On the basis of the method described in
Section 2, a total of 195 cases were obtained for the five-year (2014–2018) warm seasons, with 39 annual occurrences on average (
Table 1). It is demonstrated that LCDM increased annually over the period between 2014 and 2018; the number seen in 2018 was twice that seen in 2014. The increasing trend of LCDM possibly resulted from global warming. As demonstrated by the IPCC report of “global warming of 1.5 °C”, global temperature increased by 0.2 ± 0.1 °C per decade [
39]. In addition, Qi et al. [
40] claimed that the duration of the high-temperature heat wave in eastern China tends to be prolonged. LCDMs that do not leave the Dabie Mountains are defined as NoOut-Type, while LCDMs that move away from the Dabie Mountains are considered as Out-Type.
3.1. Spatial Distribution
The initial locations of the 195 cases reported here are summarized in
Figure 3a. Three high-elevation areas (i.e., higher than 1000 m) denoted A, B, and C (rectangles) within the Dabie Mountains, which are illustrated in
Figure 3. Initial locations are concentrated within areas to the south of A and B, respectively, as well as to the north of C (denoted as AR, BR, and CR in
Figure 3a). The elevations of these three areas fall between 400 and 800 m. The results indicate that most LCDMs do not initiate on the highest mountains, but over slopes; AR and BR are located on the southern slopes of the Dabie Mountains (will be indicated as the windward slopes in
Section 4.1), while CR falls on the northern slopes (will be indicated as the lee slopes in
Section 4.1). Maximum storm activity along the windward slopes rather than at higher elevations was also investigated in Taiwan [
41,
42,
43]. A small number of LCDM cases were generated over mountain tops within the Dabie Mountains, as CI is complex in different mountainous areas. A number of previous studies have revealed that favorable CI tends to occur on mountain tops [
44,
45,
46] not just because of elevation but also because of steeper slope angles [
46]. In earlier work, Banta and Schaaf [
1] indicated that the lee-side of mountain areas often tends to be conducive for convection generation, while Yang et al. [
47] demonstrated that most MCSs are initiated on the eastern edges of second-step terrain.
Average composite reflectivity at the time of initiation for all 195 convections, as summarized in
Figure 3b. High reflectivity falls to the south of areas A and B (
Figure 3b), consistent with AR and BR’s initial locations (
Figure 3a). At the same time, one additional area is also present at the higher elevation C as well as to the east of area B, inconsistent with CR initial locations (
Figure 3a). Higher reflectivity (i.e., composite reflectivity greater than, or equal to, 40 dBZ) also occurs frequently across these three areas (
Figure 3c), consistent with the results in
Figure 3b. These results indicate that most LCDMs, including severe ones, tend to initiate on windward slopes. Numerous previous studies have revealed the enhancement effects of windward slopes on precipitation within the Dabie Mountains [
17,
48]. In one example, Ni et al. [
49] revealed that maximum annual precipitation occurred to the southeast of the Dabie Mountains peaks, consistent with the high reflectivity in the southeast of area B (
Figure 3b,c). A limited number of studies have emphasized the impact of windward slope convection, especially within the Dabie Mountains.
3.2. Monthly and Diurnal Variations
LCDM tend to increase from April to August, occur frequently in summer (June–August) and peak in August (
Figure 4a). The monthly variations in LCDM are different from those of long-lived organized MCSs on plain areas (e.g., in the central United States), which peak in May, June, and July [
50]. The monthly variations in LCDM and long-lived organized MCSs in east China are different. For example, long-lived MCSs in other mountainous regions (e.g., over second-step terrains in the Yangtze River Valley and over central-east China) exhibit different monthly variations, with peaks in June and July [
27], perhaps related to the Meiyu Front. It is generally the case that this major rain band is controlled by the summer monsoon over the middle and lower reaches of the Yangtze River in eastern China and this peaks during middle June to middle July [
51]. Climatological studies over this area suggest that convection and associated precipitation are affected by terrain [
52,
53]. Convection on mountainous areas in France and Germany also occurred frequently in summer [
54].
In terms of diurnal variations, LCDM appears a single peak at 12:00 Beijing Standard Time (BST)—16:00 BST (
Figure 4b), dominated by afternoon convection, possibly due to maximum solar heating [
25]. This afternoon peak also indicates its heating-induced characteristics. The diurnal variations in LCDM are also different here from those seen in other areas of Central East China; in one example, MCSs in Eastern China appear to have two peaks, a main one in the afternoon and a second one in the early morning [
9]. Meng et al. [
29] revealed that the squall line initiated over the north of 30°N in East China peaks at noon (09:00–12:00 BST), while preceding landfalling tropical cyclones (pre-TC) squall lines form a peak extending from the afternoon to midnight [
28].
It can be noted that LCDM exhibits heating-induced properties with monthly and diurnal variations. Indeed, most LCDMs dissipate between the afternoon and midnight (between 15:00 and 23:00 BST), 88.2% of the total. Overall, 53.8% of this total dissipated at 17:00–19:00 BST (
Figure 4c). The lifespans of LCDM are generally shorter than six hours.
There are slight differences among AR, BR, and CR areas. LCDM in AR and BR areas peak in August, while those in CR areas peak in July (
Figure 4d). Similarly, LCDM in AR and BR areas peak at 12:00–16:00 BST, while those in CR areas peak at 11:00 and 16:00 BST (
Figure 4e). The majority of LCDMs in AR, BR, and CR areas dissipate at 17:00–20:00 BST, 15:00–19:00 BST, and 15:00–18:00 BST, respectively (
Figure 4f). LCDMs in AR and BR areas tend to exhibit similar monthly and diurnal variations, while those in CR areas appear and dissipate an hour earlier than in corresponding AR and BR areas. The results indicate that the lifespans of LCDM in these three areas are similar.
The evolutionary distribution of reflectivity ≥ 40 dBZ frequencies during the period 12:00–17:00 BST are shown in
Figure 5. Indeed, similar to the orange column in
Figure 4b, LCMD in CR areas (
Figure 3a) initiate at 12:00 BST on northern slopes (
Figure 4e). The maximum frequency on the southern slopes of AR and BR (
Figure 3a) occurred between 13:00 and 15:00 BST (
Figure 4e), an hour later than in CR areas. In general, LCDMs frequently initiate on southern slopes with a secondary maximum on northern slopes. LCDMs generally move in a northeast direction, but most dissipate within the Dabie Mountains (
Figure 5).
The Hovemöller diagram [
32,
55,
56] (
Figure 6) is used to demonstrate the diurnal variations and motion characteristics of LCMD in the N and S subdomains (
Figure 5c). It is demonstrated in
Figure 5 that LCMD initiated an hour earlier over northern slopes than that over southern slopes, the former moved to northeast plain areas, and the latter affected the southeast valley areas. The N and S subdomain cover the LCDM initiation and affecting areas. In addition, LCDM over northern slopes moves along the long side of N subdomain (in the direction of the red arrow in
Figure 5c), and that over southern slopes propagates along the short side of S subdmain (in the direction of the purple arrow in
Figure 5c). In order to demonstrate the diurnal variations and propagation of LCDM initiated over northern slopes, the Hovemöller diagram along the long side of N subdomain from west to east (the red arrow direction in
Figure 5c) was presented, and the short side of the N subdomain is averaged. For LCDM initiated over southern slopes, the Hovemöller diagram along the short side of S subdomain from north to south (the purple arrow in
Figure 5c) was presented, and the long side of S subdomain is averaged. Most LCDMs initiate on southern slopes and have a maximum frequency at 15:00 BST (
Figure 6b), similar to the results presented in
Figure 4e (purple column). However, most of them do not move away from southern slopes and the rest move southeast, along the short side of subdomain S (
Figure 5c), at most, 30km distance away from southern slopes to foothills (
Figure 6b). The diurnal propagation of LCDM over northern slopes is different, which initiates at 13:00 BST, and moves northeast, along the long side of subdomain N (
Figure 5c), away from mountainous areas. These then develop again at foothills and plains.
3.4. Related Severe Convective Weather
It is clear that LCDMs mainly produce SDHR, including all of the 195 cases studied in this analysis. Overall, just seven (3.5%) led to THWs (six of which were Out-Type), while only one generated hail (figure not shown here). SDHR ≥ 50 mm/h mostly occur within foothills at elevations around 200 m (
Figure 8a), especially in the south and north of the Dabie Mountains. High-intensity (
Figure 8a) and high-frequency (
Figure 8b) SDHR are observed to the south and north of the Dabie Mountains, consistent with high values (
Figure 3b) and frequencies (
Figure 3c) of composite reflectivity. In recent work, Tong et al. [
57] reported high SDHR frequencies over the south of the Dabie Mountains, and a similar trend was noted on Taiwan island by Lin et al. [
26], where the maximum rain frequency occurred parallel to the orientation and lower slopes of the mountains.
THWs mainly occur in valleys to the south of the Dabie Mountains (
Figure 8c). These events also tend to be less frequent in Dabie mountainous regions (
Figure 8) and have lower frequencies than those over mountainous regions in North China, especially over the Taihang Mountains [
58]; THWs also frequently occur over the eastern Rocky Mountains, and significant volumes (≥33.4 m/s) are seen over the Great Plains [
59] in the United States. The results imply that topographic configuration is an important issue influencing THWs, which will require further analysis.