Comparative Analysis of Two Tornado Processes in Southern Jiangsu

Abstract

Jiangsu is a province in China and has the highest frequency of tornado occurrences. Studying the meteorological background and mechanisms of tornado formation is crucial for predicting tornado events and preventing the resulting disasters. This paper analyzed the meteorological background, instability mechanisms, and lifting conditions of the two Enhanced Fujita Scale level 2 (EF2) and above tornadoes that occurred in southern Jiangsu on 14 May 2021 (“5.14”) and 6 July 2020 (“7.06”) using ERA5 reanalysis data. Detailed analyses of the internal structure of tornado storms were conducted using Changzhou and Qingpu radar data. The results showed that (1) both tornadoes occurred in warm and moist areas ahead of upper-level troughs with significant dry air transport following the cold troughs. The continuous strengthening of low-level warm and moist advection was crucial in maintaining potential instability and triggering tornado vortices. The 14 May tornado formed within a low-level shear line and a warm area of a surface trough, while the 6 July tornado occurred at the end of a low-level jet stream, north of the eastern section of a quasi-stationary front. (2) The convective available potential energy (CAPE) and K indices for both tornado processes were very close (391 for “5.14” and 378 for “7.06”), with the lifting condensation level (LCL) near the ground. The “5.14” showed greater instability and more favorable thermodynamic conditions, with deep southwesterly jets at the mid-level shear line producing rotation under strong convergent action (convergence center value exceeding −1 $\times {10}^{-4}$ ${\mathrm{s}}^{-1}$). In contrast, the “7.06” was driven by super-low-level jet stream pulsations and wind direction convergence under the influence of the Meiyu Front (convergence center value exceeding −1.5 $\times {10}^{-4}$ ${\mathrm{s}}^{-1}$), resulting in intense lifting and vertical vorticity triggered by a surface convergence line. (3) The “5.14” tornado process involved a supercell storm over a surface dry line experiencing tilting due to strong vertical wind shear, which led to the formation of smaller cyclonic vortices near a hook echo that developed into a tornado. The “7.06” developed on a bow echo structure within a mesoscale convective system formed over the Meiyu Front, where dry air subsidence, entrainment, and convergence of the southeast jet stream triggered a “miniature” supercell. The relevant research results provide a reference for the prediction and early warning of tornadoes.

1. Introduction

Tornadoes are small-scale convective weather phenomena with immense destructive power. Their diameters generally fall below 100 m, though stronger tornadoes can extend from several hundred meters to about one kilometer. They often bring short-term heavy rainfall, thunderstorms, strong winds, and hail. These severe weather conditions can cause significant casualties and damage [1,2,3,4,5]. Tornadoes include supercell tornadoes and non-supercell tornadoes [6,7]. Most tornadoes, especially those stronger than EF2 (Enhanced Fujita Scale level 2), occur in supercell storms. Supercell tornadoes are generated from supercell storms and are typically associated with mesocyclones. Fujita first introduced the concept of a mesocyclone, characterized by closely spaced pairs of positive and negative velocity signatures in radial velocity maps [8], usually smaller than 10 km in scale. Tornadoes are more likely to be induced by persistent mesocyclones in environments with significant 0–1 km vertical wind shear and low lifting condensation levels [9,10,11,12,13].

In China, tornadoes are low-probability weather events, with occurrences less than one-tenth of those in the United States [14,15,16]. However, the high-incidence areas of tornadoes in China, such as the eastern plains and the Pearl River Delta, are densely populated and economically developed. Even weaker tornadoes can cause significant casualties and property damage. Therefore, meteorological researchers in China have conducted investigations and studies on the regional characteristics and damage caused by tornadoes in different areas and periods [17,18,19].

Tornadoes in China mainly occur in the eastern half of the mainland, particularly in the Jianghuai, Huanghuai, and Northeast Plain areas, with Jiangsu Province experiencing the highest frequency [20]. Studies have shown that plains in Jiangsu and Guangdong have the highest probability of tornado occurrences, prompting many scholars to research Jiangsu tornadoes. Wu et al. analyzed the causes of tornado weather that occurred in Gaoyou, Yancheng of Jiangsu, and Tianchang of Anhui on 3 July 2007, noting that the tilt items in the mid-scale vorticity equation were crucial for the occurrence of tornado weather [21]. Cao et al. comprehensively analyze two consecutive tornado events related to heavy rainfall during the plum rain season (a period of continuous rainy weather occurring from mid-June to mid-July in the middle and lower reaches of the Yangtze River in China) [22]. Xu et al. conducted a statistical analysis of the spatiotemporal distribution, grade distribution, weather background characteristics, and storm morphology features of tornadoes in Jiangsu from 2006 to 2018, pointing out that over 50% of tornadoes in Jiangsu occurred within supercell mesocyclones embedded in multi-cell storm systems, with 30% occurring in quasi-linear convective systems [23]. Li et al. conducted a statistical analysis of tornadoes in China over 10 years (from 2007 to 2016), tornadoes occurring in Jiangsu and Zhejiang Provinces exhibit significant year-to-year, month-to-month, and day-to-day variation characteristics, and the differences between them are substantial [24]. Mu et al. conducted a detailed analysis of the EF2–EF3-level tornado weather process that occurred in northern Jiangsu on 22 July 2020, suggesting that a combination of a lower base height and strong shear value in mesocyclones, along with the evolution of ground-level mesocyclone scale systems, could aid in tornado warnings [25].

Currently, the foundational scientific research and forecasting techniques for tornadoes in China are still in their infancy. Studies on the weather mechanisms of tornado formation, including dynamics, thermodynamics, and cloud microphysics, are accelerating. Despite this progress, tornadoes’ small scale and sudden nature make them challenging to monitor, a persistent difficulty in meteorological disaster prevention and reduction. Therefore, enhancing the forecasting and warning capabilities for tornadoes is not only a breakthrough in academic research but also a critical need for disaster prevention.

Several urgent key scientific questions need to be addressed for Jiangsu Province. First, there are few studies on tornado cases that occur in southern Jiangsu, which are worth further exploration and research. Second, under different weather conditions, there are many differences in the environmental conditions of tornadoes. Even under similar favorable environmental conditions, the probability of tornado formation is extremely low. Therefore, the key mechanisms and factors of tornado formation should be fully understood, and the key physical factors that form tornadoes should be clearly identified. Third, under the background of the Meiyu Season, the environmental conditions conducive to tornadoes usually cover a large geographical range, but the number of tornadoes is usually only one or a few. With the development of more dense ground automatic weather station networks and high-resolution precision radar observations, there will be a deeper understanding of the detailed structural features and tornado vortices of tornado storms.

This paper, based on multi-source observation data such as radar, wind profiling, and ground automatic stations, compares and analyzes the similarities and differences in weather patterns, environmental conditions, and formation mechanisms of the two tornado processes in southern Jiangsu. This analysis aims to improve the warning capabilities for tornado processes under different types of weather backgrounds and provide a reference for the forecasting and warning of similar weather processes in the future, thereby reducing meteorological disaster losses.

2. Data and Method

2.1. Data

The data used in this paper include (1) the fifth-generation global atmospheric reanalysis data ERA5 (ECMWF Reanalysis v5) released by the European Centre for Medium-Range Weather Forecasts (ECMWF), with a spatial resolution of 0.1 × 0.1° and a temporal resolution of 1 h; (2) the minute precipitation, temperature, and 2 min mean wind direction and speed (Figure 1) of the automatic stations within Jiangsu Province from 00:00 to 12:00 on 6 July 2020; (3) the Doppler weather radar data of Changzhou (CZ) and Qingpu (QP) from 00:00 to 12:00 on 6 July 2020; (4) the wind profile data of Taicang (TC) and Wujiang (WJ) from 00:00 to 12:00 on 6 July 2020; and (5) the microwave radiometer data of Kunshan (KS) from 00:00 to 12:00 on 6 July 2020.

2.2. Method

In this paper, the conventional weather charts and ERA5 reanalysis data are used to analyze the synoptic-scale circulation background and explore the large-scale features of the heavy rain and tornado occurrence and development; the characteristics of the mesoscale low-vortex rainband are revealed through the analysis of the ground automatic stations; and the evolution features of the convective system are further confirmed by the cloud images and radar data. Finally, the distribution of thermodynamic and dynamic field physical quantities and the evolution of the low-level jet near the tornado occurrence site are calculated by using the ERA5 reanalysis data and the wind profile radar data to reveal the environmental characteristics of the convective development and evolution.

3. Overview of Processes and Synoptic Background

3.1. Weather Reality

On 14 May 2021, between 18:36 and 19:06 Beijing time, areas of Suzhou City in Jiangsu, including Wujiang, Wuzhong, Zhangjiagang, and Changshu, experienced severe weather phenomena such as intense thunderstorms, short-term heavy rainfall, stormy winds, and small hailstones. Across the city, 33 automatic meteorological stations recorded hourly rainfall exceeding 20 mm, with the highest hourly rainfall reaching 51.1 mm in Daxin Town of Zhangjiagang between 18:00 and 19:00. Nine stations registered thunderstorm winds of force 8 or above, with the highest wind speed recorded at Xiaoleishan in Taihu Lake at 31.9 m·s⁻¹ (level 11). Around 19:00, parts of Shengze Town in Wujiang District of Suzhou were struck by an EF3 tornado, with a maximum wind force of level 17, causing four deaths, injuring over 139 people, damaging power facilities and numerous houses, and resulting in severe economic losses.

On 6 July 2020, Suzhou in Jiangsu experienced widespread heavy-to-torrential rain, accompanied by short-term heavy downpours and local severe convective weather such as thunderstorm winds. The highest hourly rainfall recorded was 72.5 mm in Yushan of Kunshan, with the highest wind speed reaching 31.3 m·s⁻¹ (level 11) in Liuhe Water Source of Taicang. Around 9:10, an EF0 tornado affected Bacheng Town of Kunshan in Suzhou, near Yangcheng Lake, causing damage to 33 households, 13 of which were severely affected (referred to as the 20200706-1 tornado). At approximately 9:53, an automatic station in Liuhe Town of Taicang recorded a strong gust of 21.5 m·s⁻¹ (level 9). Between 09:54 and 10:00, Liuhe Town in Taicang was hit by a tornado of intensity level 3 (Chinese industrial standard)/EF2–EF3, with the tornado having a width of about 100 m and a path of about 1 km, moving eastward in a jumping manner (referred to as the 20200706-2 tornado). This tornado caused severe damage to houses, farmland, trees, and other property, leading to significant economic losses but no casualties.

3.2. Synoptic Background

At 8:00 a.m. on 14 May, Jiangsu was located in the divergence zone on the right side of the 200 hPa high-altitude jet stream axis. The western Pacific subtropical high (hereinafter referred to as the subtropical high) had weakened due to the eastward movement of the plateau trough. Most of Jiangsu was located in front of the trough and in the warm and humid southwest airflow around the periphery of the subtropical high, with a warm ridge extending eastward and northward in the north of Zhejiang. There was a southwest–southeast shear line existing between 32 and 33° N, and southern Jiangsu was located in the warm southwest jet stream on the south side of the warm shear. In the afternoon, the southwest jet near 30° N further strengthened and extended eastward, and the southwest wind speed at 850 hPa in southern Jiangsu increased from 12 m·s⁻¹ to 18 m·s⁻¹. The low-level jet axis was located in northern Zhejiang, with a wind speed of 20 m·s⁻¹. The divergence in the upper layers, the shear line in the low levels, and the enhancement of the jet stream formed a circulation background conducive to the development of tornadoes (Figure 2a).

Overnight on 5 July to daytime on 6 July 2020, Jiangsu was under the right side of the divergent and diffluent area of the 200 hPa high-level jet stream. The 500–700 hPa mid-latitude had an eastward-moving high-level trough, with the subtropical high distributed in a zonal pattern and its western ridge point shifted westward. The 120° E ridge line was near 25° N. The northerly airflow behind the high-level trough and the southerly warm and moist airflow on the periphery of the subtropical high converged over southern Jiangsu. Between the Jianghuai region and southern Anhui, there were northeast-to-southwest-oriented cold shear lines at both the 850 hPa and 925 hPa levels. Suzhou was situated south of the low-level shear line and at the bottom of the high-level trough, within the convergent area of the wind speed at the top end of the southwest jet stream. This provided favorable conditions for wind direction and speed convergence. At the surface, cold and warm air masses met in the middle and lower reaches of the Yangtze River, forming a northeast-to-southwest-oriented quasi-stationary front. The tornadoes occurred on the northern side of the eastern segment of this quasi-stationary front (Figure 2b).

The circulatory backgrounds of both tornadoes were remarkably similar. Southern Jiangsu was located in the warm and moist area ahead of the trough, with evident dry air transport behind the cold trough. Upper-level divergence and the development of a low-level shear line, along with continuously strengthening warm and moist advection at lower levels, played a dual role: they maintained potential instability in the stratification and induced local convergence, which triggered the formation of tornado vortices. The 14 May tornado occurred within the low-level shear line and the warm area of the surface inverted trough, whereas the 6 July tornado formed at the top end of the low-level jet stream, on the northern side of the eastern segment of a quasi-stationary front.

4. Comparative Analysis of Environmental Conditions

4.1. Sounding Curves

On 14 May 2021, at 08:00, the Hangzhou Station sounding (Figure 3a,

Table 1. Comparison of environmental parameters at Hangzhou Station at 08:00 on 14 May 2021, and at Baoshan Station at 08:00 on 6 July 2020.

Time	CAPE (J·kg−1)	CIN (J·kg−1)	K (°C)	SI	T850-500 (°C)	SREH	0–1 km Vertical Shear (m·s−1)
08:00, 14 May	391	195	37.5	−3.7	28	264	14
08:00, 6 July	378.2	0	38	1.22	23	2.1	20

) showed a convective available potential energy (CAPE) of 391 J·kg⁻¹, convective inhibition (CIN) of 195 J/kg, K index of 37.5 °C, and Showalter Index (SI) of −3.7, indicating the presence of atmospheric instability. The temperature difference between 850 and 500 hPa was nearly 28 °C, with the temperature lapse rate near the dry adiabatic rate from the surface to 925 hPa. The surface dew point depression was <2 °C, suggesting near-saturation near the surface and a steep lapse rate in temperature and humidity. A dry layer with a temperature–dew point difference of ≥5 °C was present between 700 and 450 hPa. The sounding curve exhibited a “dry–cold above, warm–moist below” trumpet mouth structure, with the lifting condensation level near the ground. The winds above the surface were consistently from the southwest, with vertical wind shear dominated by speed shear. Surface wind speeds were only 1–2 m·s⁻¹, nearly calm, with 700 hPa wind speeds reaching 18 m/s and above 650 hPa to 400 hPa wind speeds reaching 20 m·s⁻¹. The 0–1 km and 0–6 km vertical wind shear exceeded 18 m·s⁻¹, with a storm relative environmental helicity (SREH) of 264 m²·s². By 20:00, the CAPE rapidly increased to 2334 J·kg⁻¹ (Figure 3b), the CIN dropped to 0 J·kg⁻¹, and the K index increased to 41.3 °C, indicating further accumulation of unstable energy.

On 5 July 2020, at 20:00, the Baoshan Station in the Shanghai sounding (Figure 3b, Table 1) showed a deep low-level jet with wind speeds exceeding 20 m/s between 850 and 700 hPa, with the strongest southwestern jet at 700 hPa with wind speeds exceeding 24 m·s⁻¹. The strengthening of the mid-to-lower-level southwestern jet resulted in significant moistening and thickening of the wet layer, with the temperature and dew point curves being very close, indicating a near-saturated atmospheric layer throughout. The CAPE was 939.9 J·kg⁻¹, the K index was 35.8 °C, the lifting condensation level was around 1000 hPa, the 0 °C layer height was 5.6 km, and the cloud base specific humidity was 20.0 g·kg⁻¹, indicating the presence of a deep and moist warm cloud layer. The continuously strengthening southwestern jet and strong wind speed and direction convergence in the boundary layer made Suzhou situated in a warm, moist, and unstable atmosphere conducive to updrafts. Near-surface winds were weak, with 0–1 km and 0–3 km vertical wind shear reaching 20 m·s⁻¹ and 16 m·s⁻¹, respectively.

The CAPE and K index of the two tornado processes were very similar, with the lifting condensation level (LCL) near the ground. The “7.06” process had a warmer and moister overall profile, with stronger low-level vertical wind shear, while the “5.14” process had better Showalter Index, upper- and lower-layer temperature differences, and storm relative environmental helicity conditions, along with some CIN to accumulate unstable energy. This comparison indicates that tornado processes outside the plum rain belt require higher atmospheric instability.

4.2. Potential Instability

Tornadoes often require specific environmental conditions for their occurrence. Instability in China often arises from the interaction of tropical air masses with mid-latitude frontal systems. The presence of the East Asian monsoon plays a crucial role in providing the necessary moisture and heat. Below is a comparative analysis of the similarities and differences in the boundary layer environments of the two tornado processes. A decrease in θse with height reflects a state of potential instability in the atmosphere, conducive to the occurrence and development of mesoscale convective systems. As seen in the figures, on 14 May (Figure 4a), there was a distinct θse high-value center below 850 hPa in southern Jiangsu (30–32° N), with the center value exceeding 357 K. The tornado occurred in a warm and moist environment south of the θse front, with the frontal zone over southern Jiangsu tilting with height. Between 500 and 600 hPa, there was a mid-level relative dry and cold area, and below 500 hPa, there was a potential instability layer with decreasing θse with height. This warm and moist low-level atmosphere in southern Jiangsu provided favorable conditions for the development of the tornado.

On 6 July (Figure 4b), the entire atmospheric column over central and southern Jiangsu (31–33° N) was warm and moist, with the tornado occurring near a θse center with values exceeding 355 K. The tornado occurred near the θse front, with a steep slope below 775 hPa in the lower layers of the frontal zone, and a tilt with height from 775 hPa to 500 hPa, indicating a potential instability layer below 600 hPa. This warm and moist environment throughout the lower layers in southern Jiangsu favored the development of the tornado.

In both tornado events, there was potential instability in the mid and lower levels before their occurrence, with the “5.14” event exhibiting greater instability and better thermodynamic conditions. The tornado in this case occurred in the warm and moist environment south of the front, whereas in the “7.06” event, the entire layer was warm and moist, with the tornado occurring near the θse front.

4.3. Dynamic Conditions

Tornadoes often form from small-scale vortical columns as horizontal vorticity in the lower atmosphere is transformed into vertical vorticity under strong updrafts. Below, we compare and analyze the dynamic field structures of the environments surrounding both tornado processes before their occurrence. On 14 May (Figure 5a), before the tornado, there was strong convergence below 500 hPa in southern Jiangsu, with a convergence center value exceeding −1 × 10⁻⁴·s⁻¹. South of 31° N, the southwestern warm and moist airflow intensified the updraft under strong convergence, with ascending motion throughout the column below 200 hPa. The updraft, influenced by velocity and directional shear, generated vertical vorticity, initiating rotation in the horizontal direction. A strong vertical motion gradient led to a rapid increase in cyclonic rotation in the wind field, with a positive vorticity center existing below 850 hPa in southern Jiangsu, and the center value exceeding 8 × 10⁻⁵·s⁻¹. This indicates strong development of low-level cyclonic vorticity south of the shear convergence area before the tornado, with the vorticity tilting slightly southward below 775 hPa. The strong upward motion area between 700 and 500 hPa, combined with positive vorticity and convergence areas, was conducive to the formation of the parent storm of the mid-level tornado.

On 6 July (Figure 5b), strong convergence existed below 700 hPa near the Jiangsu River, with a convergence center value exceeding −1.5 × 10⁻⁴·s⁻¹. Near 32° N, strong ascending motion occurred throughout the layer under the influence of strong convergence, with the most intense upward motion near the shear line. There was intense positive relative vorticity below 600 hPa, with a vorticity center value exceeding 4 × 10⁻⁴·s⁻¹. Below 600 hPa, the relative vorticity did not show a significant tilt with height. The addition of the ascending branch of a secondary circulation over the shear line was favorable for further stretching of vertical vorticity in the updraft, leading to tornado formation.

Comparatively, the “5.14” event was mainly due to rotation caused by the deep southwestern jet stream under strong convergent action on the mid-level shear line, while the “7.06” event was due to wind speed pulsations and directional convergence caused by the super-low-level jet stream under the influence of the plum rain front, leading to intense uplift and vertical vorticity.

4.4. Triggering Mechanism

Surface convergence lines often act as triggering mechanisms for mesoscale convective systems. Energy convergence along these lines can lead to maximal upward movement, further developing cumulus systems. Surface convergence lines are also important systems for the development of tornadoes. At 18:00 on 14 May (Figure 6a,i), near Suzhou, a surface convergence line formed by northerly and southeasterly winds and a convergence center with a divergence of 40 × 10⁻⁶·s⁻¹ was present in the vicinity of Huzhou. As the thunderstorm cell approached the convergence line, the encounter between the storm outflow boundary and the surface convergence line intensified the lifting action, leading to a strong development of the thunderstorm cell (Figure 6b,c). By 18:30 (Figure 6d), a mesoscale cyclonic rotation started appearing on the surface convergence line, indicating the supercell storm that was the parent body of the tornado. By 18:40, as the cyclonic wind field moved eastward, it intensified and contracted in size, forming a small-scale vortex southeast of Taihu Lake in the southern area of Wujiang. By 18:50, the wind directions recorded by the three nearest automatic stations to the tornado site showed a convergent distribution towards the tornado site, indicating that the tornado occurred in the rising area of the convergence center. When the vortex encountered the convergent uplift area, it possibly led to vortex stretching, accelerating rotation and forming a tornado.

On 6 July, the surface along southern Jiangsu was controlled by a broad plum rain front, with Suzhou located on the warm side of the front. At 08:00, there was a northeast–southwest-oriented surface convergence line from the north of Kunshan to the middle of Taihu Lake. By 8:30, multiple mesoscale convergence centers formed on the convergence line, corresponding to the development of multiple mesoscale convective clusters along the precipitation echo band of the plum rain front, creating a training effect. From 9:00 to 9:30 (Figure 6e–g,j), the convergence line shifted northward to Taicang on its eastern side, where the convergence center and cyclonic rotation in the wind field near Taicang noticeably intensified, providing favorable conditions for the genesis of the parent storm of the tornado (Figure 6h).

Comparing the sea-level pressure fields and dew point distributions of the two tornado processes, it is evident that they are very similar, with a northeast–southwest-oriented asymmetric distribution. The low-pressure center on the eastern side of the isobars was more densely packed, corresponding to a large wind speed area from the southwest to south on the wind field. In both tornado processes, the central pressure of the low-pressure system was below 950 hPa. The tornadoes occurred in the north or northeast quadrant of the low-pressure center, i.e., on the left front side of the eastward-moving low-pressure system, corresponding to the convergence center of the 10 m wind vectors. Comparing the dew point distributions of the two processes, the tornadoes occurred on the moist side near the dry line. The “5.14” process had a larger dew point horizontal gradient, indicating a stronger dew point front, primarily triggered by the surface dry line. In contrast, the “7.06” process was primarily triggered by the surface convergence line.

4.5. Vertical Wind Shear

The 0–1 km low-level vertical wind shear is a key dynamic condition for tornado formation. In both tornado processes, the tornadoes occurred near the high-value centers of the 0–1 km low-level vertical wind shear, which were northeast–southwest oriented, corresponding to the location of the southwestern jet stream (strong wind speed band). In the “7.06” process, the vector difference in 0–1 km wind speed exceeded 16 m·s⁻¹, with the tornado occurring near the high-value center of the 0–1 km low-level vertical wind shear, within a latitudinal range of −1° to 0° and a longitudinal range of −2° to 0° (Figure 7a). In the “5.14” process, the 0–1 km wind vector difference was slightly lower, around 10–12 m/s, with the tornado occurring at a secondary high-value center of the 0–1 km vertical wind shear, one relative longitude to the left of the center. The 0–3 km mid-level vertical wind shear high-value areas were also northeast–southwest oriented (Figure 7b). In the “5.14” process, the mid-level vertical wind shear vector difference center values were 20–22 m·s⁻¹, while in the “7.06” process, the 0–3 km vertical wind vector difference center values were 18–20 m·s⁻¹.

In both tornado processes, low-level vertical wind shear generated horizontal vorticity, and the mid-tropospheric jet stream produced strong updrafts. In the “7.06” process, the tornado formed due to the super-low-level jet stream’s wind speed pulsations and directional convergence under the influence of the plum rain front, causing intense uplift and vertical vorticity (Figure 7c). In contrast, the “5.14” event’s tornado was mainly generated by the buoyancy vorticity on the surface dry line, with the deep southwestern jet stream causing rotation under the strong convergent action of the mid-level shear line. This process transformed horizontal vorticity into vertical vorticity, forming a very small-scale vortical column, or a tornado (Figure 7d).

5. Comparison of Key Environmental Parameters for Tornadoes

The Significant Tornado Parameter (STP) is often used in the United States to forecast the likelihood of tornado occurrences, with an STP greater than 1 indicating a high probability of strong tornadoes (EF2 and above). Comparing the distributions of the Significant Tornado Index (STP) in both cases, the maximum STP values were greater than 1.5 in both tornado events. The “5.14” tornado process had a broader STP high-value area, centered within a latitude range of −1° to 1° and a longitude range of −1° to 1° relative to the tornado site (Figure 8a). In the “7.06” process, the strong STP center was located within a latitude range of −2° to 1° and a longitude range of −1° to 1° relative to the tornado site, positioned further south relative to the tornado center. Thus, STP indicates the possibility of EF2 to EF3 tornado touchdowns (Figure 8b).

SREH represents the environmental vorticity entering a convective storm, characterizing the rotational potential produced by storm motion in a vertically sheared environment. Comparing the lower-layer SREH and the relative position of the tornado damage centers in both tornado processes, the “5.14” event had a high-value center of low-layer SREH near the tornado center, exceeding 135 m²·s⁻² (Figure 8c). In the “7.06” event, there were high-value centers of storm relative helicity on the west side and southeast quadrant of the tornado center. The western center, with a value exceeding 255 m²·s⁻², extended to the south side of the tornado center, located near the relative distance of −1.5° to −0.5° longitude and 0° latitude, corresponding to the location of the super-low-level southwest strong wind belt. The southeast quadrant center, with a value over 120 m²·s⁻², was located at the relative distance of 1° to 2° longitude and −1° to −2° latitude of the tornado damage center (Figure 8d).

Tornado formation and development depend on favorable environmental moisture, unstable stratification, and dynamic conditions. The analysis above reveals that both tornadoes occurred under moderate 0–1 km and 0–3 km vertical wind shear conditions. Appropriate mid-to-lower-level vertical wind shear is conducive to the transformation of horizontal vorticity into vertical vorticity and the organized development of convective systems. It also aids in the downward extension of mid-level mesocyclone structures to some extent. In both events, the lifting condensation levels were low, conducive to triggering deep moist convection. Both occurred near high-value centers of storm relative helicity, with the maximum Significant Tornado Index values exceeding 1.5, indicating good dynamic conditions in both tornado processes. The difference is that the “5.14” event had an unstable stratification of “dry–cold above, warm–moist below”, with stronger thermal instability and slightly weaker environmental moisture conditions. In contrast, the “7.06” event had a deeper moist layer, better environmental moisture conditions, and slightly weaker instability.

6. Comparison of Storm Evolution in Both Tornado Processes

On 14 May, between 17:18 and 17:36, new convective cells were triggered in the southern part of Suzhou, with initial center intensity exceeding 55 dBz (Figure 9a). Qingpu radar first detected a tornado vortex signature (TVS), with a shear intensity of 66.5 × 10⁻³·s⁻¹, a base height of 1.3 km, and the strongest shear height at 7.6 km (Figure 9b). Vertical profiles showed typical supercell features like echo overhangs and bounded weak echo regions. The supercell storm further intensified as it moved east, with center intensity exceeding 70 dBz and multiple mesocyclone scans detecting TVS (Figure 9c). The TVS base height dropped from 1.2 km to 0.7 km, with gradually increasing shear intensity, indicating the formation of the parent storm of the tornado. Between 18:30 and 18:48, as the storm moved eastward to Wujiang, the maximum vertical integrated liquid (VIL) exceeded 21 kg·m⁻², with strong echo cores shifting rightward with height (Figure 9d). The echo top exceeded 18 km, with the echo top center north of the tornado site, suggesting that the convective storm tilted under strong vertical wind shear. Additionally, the uneven horizontal distribution of the top height indicated significant horizontal gradients in the updraft intensity. At 18:54, the TVS base height dropped to 0.4 km, with the shear value rapidly increasing from 73.4 × 10⁻³·s⁻¹ to 165.7 × 10⁻³·s⁻¹. At the moment the tornado occurred at 19:00, the hook echo structure became clearer, and smaller-scale cyclonic vortices near the hook echo indicated intense storm rotation. The TVS base height further dropped to 0.3 km, and the shear value increased to 169.1 × 10⁻³·s⁻¹. Vertical profile imagery showed strong echoes of 60 dBZ and above touching the ground, with ground-level positive and negative velocities of 20 m·s⁻¹ and −20 m·s⁻¹, respectively, resulting in a velocity difference of 44 m·s⁻¹. These characteristics indicated a tornado touchdown (Figure 9e,f).

On the morning of 6 July from 05:00 to 07:00, as the convective cell moved downstream, it developed into a vortical convective cloud cluster in Kunshan (Figure 3). Between 08:52 and 08:57, the echo center intensity exceeded 55 dBz, with new convective cells continuously forming at the tail end. At the lowest elevation angle, a mid-level mesocyclone with a diameter of 6–8 km appeared, with a rear-flank downdraft (RFD) continuously developing. The supercell intensified into a bow echo with a mid-level radial convergence (MARC) (Figure 3). At 09:03, vortex features appeared as the tail end convective cells merged, with clear radial convergence zones visible on the radial velocity image, marking a strong inflow area into the lower level of the storm. Vertical profiles showed that the strong echo cores of over 40 dBZ were all below 6 km in height, with low centroid strong echoes and rightward shift with height, indicating strong inflow air entering the updraft. A clear V-notch was visible at the rear between 1.5 and 3.3° elevation angles, not apparent below 1.5°, indicating strong downdrafts. The radar detected mesocyclones and TVS, with a tornado occurring in Bacheng, Kunshan. The small-scale rotation features were not apparent, indicating a weaker intensity of the supercell at that time. Subsequently, as dry and cold air was entrained, the vortex in the northern end of the bow echo continuously developed, and the vortex echo began to show a hollow structure, with the mesocyclone diameter rapidly decreasing from 6 to 8 km to 3 to 4 km. At 09:44, a TVS appeared on the radial velocity image, with a tornado occurring near Liuhe in Taicang (Figure 9g,h).

Table 2. TVS parameters of Nantong Radar Station on 6 July 2020.

Time	Bottom Height/km	Top Height/km	Shear Value/ 10−3 s−1	Max Shear Height/km
8:24	1.2	4	40.8	1.2
8:36	0.7	1.8	50.1	0.7
8:42	0.8	1.8	50	0.8
8:48	0.8	2.9	36	0.8
8:54	0.8	1.8	55.4	0.8
9:00	0.8	2.8	44.1	1.8
9:06	0.7	2.8	35.8	0.7
9:12	0.9	2.2	43.8	0.9
9:18	0.9	2.1	54.3	0.9
9:24	0.9	2.1	44.6	0.9
9:30	0.8	1.9	42.3	0.8
9:42	0.8	2.7	50.3	0.8
9:48	0.8	2	29.6	2

shows the TVS features identified by the Nantong radar on the morning of 6 July 2020, from 08:24 to 09:48. Before the tornado in Bacheng, Kunshan, the shear value continuously decreased in three consecutive volume scans from 08:36 to 08:48. At the time of the tornado at 08:54, the TVS base height and maximum shear height dropped from 1.2 km at 08:24 to 800 m, indicating a continuous descent of the tornado vortex, with a maximum shear intensity of 55.4 × 10⁻³·s⁻¹. Between 09:12 and 09:30, the TVS base height remained at 700–800 m, with the top height continuously decreasing, the maximum shear height staying near the TVS base height, and the shear value decreasing from 54.3 × 10⁻³·s⁻¹ to 42.3 × 10⁻³·s⁻¹. At 09:42, before the Liuhe tornado in Taicang, the Nantong radar again detected TVS, with a shear intensity of 50.3 × 10⁻³·s⁻¹. The maximum shear height was near the base height, indicating a touchdown of the tornado vortex.

Table 3. TVS parameters of Qingpu Radar Station on 14 May 2021.

Time	Bottom Height/km	Top Height/km	Shear Value/ 10−3 s−1	Max Shear Height/km
17:18	1.3	7.6	66.5	6.3
17:36	1.2	7	42.3	5.8
17:42	1.1	6.6	32.4	5.5
17:48	1.1	6.3	31.2	5.2
17:54	0.9	5.5	41.6	4.6
18:00	0.9	5.4	44.6	4.5
18:06	0.9	5.4	56	4.5
18:12	0.7	4.7	53.5	4
18:24	0.6	4.1	55.2	3.5
18:36	0.5	3.3	73.4	2.8
18:54	0.4	2.1	165.7	1.7
19:00	0.3	2.1	169.1	1.8

shows the TVS features identified by the Qingpu radar from 17:18 to 19:00 on 14 May 2021. From 18:00 to 18:48, the TVS base height remained at 1 km, while the top height continuously dropped, and the maximum shear height rapidly descended from 6 km to 2 km, indicating a continuous lowering of the tornado vortex height. At 18:54, the moment the tornado occurred, the TVS base height dropped to 0.4 km, and the shear value rapidly increased to 165.7 × 10⁻³·s⁻¹. The strongest shear height at 18:54 dropped to near the base of the TVS, indicating that the tornado vortex was beginning to touch the ground. At 19:00, when the tornado formed, the hook echo structure became more distinct, and the TVS base height further dropped to 0.3 km, with the shear value increasing to 169.1 × 10⁻³·s⁻¹. This shows that the low base height and strong shear of the TVS are important indicators for tornado warnings.

In summary, during the “5.14” tornado process, the supercell storm over the surface dry line tilted due to strong vertical wind shear. This strong rotation led to the formation of smaller-scale cyclonic vortices near the hook echo, which then developed into a tornado. The tornado in this process had stronger rotation and a lower base height. In the “7.06” process, the tornado developed over a plum rain front-formed bow echo and comma-shaped mesoscale convective system. The “comma-shaped” convective system, influenced by dry air subsidence, entrainment, and the convergence of the southeast jet stream, stimulated a “miniature” supercell. Although the rotation intensity of the “7.06” process was slightly weaker than that of the “5.14” process, the maximum shear height remained near the base height, indicating a low height of the strong rotation.

7. Conclusions and Discussion

This paper focuses on the two EF2-and-above-grade tornadoes that occurred in southern Jiangsu on 14 May 2021 and 6 July 2020. Using ERA5 reanalysis data, we analyzed the meteorological background, instability mechanisms, and lifting conditions, and conducted a detailed analysis of the internal structure features of tornado storms through Changzhou and Qingpu radars. The following conclusions were reached:

1. The circulation backgrounds of the two tornadoes were very similar, with both occurring in warm and moist regions ahead of upper-level troughs, followed by dry air transport behind cold troughs. Continuous enhancement of low-level warm and moist advection was beneficial in maintaining a potentially unstable stratification and played a key role in triggering tornado vortices. The “5.14” tornado occurred within the low-level shear line and warm region of a surface inverted trough, while the “7.06” tornado occurred at the top end of a low-level jet stream near the northern side of the eastern segment of a quasi-stationary front.

2. The CAPE and K indices of both tornado processes were very similar, with the lifting condensation level (LCL) near the ground. There was potential instability in the mid and lower layers before both tornadoes, with the “5.14” process exhibiting greater instability and better thermodynamic conditions. The “5.14” process was mainly due to rotation caused by the deep southwestern jet stream under the strong convergent action of the mid-level shear line, triggered by the surface dry line. The “7.06” process was caused by wind speed pulsations and directional convergence under the influence of the plum rain front, resulting in intense uplift and vertical vorticity, triggered by the surface convergence line.

3. During the “5.14” tornado process, the supercell storm over the surface dry line tilted due to strong vertical wind shear. This strong rotation led to the formation of smaller-scale cyclonic vortices near the hook echo, which subsequently developed into a tornado. In the “7.06” process, the tornado developed over a bow echo and comma-shaped mesoscale convective system formed over the plum rain front. The “comma-shaped” convective system, influenced by dry air subsidence, entrainment, and the convergence of the southeast jet stream, stimulated a “miniature” supercell.

In summary, both tornado events were complex meteorological phenomena influenced by a range of factors including atmospheric instability, moisture content, and specific dynamics such as wind shear and vortical structures. This comparative analysis offers valuable insights into the meteorological conditions conducive to tornado formation and provides an important reference for future tornado forecasting and warning systems, aiming to mitigate meteorological disaster losses.

This paper mainly compares two severe tornado events, enhancing the understanding of the occurrence, development, and evolution of tornado processes in Jiangsu Province. However, there are some limitations: (1) Under the background of global warming, whether severe tornadoes have increased is a point of concern, which relates to the relatively small sample size of tornadoes [25,26,27,28]. However, previous work has indicated that global warming might not directly affect the occurrence of tornadoes within Jiangsu, since no significant linear correlation could be found between the surface temperature and the tornadoes’ occurrence frequency [25]. Whether numerical models can simulate severe tornado processes depends on the understanding of tornado occurrences [29,30,31,32]. The next step involves using the WRF model to conduct numerical simulations. The differences in severe storm occurrences between China and the United States are also a topic worthy of attention [14,33]; for example, the primary instability mechanism involves the presence of a strong low-level jet stream that brings warm, moist air into the central U.S. Instability in China often arises from the interaction of tropical air masses with mid-latitude frontal systems. The presence of the East Asian monsoon plays a crucial role in providing the necessary moisture and heat.