m s-1) and pseudo-equivalent potential temperature (θse, shaded; K) at 950 hPa at (a) 0000 BT 21 May 2020 and (b) 2300 BT 21 May 2020. (c) Area-mean (22.5°–24.5°N, 112.5°–114.5°E) local variation of convective instability (A: black solid line;10-8 K Pa-1 s-1), θse advection variation with height (B: red solid line), the effect of divergence on convective instability (C: green solid line), and the vertical transportation of convective instability (D: blue solid line) from 2000 BT 20 to 0800 BT 22 May 2020.

6. Summary and discussion

In this study, we investigated the structures of LLJs and their influences on the water vapor and convective instability conditions during two heavy rainfall events that occurred over South China on 20–22 May 2020. The major findings can be summarized as follows (illustrated schematically in Fig. 12):

(1) South China was affected by a low-level vortex and a low-level shear line during the two processes. The first rainfall event was a warm-sector heavy rainfall event that occurred south of the shear line, while the second rainfall event was a frontal heavy rainfall event caused by the cold front and shear line. Divergence on the east side of the South Asian high and positive vorticity advection in front of the shortwave troughs at 500 hPa provided favorable synoptic-scale lifting conditions for heavy precipitation. Before the two heavy rainfall events occurred, favorable water vapor and thermodynamic conditions were already in place over the PRD, which had resulted from the onset of the SCS summer monsoon.

(2) Double LLJs existed over South China during both heavy rainfall events. The coupling of the convergence at the exit of the BLJ and the divergence at the entrance of the SLLJ provided favorable dynamic conditions for the first heavy rainfall event. Previous studies (Du and Chen,2018, 2019; Li and Du, 2021) have suggested that warmsector heavy rainfall in South China is mainly concentrated over coastal areas, and the coupling of SLLJs and BLJs is an important trigger mechanism. The results of this study show that the coupling of double LLJs also plays important roles in inland warm-sector heavy rainfall, and has a similar mechanism to coastal heavy rainfall. As the cold front moved southwards, the strong convergence between the LLJs and northerly winds was the main lifting reason for the second heavy rainfall event,with the convergence center located near the 850–800-hPa level.

(3) Water vapor was mainly transported by the BLJ from the ocean to the PRD during the two processes.During the first event, water vapor convergence was concentrated in the boundary layer, while the lower and middle troposphere had strong vapor convergence during the second event. The difference in intensity, location, and direction between the BLJ and SLLJ resulted in the positiveθseadvection in the boundary layer being significantly larger than in the lower and middle troposphere, which was an important reason for the maintenance of convective instability over the PRD.

The structures and roles of the BLJ and SLLJ during these two heavy rainfall events in the pre-summer rainy season over South China in 2020 are discussed in this study. The analyses are relatively preliminary. The combined roles of LLJs with other factors (e.g., topography,cold pool, etc.) were not fully considered. The interaction between LLJs and heavy precipitation also deserves further analysis. The pre-summer rainfall over South China is closely associated with LLJs, and more in-depth studies on the roles of LLJs will improve the level of scientific understanding and operational forecasting of the rainfall during the pre-summer rainy season over South China.