Observational and Mechanistic Analysis of a Nighttime Warm-Sector Heavy Rainfall Event Within the Subtropical High over the Southeastern Coast of China

The warm-sector heavy rainfall was proposed from the results of rainfall experiments conducted in southern China, revealing that, distinct from large-scale systematic precipitation, it typically occurs in warm areas 200–300 km away from the frontal surface ^[1]. In other regions of China, warm-sector rainfall has even been observed at the edges and interiors of the subtropical high ^[2-3]. Warm-sector rainfall is characterized by high intensity, long duration, large levels of accumulation, and a concentrated location. The convective systems responsible for warm-sector rainfall tend to exhibit low centroid echoes of tropical oceanic precipitation ^[9-10], which results in rearward propagation and echo training ^[11-14]. Consequently, the precipitation produced by these systems generally exceeds that caused by frontal systems ^[15]. Numerous studies have investigated the causes of warm-sector heavy rainfall, finding that it typically occurs in environments with large convective available potential energy (CAPE), small convective inhibition (CIN), low lifting condensation level (LCL), and abundant water vapor ^[16-19]. Due to the weak synoptic forcing background, warm-sector rainfall is often influenced by complex physical processes caused by topography, ocean-land interactions, and other factors ^[20-21]. The warm advection in the lower troposphere and the cold advection in the upper troposphere are beneficial to the accumulation of unstable energy. The southwesterly low-level jet (LLJ) supplies large quantities of water vapor for rainfall, and the convergence caused by low-level winds and the pulsing of the LLJ play crucial roles in the formation and development of heavy rainfall ^[22-24]. Despite the numerous research findings on the causes of warm-sector heavy rainfall, the predictive capabilities of major numerical weather prediction models for such events remain limited, particularly in regard to CI preceding heavy rainfall events.

CI consistently marks the onset of severe convective weather, the mechanism of which is a central area of interest in mesoscale meteorology, with numerous related studies having been conducted both domestically and internationally ^[25-28]. In 2002, the United States completed a renowned research project called the International H2O Project (IHOP_2002) ^[26]. One of its scientific objectives was to study the mechanism of CI caused by the boundary layer convergence lines. Existing research findings indicate that there is significant convergence and upward motion near these convergence lines, which can reduce CIN and enhance the moisture content of air parcels, thereby creating a favorable environment for CI ^[29-30]. Moreover, factors such as temperature and humidity disturbance ^[31-32], instability ^[33-34], horizontal convective rolls ^[35-37], the ambient wind shear profile ^[38-41], and topography ^[42-45] also have effects on CI.

Compared to warm-sector heavy rainfall in southern China, along the southeastern coast of China, it may exhibit different triggering mechanisms owing to the variations in, for example, geomorphic features, land-sea boundaries, and weather systems. However, there has been relatively little research conducted on the southeastern coast of China. Consequently, it is essential to further investigate the mechanisms underlying warm-sector heavy rainfall in this region. In August 2021, a warm-sector heavy rainfall event under the control of the subtropical high occurred over the southeastern coast of China. The maximum hourly rainfall exceeded 100 mm, resulting in severe waterlogging in the urban area of Xiamen. Notably, none of the major numerical weather prediction models were able to predict this event. Thus, the question arises: how was the mesoscale convective system (MCS) that led to the heavy rainfall initiated and developed under the influence of the subtropical high? To address this question, this study used multi-source observational data and reanalysis data from the fifth major global reanalysis produced by ECMWF (ERA5) to study the evolution of the environmental conditions of the rainfall event and the development of the MCS. The aim was to provide a technical reference for forecasting similar warm-sector heavy rainfall events in the future.

The rainfall event had the characteristics of local heavy rainfall, high cumulative precipitation, and extreme rainfall intensity, so multi-source observational data and ERA5 reanalysis data were used in this study. Specifically, the following five types of data were employed (Fig. 1) : (1) observational data from approximately 400 regional automatic weather stations in southern Fujian; (2) observational data from three S-band Doppler weather radars in Xiamen, Quanzhou and Zhangzhou; (3) observation data from three X-band phased array weather radars in Xiang'an District, Tong'an District and Haicang District of Xiamen; (4) sounding data produced by the fusion of microwave radiometer and wind profiler weather radar in Xiang'an district; and (5) ERA5 hourly reanalysis data with a spatial resolution of 0.25° × 0.25°.

Figure 1.  Distribution of observation equipment in Southern Fujian. Terrain height, contours, units: m; automatic station, black dots; microwave radiometer, red diamond; wind profile radar, red square; S-band radar, red circle; phased array radar, red cross.

In order to study the structural characteristics of the wind field inside the MCS, this study used the method of multi-Doppler radar retrieval in earth coordinates ^[46] to generate three-dimensional wind retrieval data. The spatial resolution of the retrieval data is 0.005° × 0.005°, and the vertical resolution is 500 m.

From the night of August 10 to the early morning of August 11, 2021, an extreme rainfall event took place along the southeastern coast of China under a backdrop of weak synoptic forcing. The rain band displayed a northeast-southwest orientation. The maximum precipitation area was concentrated southeast of Xiamen (Fig. 2), with the largest amount at Binhai Street Station, reaching 154 mm. The temporal evolution of the 5-min rainfall amounts exhibited a single-peak pattern, peaking between 02:00 and 05:00 Beijing time (BJT), and the maximum 5-min rainfall exceeding 7 mm (Fig. 3). At 03:00 BJT, the number of stations with hourly rainfall surpassing 50 mm reached 17, and the maximum hourly rainfall also occurred at Binhai Street, with an hourly rainfall of 108.2 mm. This rainfall event not only displayed characteristics of significant cumulative precipitation but also demonstrated features of sudden onset, localized heavy rainfall, and extreme rainfall intensity.

Figure 2.  Cumulative precipitation from 20:00 BJT August 10 to 08:00 BJT August 11, 2021. Terrain height, contours, units: m; precipitation, colored dots, units: mm.

Figure 3.  Features of meteorological variables at a 5-min temporal resolution at Meihailing Station from 00:00 to 06:00 BJT August 11, 2021. Precipitation, green column, units: mm; atmospheric pressure, black curve, units: hPa; temperature, red curve, units: ℃; dew point temperature, blue dotted line, units: ℃.

From the 0.5° elevation reflectivity of the S-band dual-polarization radar in the Haicang district of Xiamen (Fig. 4), the evolutionary characteristics of convection during the heavy rainfall event can be observed. Dispersed convective cells emerged along the coast of Zhangzhou at 01:00 BJT on August 11, subsequently strengthening and moving northeastward. By 02:00 BJT, the dispersed convective cells along the coast of Zhangzhou had organized and developed into a relatively complete linearly shaped MCS, starting to affect the southeastern coastal areas of Xiamen. Following that, the convective system with reflectivity exceeding 40 dBZ persisted in impacting the southern areas of Xiamen for approximately two hours. The MCS then moved northeastward and began to affect the northern areas of Xiamen at 04:00 BJT. After 05:00 BJT, the intensity of the MCS weakened, and the range of reflectivity above 40 dBZ decreased substantially. The original linear shape of the convective system also became unstructured, signaling the precipitation event′s conclusion.

Figure 4.  Evolution of the 0.5° elevation reflectivity of the S-band dual-polarization radar in Haicang district from 01:00 to 05:00 BJT August 11. Radar reflectivity, contours, units: dBZ.

According to the analysis of atmospheric circulation characteristics (Fig. 5), the subtropical high exhibited a strong intensity at 20:00 BJT prior to the heavy rainfall event. The area of southern Fujian was situated near the ridge of the subtropical high, with a geopotential height of 5890 gpm. On the 850–925 hPa isobaric surface, a warm shear line was observed in the mid-lower level reaches of the Yangtze River. The area of southern Fujian was positioned on the south side of the shear line and was influenced by the southwesterly wind.

Figure 5.  Synoptic-scale characteristics of the background circulation at (a) 20:00 BJT August 10; (b) 02:00 BJT August 11, including the 500 hPa geopotential height (contours, units: gpm), 925 hPa wind field (black arrows, units: m s^–1), precipitable water (color shading, units: mm), the warm shear line (blue dotted lines).

At 02:00 BJT, the extent of the subtropical high decreased slightly, but the area of southern Fujian remained close to the ridge line. The shear line on the 850–925 hPa isobaric surface gradually moved seaward and developed during the heavy rainfall event. A low-pressure system with an inverted trough developed in the surface layer (figure omitted). These changes caused the isobaric lines to rotate and increased the velocity of the southerly wind. The wind field in the Taiwan Strait exhibited similar evolutionary characteristics, with wind velocities stronger than those in the coastal areas. This wind field distribution established a convergence zone at the land-sea boundary along the coast of southern Fujian. From the perspective of the water vapor conditions, a persistent precipitable water (PW) area (exceeding 60 mm) extended from the northeastern South China Sea to the southern Taiwan Strait, providing an abundant water vapor source for this heavy rainfall event, in conjunction with the continuous water vapor transport brought by the southerly wind. Additionally, from 20:00 BJT August 10 to 08:00 BJT August 11, the main body of the South Asian High at 200 hPa was situated over the Tibetan Plateau and extended eastward in a zonal pattern. The coastal area of Fujian was influenced by the northeastern jet stream on the southern side of the eastern ridge, which provided favorable outflow conditions in the upper levels (figure omitted).

Based on the above analysis, it can be concluded that this event was a nighttime warm-sector heavy rainfall occurring within the range of the subtropical high, without significant cold air influence or systematic synoptic-scale uplift. The abundant water vapor combined with convergence caused by the low-level wind provided the fundamental conditions for the heavy rainfall. However, such synoptic-scale conditions alone cannot directly result in extreme precipitation intensity. From the precipitation distribution predicted by global numerical weather prediction models, it is apparent that only light rain was forecasted for the coastal area of southern Fujian (figure omitted). The question therefore remains: how was the convection triggered, and how did it organize into a linearly shaped MCS, ultimately leading to heavy rainfall? Changes in atmospheric stratification may have played crucial roles in this process.

Numerous studies have demonstrated that the development of convective systems is closely related to environmental conditions, such as atmospheric stratification instability and abundant water vapor, among others ^[47-50]. The preceding analysis revealed that the heavy rainfall event occurred under conditions of weak synoptic forcing. However, it is essential to investigate what kind of changes occurred in the atmospheric stratification during this event, necessitating further analysis.

This study utilized the sounding data (Fig. 6) generated through the fusion of a microwave radiometer and wind profiler weather radar in the Xiang'an district of Xiamen before the precipitation event to analyze the vertical evolution characteristics of atmospheric stratification. At 20:00 BJT on August 10, due to the absence of dry cold air, there was a consistent southwesterly wind in the mid-lower levels, resulting in very close vertical profiles of temperature and humidity. The K-index reached 35 ℃, the LCL was at 1005 hPa, and the values of CAPE and CIN were 1749 J kg^–1 and 0 J kg^–1, respectively. These conditions were not conducive to the accumulation of significant unstable energy and the development of deep convection. The actual weather primarily involved short-duration rainfall without downbursts or hail.

Figure 6.  Sounding data produced by the fusion of microwave radiometer and wind profiler weather radar in Xiang'an district.

At 23:00 BJT, the K-index reached 37 ℃, and the values of CAPE and CIN were 1495 J kg^–1 and 0 J kg^–1, respectively. At 01:00 BJT on August 11, the strengthening of the southerly wind at the lower level brought the vertical profiles of temperature and humidity even closer, raising the K-index to 39 ℃ and lowering the LCL to 1008 hPa. The values of CAPE and CIN were 1297 J kg^–1 and 0 J kg^–1, respectively. The minimal CIN, combined with the very low LCL, suggest that the atmospheric conditions were conducive to convection initiation, with only minor lifting required ^[51].

Over time, as the southerly wind component in the lower levels strengthened, the temperature and dew point curves became increasingly closer, and the sounding curves maintained an elongated shape. Despite the decrease in the value of CAPE due to the decline in surface temperature, it remained above 1000 J kg^–1. These results indicate more abundant moisture conditions and increased atmospheric instability, making short-duration heavy rainfall more likely to occur.

In addition to favorable environmental conditions, CI is also closely related to the wind field. To analyze the role of ambient wind during the precipitation event, this study used ERA5 reanalysis data to examine the evolutionary characteristics of the wind field before and after CI.

At 00:00 BJT on August 11, the coastal area of southern Fujian was affected by a southerly wind at the 850–925 hPa isobaric surface, with a relatively low wind velocity of only 5–6 m s^–1. A southwesterly LLJ was observed in the coastal area north of Xiamen. As a result, the coastal area south of Xiamen was located in the LLJ's inlet area, corresponding to a divergence zone from 925 hPa to 850 hPa (Fig. 7b-c). The vertical section profile along 118°E (Fig. 8) reveals that the upper region of the seawaters south of Xiamen comprised a divergence zone, lacking dynamic lifting conditions for CI. Notably, an extensive southerly wind in the lower layer was present over the sea waters (22°–24°N). The maximum wind was located in the boundary layer (900–1000 hPa), with the velocity reaching 7 m s^–1. The front side of the maximum wind corresponded to a strong convergence zone, which dipped southward and extended upward to 700 hPa. Additionally, there was a divergence center over 25°N at 825–950 hPa, caused by the LLJ's inlet area. Previous studies have found that the LLJ is an important weather system for warm-sector rainfall events in southern China ^[15]. It can supply water vapor and unstable energy for heavy rainfall ^[52], and some rainfall events exhibit interaction between the synoptic-system-related low-level jet (SLLJ) and the boundary-layer jet (BLJ) ^[53-54]. In this rainfall event, the interaction between the LLJ and the strong wind area in the boundary layer was also observed.

Figure 7.  Wind field and divergence at the height of 700–925 hPa from 00:00 to 02:00 BJT August 11. Wind field, black arrows and contours, units: m s^–1; Divergence, contour line, the solid line is positive, and the dotted line is negative, units: 10^–5 s^–1; The red triangles denote the location of Xiamen; The black straight line in Fig. 7 c denotes the location of the cross section in Fig. 8.

At 01:00 BJT, the southerly wind at 925 hPa shifted northward and increased to 6–7 m s^–1, transforming the coast of southern Fujian into a southwest-northeast convergence zone under its influence (Fig. 7f). The vertical cross-sectional profile demonstrates that the intensity of the low-level southerly wind increased and advanced northward (Fig. 8b). Correspondingly, the convergence zone along the leading edge of the high wind area also intensified and moved northward to between 24°N and 25°N. Meanwhile, the divergence center resulting from the inlet area of the LLJ within the 925–850 hPa layer persisted near 25°N. Under the interaction between the convergence and divergence centers, the divergence field configuration over Xiamen underwent the following changes: The 925–1000 hPa isobaric surface corresponded to the convergence area, whereas the 700–925 hPa isobaric surface corresponded to the divergence area. As a result of this divergence field configuration, upward motion began at 975 hPa, extending upwards to approximately 800 hPa (Fig. 8b). This demonstrates that the superposition of the convergence zone in the boundary layer and the divergence zone in the lower layer supplied dynamic lifting conditions for CI. At this time, new convective cells were continuously triggered in the coastal area south of Xiamen.

Figure 8.  Vertical section along 118°E from 00:00 to 02:00 BJT August 11. Wind field, black arrows, units: m s^–1; wind velocity, contour line, units: m s^–1; divergence, contours, units: 10^–5 s^–1; the red triangles denote the location of Xiamen.

At 02:00 BJT, the southerly wind further increased and moved northward, extending the 5 m s^–1 range upward to 800 hPa. The intensity and height of the convergence zone over Xiamen were enhanced, as evidenced by the deep convergence zone at 700–1000 hPa (Fig. 8c). At this time, the dispersed convective cells along the coastal area of southern Fujian had organized and developed into a relatively complete, linearly shaped MCS under the influence of the deep convergence zone.

In conclusion, under the backdrop of weak synoptic forcing, the initiation and progression of convection cells were intimately connected to the evolution and vertical structure of the ambient wind. While the southerly wind velocity in the boundary layer may not have met the criteria for an LLJ, it was still capable of generating a significant convergence zone and surface convergence line as it intensified and advanced northward. Moreover, when combined with the boundary layer convergence zone, the low-level divergence zone created by the LLJ was able to provide the necessary lift and upward motion for CI. This distribution of the divergence field bears resemblance to the heavy rainfall event that took place along the south coast of China during May 10–11, 2014 ^[54].

Through the research presented above, we were able to find that the increase in southerly wind played a significant role in the CI. However, after that, did any changes occur in the mesoscale and microscale structures of convective systems, leading to the rapid development of the MCS? To address this question, we employed a multi-Doppler radar retrieval technique in earth coordinates ^[46] to generate three-dimensional wind retrieval data (including three S-band weather radars in Xiamen, Quanzhou, and Zhangzhou and three X-band phased array weather radars in Xiamen). The retrieval data were then utilized to conduct a comprehensive analysis of the evolution of the wind field characteristics within the convective systems (Fig. 9).

Figure 9.  Distribution of reflectivity and the horizontal wind field retrieved by means of multi-Doppler radar retrieval in earth coordinates at 1.5 km. Black arrows, units: m s^–1; radar reflectivity, contours, units: dBZ; the shear line, light blue lines.

At 01:18 BJT on August 11, convective cells were isolated and scattered along the coast of southern Fujian, initially exhibiting a weak southwesterly wind. As these cells developed and moved northward, a consistent southwesterly wind persisted, with the velocity increasing to approximately 4 m s^–1. Then, the range of reflectivity above 40 dBZ continued to expand, leading the convective cells to merge and evolve into a linearly shaped MCS by 02:30 BJT. Concurrently, the southwesterly wind on the south side of the MCS shifted to a southerly direction, and the wind velocity experienced a significant increase. This observation aligns with the conclusions drawn from the ERA5 data analysis.

The primary distinction is that the southerly wind formed a southwest-northeast aligned shear line with the southwesterly wind ahead of the MCS, generating a significant convergence zone in the vicinity of the shear line. The analysis reveals that the shear line within the convective systems was maintained throughout the heavy precipitation process. At the rear of the shear line, convective cells continuously formed, merging with the MCS and causing it to develop and strengthen further. This also led to the subsequent propagation of the MCS, resulting in echo training impacting Xiamen. The wind field structure bears similarities to that of the MCS during the torrential rainfall event in Xiamen on May 7, 2018 ^[24].

By 04:06 BJT, the MCS had advanced northward, causing the wind in the southern region of Xiamen to shift to a southwesterly direction. The wind velocity decreased to approximately 4 m s^–1, and the shear line became incomplete. Consequently, the dynamic lifting mechanism was absent, leaving the southwesterly wind to serve solely as a water vapor channel. This condition prevented the formation of new convective cells in the south of Xiamen, leading to a substantial decrease in precipitation intensity.

From the above analysis, it is evident that the strengthening of the low-level southerly wind led to the formation of a southwest-northeast shear line in conjunction with the southwesterly wind within the convective systems. The shear line acted as a meso-to small-scale system in this rainfall event, providing dynamic lifting essential for the development of the MCS.

Through the comprehensive analysis of the horizontal wind field within the convective systems, we determined that the persistent presence of the shear line was one of the influential factors contributing to the organization of the MCS. However, did the vertical direction of the convective systems also exhibit meso-to small-scale systems that may have altered the dynamic uplift conditions? To address this question, we utilized multi-Doppler radar retrieval data to construct a vertical cross-section along the shear line (corresponding to the northwest-southeast-oriented black line depicted in Fig. 9e).

At 02:30 BJT, the linearly shaped MCS began to impact the southeastern region of Xiamen Island (Fig. 10). The low-level southerly wind was slightly stronger, forming a southerly convergence zone and a positive vorticity center below 4 km. At 02:54 BJT, the southerly wind further intensified to approximately 10 m s^–1. The updrafts also increased significantly and extended upward to around 9 km. This resulted in the formation of a conspicuous convergence zone in the middle of the MCS, extending up to 6 km, while the area above 6 km corresponded to a divergence zone. Simultaneously, the positive vorticity center within the convective systems evolved into an inclined vorticity column with a height exceeding 10 km, and the convective systems intensified. The range of 50 dBZ reflectivity substantially expanded, and the range of 30 dBZ reflectivity also extended upward to 10 km. Moreover, the height of the 40 dBZ reflectivity within the MCS was below 5 km (corresponding to the height of the 0℃ layer), indicating that it exhibited low centroid echo characteristics and was typical of tropical oceanic precipitation with high precipitation efficiency ^[55].

Figure 10.  Evolution of the vertical structures along the strong echo center: (a) evolution of the reflectivity (color shading; units: dBZ), V-component wind and W-component wind (vector arrows; units: m s^–1), and the secondary circulation (circular blue arrows); (b) evolution of the divergence (color shading; units: 10^–5 s^–1) and the vorticity (contours; units: 10^–5 s^–1. The red plus signs denote the positive vorticity centers).

At 03:18 BJT, the convergence zone within the convective systems persisted, and the updrafts further strengthened and extended upward to more than 12 km, nearly penetrating the height of the troposphere. The range of 30 dBZ reflectivity also developed upward to about 12 km. Notably, a counterclockwise secondary circulation formed in the mid-upper portions of the convective systems at this time. The ascending branch of the secondary circulation corresponded to strong updrafts and a deep positive vorticity zone, suggesting the presence of substantial helicity within the convective systems. Strong helicity may contribute to a more organized storm structure, which may help maintain the storm′s intensity by reducing the dissipation of its kinetic energy ^[56]. Concurrently, the convergence zone rapidly developed to the height of 10 km, which was observed to be associated with the secondary circulation. This observation indicates that under the influence of the secondary circulation, the updrafts became connected with the upper-level northeastern jet associated with the South Asian High. This interaction created a pumping action in the upper atmosphere, continuously reducing pressure in the lower layer and further enhancing updrafts in the mid-lower levels.

At 03:24 BJT, the convective systems moved forward slowly, with new convective cells continuously forming on its inflow side and merging with it. The width of the region exhibiting reflectivity greater than 45 dBZ in the horizontal direction also expanded to its maximum extent. Throughout the occurrence of the heavy rainfall, the convergence zone within the convective systems and the secondary circulation in the mid-upper levels were consistently maintained. At 04:24 BJT, the lower-level convergence zone weakened significantly, the secondary circulation in the mid-upper levels became incomplete, and the central intensity of the convective systems decreased (figure omitted). Consequently, the precipitation intensity in the southeast of Xiamen Island also decreased significantly.

In summary, with the strengthening of the low-level southerly wind, the effects of convergence and lifting contributed to the continuous development of the MCS. During this process, the vertical updrafts in the front part of the MCS kept developing and strengthening, while a secondary circulation was established and maintained in its mid-upper levels, forming a tilted positive vorticity column. Additionally, the distribution of the divergence field changed, providing favorable dynamic lifting mechanisms for the occurrence of extreme precipitation.

In this study, we utilized multi-source observational data and ERA5 reanalysis data to conduct a detailed observational and mechanistic analysis of a heavy rainfall event caused by a linearly shaped MCS over the southeastern coast of China in August 2021. The findings are as follows:

First, this precipitation event was a nighttime warm-sector heavy rainfall that occurred in the subtropical high without the influence of dry, cold air or systematic uplift from synoptic-scale systems. The rain band exhibited a northeast-southwest alignment characterized by large cumulative rainfall, sudden onset, localized heavy rainfall, and extreme rainfall intensity. High humidity, abundant water vapor, and low-level southerly wind convergence provided the basic conditions for the event.

Second, under weak synoptic forcing, the initiation and development of convection were closely related to the evolutionary characteristics and vertical configuration of the ambient wind. The southerly wind was able to produce a considerable convergence zone in the process of strengthening and pushing northward. In conjunction with the boundary layer (925–1000 hPa) convergence zone generated by the southerly wind, the low-level (700–925 hPa) divergence zone generated by the LLJ provided lifting and pumping action for CI. As both the boundary layer and lower layer transformed into convergence zones, dispersed convective cells organized and developed into a linearly shaped MCS under the influence of the deep convergence zone.

Third, results from the multi-Doppler radar retrieval data analysis revealed that the shear line (between the southwesterly wind and the southerly wind in the MCS) was an important meso-to small-scale system in this event, providing dynamic lifting for the development of the MCS. In the vertical direction, the intensification of the low-level southerly inflow facilitated the organization and development of the MCS, while the upper-level northeastern jet stream, associated with the South Asian High, contributed to the formation of the secondary circulation. The emergence of this secondary circulation brought about changes in the wind field and dynamic conditions within the MCS. These changes included the extension of vertical updrafts to the top of the MCS, the formation of a tilted positive vorticity column inside the MCS, and the establishment of a divergence field configuration characterized by low-level convergence and upper-level divergence. All these factors were conducive to the occurrence of this heavy rainfall.

In conclusion, although numerous studies have investigated the formation mechanisms of warm-sector heavy rainfall events in South China, our analysis of this specific event has unveiled some unique characteristics. In this intense precipitation event, we have identified interactions among distinct scales of meteorological systems. The boundary layer convergence induced by synoptic-scale systems, coupled with the influence of specialized meso-to small-scale systems within the convective systems (such as mesoscale shear lines and the establishment of secondary vertical circulation), intensified the connection with the upper-level anticyclonic outflow region associated with the South Asian High (Fig. 11). It is through the combined action of these diverse-scale systems that this heavy precipitation event was manifested. Therefore, in practical forecasting, particularly for intense precipitation under weak synoptic conditions, beyond scrutinizing the synoptic-scale circulation patterns, due consideration must also be directed toward potential synergistic effects brought about by the formation of mesoscale systems subsequent to CI. Moving forward, we will explore a range of similar cases occurring in Fujian, with the goal of pinpointing the shared characteristics of these heavy rainfall events. This will ultimately provide valuable mechanistic insights that can be used to enhance forecasting accuracy and facilitate more effective early warning measures.

Figure 11.  Conceptual model of the MCS.