Thunderstorm Structure and Lightning Properties in South China and over the South China Sea: A Comparative Study

1.   Introduction

Thunderstorms are convective events that are often accompanied by severe weather such as lightning, hail, and strong winds. Therefore, thunderstorms are a primary focus of meteorological warnings and forecasting. Comprehensive understanding of the differences between land and oceanic thunderstorms can advance our knowledge on the mechanisms of thunderstorm formation and deve-lopment. Furthermore, improved understanding of the differences between land and oceanic thunderstorms can help improve techniques for nowcasting and forecasting thunderstorms, particularly in ocean areas where meteo-rological observations are limited.

Previous studies found substantial differences between land and oceanic thunderstorms in terms of occurrence probability and convective structure. The occurrence of thunderstorms over land is much greater than that over ocean areas because of the presence of strong updrafts and intense convection in land storms (Heymsfield et al., 2010). Wu et al. (2020) investigated the geographical distribution of convective storms globally using 16-yr Tropical Rainfall Measurement Mission (TRMM) data. They found that convective storms are distributed mainly in land areas, i.e., the tropics, the northern part of South America, and the Maritime Continent. Qie et al. (2014) found that the convective properties and structural features of deep convective systems (with 20-dBZ echo top heights exceeding 14 km) vary substantially between regions of different terrain heights. The occurrence of deep convective systems and intense deep convective systems (with 40-dBZ echo top heights exceeding 10 km) is more frequent over land regions than over the ocean. In contrast, oceanic thunderstorms exhibit relatively weak convection and updrafts, with rapid reduction in the vertical reflectivity profile above the freezing level (Zipser and LeMone, 1980; Jorgensen and LeMone, 1989). Bang and Zipser (2016) investigated the differences in the size of thunderstorms between land and ocean areas. Their results showed that the thunderstorm area gradually increases from land to the deep ocean, being approximately 10 times larger over the central Pacific than in land areas of Congo. The proportion of convective precipitation gradually decreases from 80% in deep ocean thunderstorms to approximately 40% in land thunderstorms [see Fig. 8 in Bang and Zipser (2016)]. Despite the evidence of a gradual evolution of thunderstorm features in terms of a spatial pattern from land, to offshore areas, and then to the deep ocean, no studies have so far been conducted specifically on the thunderstorms over transi-tional water areas.

Another notable disparity between land and oceanic thunderstorms is the formation of the precipitation process. Oceanic thunderstorms generally possess a stronger warm rain process, characterized by raindrop collision and descent prior to reaching the freezing level (Szoke and Zipser, 1986). Earlier studies indicated that thermodynamic factors are the major contributors to the differences between land and oceanic thunderstorms (Lucas et al., 1994; Stolz et al., 2015). A recent study utilized a global atmospheric model to predict the trends in convective properties between ocean and land areas in the tropics (Wu et al., 2023). It was found that a warmer and wetter atmosphere will lead to increase in both the occurrence frequency and the cloud top height of overshooting deep convection over oceanic areas, but cause only a relatively weak increase in deep convection frequency over tropical land areas.

In addition to the structure of thunderstorms, there are notable differences in the properties of lightning activity between land and oceanic thunderstorms. Previous studies reported markedly higher frequency and density of lightning over land, with 10 times more lightning occurrences over land areas than over the ocean (Christian et al., 2003). Compared with oceanic thunderstorms, land thunderstorms are more likely to produce lightning when they exhibit similar brightness temperatures or radar reflectivity at the same altitude (Cecil et al., 2005). However, oceanic thunderstorms are capable of producing lightning when the vertical velocity exceeds a threshold of 6–7 m s⁻¹ (Zipser and Lutz, 1994). Using data from the U.S. National Lightning Detection Network (NLDN), Nag and Cummins (2017) revealed the differences in lightning properties between ocean and land areas by examining the downward leader characteristics of negative first return strokes. They found that the median duration of negative first stroke leaders was considerably (5–11-ms) shorter for oceanic lightning than for lightning over land. This shorter duration might be related to the higher oceanic return stroke peak currents, indicating that the cloud charge structures of oceanic storms are different to those of land thunderstorms. Some other recent studies on lightning properties revealed that oceanic lightning flashes exhibit higher current, increased length, longer duration, and more intense radiance than land flashes, despite the lower frequency of oceanic lightning activity (Said et al., 2013; Bang and Zipser, 2015; You et al., 2019).

The dynamic process of a thunderstorm somehow determines the differences observed in lightning properties between land and ocean areas. The weak updrafts within oceanic convective systems attenuate the charging process of ice particles, thus preventing vigorous development of lightning activity in oceanic thunderstorms (Williams and Stanfill, 2002). However, oceanic thunderstorms have larger charge regions, resulting in a greater amount of charge to be neutralized and therefore leading to larger flash size and higher flash energy (Chronis et al., 2015). Statistical analyses based on thunderstorm dynamics (Bruning and MacGorman, 2013; Zheng and MacGorman, 2016; Zheng et al., 2016, 2018; You et al., 2019) revealed that land thunderstorms characterized by stronger dynamical processes tend to be dominated by “charge pockets,” which generate high-frequency and small-scale lightning flashes. In contrast, oceanic thunderstorms with weaker dynamical processes tend to have charge regions with greater horizontal extent, which produce less frequent but larger-scale lightning flashes.

Most previous related studies focused on the differences in thunderstorms between land and deep ocean areas, revealing notable variations in the magnitude of lightning frequency and the convective properties of thunderstorms. However, the question of whether transitional zones exist between land and deep ocean areas with regard to lightning activity and thunderstorm pro-perties has yet to be investigated in a comprehensive way. The South China Sea (SCS) lies between the Asian continent and the Northwest Pacific Ocean, and the thunderstorms in the SCS are affected by both oceanic and land systems. Additionally, among the four major ocean areas adjacent to China, the SCS experiences the most intense lightning (Yuan and Qie, 2004; Ma et al., 2021). South China is a region of Chinese mainland with the highest frequency of thunderstorm and lightning activity. South China and the SCS are both affected by the East Asian monsoon and are in close geographical proximity; therefore, they represent a good case for comparative study of the differences in thunderstorm and lightning between land and transitional water areas to the deep ocean. In this study, lightning and precipitation feature data from the TRMM satellite are used to compare the thunderstorm structure and lightning properties between South China and the SCS. First, statistical analyses of the thunderstorm structure and lightning properties of the two regions are conducted. Second, the differences in thunderstorm structure between the two regions are compared. Finally, a schematic summary highlighting the differences in properties is proposed, and the impact of thunderstorm structure on lightning flash properties over South China and the SCS is revealed.

2.   Data and methods

The data used in this study are obtained from the TRMM satellite observations. The TRMM satellite was operational from November 1997 to April 2015, with a circular orbit of 350 km, inclination angle of 35°, and an observational range extending 35° north and south of the equator. The instrument payloads onboard the TRMM satellite included the Precipitation Radar (PR), the TRMM Microwave Imager (TMI), the Visible and Infrared Scanner (VIRS), and the Lightning Imaging Sensor (LIS) (Kummerow et al., 1998). The TRMM dataset provides detailed measurements of precipitation in rainstorms in the tropical and subtropical regions, and the data are used widely to study the properties of global or regional thunderstorms and lightning activity (Cecil et al., 2005; Albrecht et al., 2016; Liu et al., 2020).

This study employs the TRMM data from 1998 to 2014; however, the period of August 2001 is excluded owing to the satellite’s orbit boost during that time. Two regions, i.e., South China and the SCS, are selected for comparative study. According to the spatial distributions of flash density and thunderstorm density, as reported by Ding et al. (2023) (see their Figs. 3, 5), the region with the highest values of lightning density over the Chinese mainland is selected as the South China domain in this study, and the region of the SCS with the highest values of lightning density and thunderstorm density is selected as the SCS domain. Figure 1 illustrates the spatial extent of the two domains focused on by this study.

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Fig  1.  Geographic locations of the study domains: South China (SC) and the South China Sea (SCS), outlined by red boxes.

2.1   Lightning flash data

The lightning flash data are obtained from the TRMM LIS orbital observations. The LIS products consist of event, group, flash, and area, and this study has used the flash data. Previous studies reported that the LIS detection efficiency is approximately 73% ± 11% during daytime and 93% ± 4% at night (Boccippio et al., 2002). In addition to flash location information (longitude and latitude), four other flash property parameters are used in this study: (1) flash duration (unit: s), i.e., the time difference between the first and last event that constitute a flash; (2) flash radiance (unit: J sr⁻¹ m⁻² μm⁻¹), i.e., the sum of the optical radiance energy of all events in a flash; (3) flash footprint (unit: km²), i.e., the area of spatial expansion of a flash; and (4) flash length (unit: m), i.e., the distance of the horizontal extension of a lightning channel, obtained by calculating the distance between the two farthest events in a flash. All data are quality controlled by excluding extreme data samples. Extreme samples refer to the flashes with length > 1000 km or duration > 3 s, typically caused by algorithmic errors, and the flashes consisting of only a single event with duration of 0 s. The data quality control method employed in this study is consistent with the approach described by You et al. (2019).

2.2   Thunderstorm data

The thunderstorm data are obtained from the TRMM Radar Precipitation Feature (RPF) dataset. This dataset was developed by the University of Utah based on observations from the PR, LIS, TMI, and VIRS instruments onboard the TRMM satellite (Liu, 2007; Liu et al., 2008), and it is used to define cloud and precipitation features. The definition of the RPF refers to the pixels with a precipitation rate of > 0 mm in Level 1 of TRMM 2A25 data, and it contains at least 4 consecutive PR rainfall pixels. This study mainly uses the frequency of the RPF and the PR reflectivity data.

The research samples comprise the RPF samples containing at least one lightning flash, which are referred to as thunderstorms. The total RPF sample size in this study is 3775, i.e., 1852 RPF samples for South China and 1923 RPF samples for the SCS. The thunderstorm pro-perty parameters used in this study comprise the following: 20-/30-/40-dBZ echo top height (defined as the height of the 20-/30-/40-dBZ echo), 37-/85-GHz polarization-corrected temperature (PCT), 20-dBZ area, precipitation area, 20-/30-/40-dBZ volume, flash frequency, and flash density. Among these parameters, the 20-/30-/40-dBZ echo top and the 37-/85-GHz PCT are extracted directly from the RPF dataset. The 20-dBZ area and the precipitation area are calculated by multiplying the pixel area by the number of pixels with PR reflectivity of > 20 dBZ or by the number of pixels with rainfall > 0 mm, respectively. The pixel area was 17.92 km² before the TRMM orbit boost and 20.35 km² after the orbit boost. The RPF dataset provides the number of pixels with reflectivity of ≥ 20/30/40 dBZ at 1-km intervals within the height range of 1–16 km. Echo volumes of varying intensities are calculated by multiplying the number of pixels per kilometer by the corresponding area, and then being integrated vertically to obtain the sum results. In this study, the number of lightning flashes indicates the total number of lightning flashes produced by a thunderstorm. The frequency of lightning flashes indicates the number of lightning flashes per unit time (i.e., 1 h or 1 min). The flash frequency of a thunderstorm is obtained by dividing the number of flashes in the RPF samples by the observation time. The flash density of a thunderstorm is obtained by dividing the flash frequency by the 20-dBZ area. The area with reflectivity of > 20 dBZ is defined as the thunderstorm area, consistent with that in Liu et al. (2008).

2.3   Statistical methods for correlation

The commonly used Pearson and Spearman correlation coefficients are selected for the statistical analysis performed in this study. The Pearson correlation coefficient assesses the degree of linear association between variables, whereas the Spearman correlation coefficient evaluates both the strength and the direction of monotonicity in the relationship. These two correlation coefficients are used to examine the correlations between lightning flash properties (e.g., flash frequency, flash density, and duration) in South China and over the SCS. The objective is to assess the adherence of the data to a normal distribution, and to determine the linearity and monotonicity of the relationship between variables.

3.   Results

3.1   Differences in lightning properties between South China and the South China Sea

Flash frequency and flash density indicate the intensity of deep convection in thunderstorms. Figure 2 shows the probability distribution of flash frequency and flash density for thunderstorms in South China and over the SCS. Approximately 42% of thunderstorm samples over the SCS have a flash frequency of < 1 fl min⁻¹ (Fig. 2a), and nearly 70% of thunderstorms exhibit a flash density of < 0.1 fl km⁻² h⁻¹ (Fig. 2b). The average values of flash frequency and flash density over the SCS are 3.56 fl min⁻¹ and 0.12 fl km⁻² h⁻¹, respectively. In compari-son, the lightning activity of thunderstorms in South China exhibits markedly higher intensity, with average values of flash frequency of 7.22 fl min⁻¹ and flash density of 0.28 fl km⁻² h⁻¹, which are twice as high as those over the SCS. It is worth noting that only the thunderstorms in South China reach flash frequency of > 12 fl min⁻¹ and flash density of > 0.8 fl km⁻² h⁻¹. Therefore, the lightning-generating capacity of land thunderstorms is substantially higher than that of oceanic thunderstorms.

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Fig  2.  Probability distributions of lightning flash parameters of thunderstorms in South China and over the South China Sea: (a) flash frequency and (b) flash density.

The lightning extension properties, as indicated by flash duration and flash length, are related to the convective intensity of thunderstorms (Zheng et al., 2021). The probability distributions of flash properties for thunderstorms in South China and over the SCS are illustrated in Fig. 3. It is evident that the probability distribution of flash duration is similar in both regions, with identical mean and median values of 0.3 and 0.26 s, respectively (Fig. 3a). However, the flash properties of land thunderstorms differ markedly from those of oceanic thunderstorms in terms of flash length, radiance, and footprint. The peak flash length over the SCS is concentrated between 10 and 20 km, with a median value of 18.27 km and a mean value of 20.84 km. In contrast, the peak flash length in South China is distributed within the range of 6–16 km, with a median value of 14.88 km and a mean value of 17.36 km (Fig. 3b). These findings suggest that the SCS lightning has markedly greater extension than the lightning occurring over the land of South China.

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Fig  3.  Probability distributions of flash properties in South China and over the South China Sea: (a) flash duration, (b) flash length, (c) flash radiance, and (d) flash footprint.

In addition to flash length, flash footprint can also be used to represent the scale of extension of lightning. The maximum flash footprint over the SCS is 120–300 km², whereas that in South China is 80–200 km². The median flash footprint over the SCS is approximately 73 km² larger than that in South China (Fig. 3d), suggesting that the size and area of oceanic lightning are larger than those of land lightning. Both the flash length and the flash footprint exhibit the pattern of a lognormal distribution, i.e., the probability of lightning occurrence first increases and then gradually diminishes as the size of the lightning grows, with a low probability of extreme large lightning. The median values for flash radiance over the SCS and in South China are 0.39 and 0.26 J m⁻² sr⁻¹ μm⁻¹, respectively, while the corresponding average values are 1.07 and 0.78 J m⁻² sr⁻¹ μm⁻¹, respectively. The average flash radiance over the SCS is larger than that in South China. Approximately 39% of lightning flashes that occur over the SCS exhibit radiance of < 0.2 J m⁻² sr⁻¹ μm⁻¹, while the corresponding proportion in the South China samples is nearly 50% (Fig. 3c). Zhou et al. (2022) found that the spatial scale and the optical radiance of lightning flashes are greater over the Northwest Pacific Ocean compared with those over land, consistent with our results from the SCS and South China. Additionally, our results are also consistent with a previous study on lightning in East Asia and the western Pacific, which found that flash size and flash radiance are the largest over open sea, followed by offshore areas, and the smallest over land (You et al., 2019). Compared with the results of You et al. (2019) and Zhou et al. (2022), we found that the average values of lightning size and lightning radiation over the SCS lie between the values reported for the Pacific Ocean and East Asia. This indicates that oceanic lightning over the SCS has distinct characteristics, and highlights the gra-dual transition of lightning properties of thunderstorms from those over land, to the SCS, and then to deep ocean areas.

The correlation coefficients among different flash properties in South China and over the SCS are presented in Table 1. Overall, the correlations between flash properties in South China and those over the SCS show a high degree of similarity. The correlations between flash length and flash footprint exhibit the highest values, with Pearson (Spearman) correlation coefficients in the range of 0.92–0.93 (0.94–0.95). The correlations between flash length and radiance, and those between flash footprint and radiance are next highest, with Pearson (Spearman) correlation coefficients of 0.50–0.59 (0.78). The lowest correlations are observed between the flash duration and other properties, with Pearson (Spearman) correlation coefficients in the range of 0.27–0.42 (0.33–0.58). The above results are consistent with previous studies based on satellite lightning observations, indicating that the correlations between flash size and optical radiance are substantially higher than those with flash duration (Peterson and Liu, 2013; Peterson et al., 2017).

Table  1.  Pearson (Spearman) correlation coefficients among flash properties over the South China Sea and in South China

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	South China Sea				South China
	Length	Footprint	Radiance		Length	Footprint	Radiance
Duration	0.29 (0.33)	0.27 (0.37)	0.42 (0.53)		0.37 (0.40)	0.33 (0.43)	0.33 (0.58)
Length		0.93 (0.95)	0.57 (0.78)			0.92 (0.94)	0.50 (0.78)
Footprint			0.59 (0.78)				0.56 (0.78)
* All values achieved the significance level of 0.05.

 | Show Table

DownLoad: CSV

3.2   Differences in thunderstorm structure between South China and the South China Sea

3.2.1   Horizontal scale of thunderstorms

Figure 4 shows the probability distribution of thunderstorm precipitation area in South China and over the SCS, obtained by multiplying the number of rainfall pixels by the corresponding pixel area. It is evident that the precipitation area of thunderstorm samples in South China falls predominantly within the lower end of the size spectrum, with peak values distributed within the range of 316.23–1333.52 km². The distribution of thunderstorm samples over the SCS tends to incline toward larger areas, with peak values in the range of 865.96–2371.37 km². The median precipitation area of thunderstorms over the SCS (2299.55 km²) is approximately twice that of thunderstorms in South China (1037.85 km²).

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Fig  4.  Probability distribution of thunderstorm precipitation area detected by precipitation radar (PR) in South China and over the South China Sea.

The area with reflectivity of ≥ 20 dBZ is selected to compare the horizontal scale of thunderstorms over land and over the transitional water area, and the results are illustrated in Fig. 5. Over 64% of the thunderstorm samples over the SCS have a 20-dBZ area of > 1000 km², while this proportion is < 50% in South China. For extreme thunderstorms with a 20-dBZ area exceeding 20,000 km², only offshore thunderstorms are capable of reaching such an intensity. In terms of both the median and the mean values of the 20-dBZ area, it is evident that the area of thunderstorms in South China is only half that of the thunderstorms observed over the SCS. This suggests that the horizontal scale of transitional water thunderstorms also appears to be substantially larger than that of land thunderstorms, and that notable differences exist in the structure of thunderstorms between those in South China and those over the SCS.

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Fig  5.  Probability distribution of thunderstorm area with radar reflectivity greater than or equal to 20 dBZ in South China and over the South China Sea.

The probability distributions of precipitation area in thunderstorms in South China and over the SCS are depicted in Fig. 6. For thunderstorms of South China, seasonal variations are not obvious, with peak precipitation areas of approximately 10³ km² (Fig. 6a), consistent with the findings derived from Fig. 4. Conversely, there is a shift toward larger peak areas in seasonal variation over the SCS. Compared with spring, summer, and autumn, the thunderstorm precipitation area in winter is the largest, with a peak distributed at 10⁴ km² (Fig. 6b), which is an order of magnitude larger than the peak area (10³ km²) of winter thunderstorms in South China. Zhou et al. (2021) also observed a trend of increase in the average area of thunderstorms over the SCS during winter. This can be attributed to the smaller normalized convective available convective energy and the larger vertical wind speed in winter convective systems over the ocean, which are factors that are not conducive to the development of deep convection (Bang and Zipser, 2016).

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Fig  6.  Seasonal variations in precipitation area of Radar Precipitation Feature (RPF) samples with lightning (DJF: winter; MAM: spring; JJA: summer; and SON: autumn) in (a) South China and (b) the South China Sea. Sample sizes in winter, spring, summer, and autumn for South China (South China Sea) are 35 (74), 442 (539), 1122 (730), and 253 (580), respectively.

Figure 7 shows the distribution of thunderstorm precipitation types, represented by the ratio of convective precipitation to total precipitation. The median proportion of convective precipitation in thunderstorms in South China is 0.85, while the corresponding value is 0.77 for SCS thunderstorms. This indicates that the proportion of convective precipitation in thunderstorms in South China is higher than that in SCS thunderstorms. According to Bang and Zipser (2016), the ratio of convective precipitation in Congo, one of the three lightning “chimney regions” on land, is 86%, whereas it is 62% over the Pacific Ocean, highlighting the substantial difference in convective precipitation between land and ocean areas. This study found that the median value of convective precipitation in South China is 85%, similar to that reported for the Congo region. In contrast, the median value of convective precipitation over the SCS is 77%, falling between the proportions observed over land and deep ocean areas, and indicating the transitional characteristics of thunderstorm properties over the SCS.

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Fig  7.  Probability distribution of the ratio of convective precipitation to total precipitation in South China and over the South China Sea. Value of 1.0 (0.0) on the x-axis indicates that all precipitation is convective precipitation (stratiform precipitation). Precipitation data are obtained from TRMM 2A25.

Convective precipitation tends to occur during the developing and mature stages of the associated weather systems, while stratiform precipitation is more prevalent during the mature and dissipating stages (Houze, 1997; Romatschke and Houze, 2010; Bang and Zipser, 2016). Land thunderstorms are capable of generating precipitation in their early stages of development, whereas ocea-nic thunderstorms often produce precipitation during the mature stage of the convective system. Additionally, lightning discharges have the potential to substantially enhance the convective precipitation of land thunderstorms, whereas the precipitation of oceanic thunderstorms is mostly a combination of convective and stratiform precipitation (Mudiar et al., 2021). This to some extent explains the differences observed in convective precipitation between thunderstorms in South China and over the SCS, and the smaller precipitation area and higher convective precipitation ratio of land thunderstorms. It also indicates that thunderstorms over the SCS would produce lightning when they reach a more mature stage compared with land thunderstorms in South China.

3.2.2   Thunderstorm development intensity

The height of strong reflectivity observed by the TRMM satellite radar represents the maximum height that precipitation particles can reach, and it is closely associated with convective intensity. Figure 8 illustrates the probability distribution of echo top heights in the range of 20–40 dBZ for thunderstorms in South China and over the SCS. There is evident disparity in the distribution of the 20-dBZ echo top heights between South China and the SCS (Fig. 8a), with the peak height for South China being 10–14 km and the corresponding value for the SCS being 12–16 km. The average and median values of the 20-dBZ echo top heights over the SCS (in South China) are 12.86 (11.61) km and 13.25 (11.5) km, respectively, indicating that thunderstorms in transitional water areas reach higher altitudes compared with thunderstorms over land by an average of 1.25 km (median: 1.75 km).

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Fig  8.  Probability distributions of different reflectivity echo top heights of thunderstorms in South China and over the South China Sea: (a) 20-dBZ echo top, (b) 30-dBZ echo top, and (c) 40-dBZ echo top.

For 30- and 40-dBZ echo top heights, the difference between land and ocean areas is not statistically significant. The median values of 30-dBZ echo top heights over the SCS and in South China are 9.25 and 9.0 km, respectively (Fig. 8b), i.e., those over the ocean areas are slightly higher than those over land. It is worth noting that as the reflectivity increases, the difference in reflectivity heights between land and ocean areas dimi-nishes. When the reflectivity reaches 40 dBZ (Fig. 8c), the average reflectivity height over land (6.25 km) is larger than that over ocean areas (6.09 km). Notably, only thunderstorms in South China are capable of producing a 40-dBZ echo top height that exceeds 10 km. This suggests that the intensity of land thunderstorms is much stronger and that the vertical extent of their deep convection is higher than the corresponding values of oceanic thunderstorms.

In addition to the reflectivity echo top height, the volume of different reflectivity echoes can also serve as an indicator of the intensity of thunderstorm development. Figure 9 shows the probability distribution of 20-/30-/40-dBZ reflectivity volumes in thunderstorms. The probability of occurrence decreases with increase in the reflectivity volume in both regions (Figs. 9a–c). Approximately 40% of thunderstorms in South China exhibit a 20-dBZ volume of < 4000 km³, nearly 50% display a 30-dBZ volume of < 2000 km³, and nearly 50% have a 40-dBZ volume of < 500 km³. The volumes of different radar reflectivities of thunderstorms over the SCS are larger than those in South China. When the 20-dBZ volume is > 50,000 km³, the 30-dBZ volume is > 40,000 km³, or the 40-dBZ volume is > 8000 km³, only thunderstorms over the SCS can attain such large values. In terms of the 20- and 30-dBZ volumes, the median and mean values in South China are only half those over the SCS. The mean value of the 40-dBZ volume in South China is approximately half that over the SCS, while the median value in South China closely resembles that over the SCS. The 40-dBZ echo top height over the SCS is slightly lower than that in South China (Fig. 8c), but the mean 40-dBZ volume is twice that in South China (Fig. 9c), which is attributable to the larger area of thunderstorms over ocean areas.

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Fig  9.  Probability distributions of different reflectivity volumes of thunderstorms in South China and over the South China Sea: (a) 20-dBZ volume, (b) 30-dBZ volume, and (c) 40-dBZ volume.

Brightness temperatures at 85 and 37 GHz in the microwave spectrum are designated indicators of ice scattering signals owing to their sensitivity to scattering by ice particles within clouds. The magnitude of the ice scattering signal depends on the wavelength, vertical distribution, phase, concentration, and size of the hydrometeors. As the number of ice particles in the cloud increases, there is corresponding increase in the scattering intensity and reduction in the brightness temperature of the channel received by a satellite. The PCT reduces the effect of the underlying surface through combining the vertical and horizontal polarization channels (Spencer et al., 1989). The 85- and 37-GHz PCTs can be used to characterize the development intensity and spatial extent of thunderstorms (Cecil et al., 2005; Liu et al., 2012). The probability distributions of the 85- and 37-GHz mini-mum PCTs are illustrated in Fig. 10. The peak values of the 85-GHz PCT in South China are distributed at 180–240 K, while the corresponding peaks over the SCS are distributed at 150–200 K, i.e., the values over ocean areas are lower than those over land (Fig. 10a). The median (mean) value over the SCS is approximately 30 K (20 K) lower than that in South China. Figure 10b shows that the differences in the 37-GHz PCT between land and ocean areas are not statistically significant.

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Fig  10.  Probability distributions of the minimum polarization correction temperature (PCT) in thunderstorms in South China and over the South China Sea: (a) minimum 85-GHz PCT and (b) minimum 37-GHz PCT.

Because the 85-GHz wavelength (3.5 mm) is shorter than the 37-GHz wavelength (8.1 mm), it is more sensitive to smaller-sized ice particles. Particles with diameter of hundreds of micrometers are in the Mie spectrum at 85 GHz, but the diameter of particles in the 37-GHz Mie spectrum expands to the scale of millimeters (Cecil et al., 2002). The results show that the mean 37-GHz PCT values over the SCS and in South China are comparable, but that the mean 85-GHz PCT value over the SCS is lower, indicating a higher concentration of smaller-sized ice particles in oceanic thunderstorms. This is associated with the larger area and higher vertical development of oceanic thunderstorms (Figs. 5, 8), although the concentration of large ice particles is comparable between thunderstorms in both regions. Tropical convective clouds are characterized by high cloud top heights, and ice-phase processes play an important role in the formation of precipitation (Wang and Yin, 2011). Hu et al. (2014) analyzed the raindrop spectrum during convective precipitation over the SCS and found that particles with diameter of < 1 mm accounted for a large proportion of the raindrops, showing notable distinctions in raindrop spectra when compared with convective precipitation over land.

4.   Discussion

Despite the complex formation mechanism of thunderstorms and lightning, the above results to some extent reveal the differences in the structure and lightning properties of thunderstorms between South China and the SCS. Figure 11 presents a conceptual diagram highlighting these property distinctions.

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Fig  11.  Conceptual diagram highlighting the differences in thunderstorm structures and lightning properties between thunderstorms in South China and over the SCS. Mean values of echo top height are extracted from Fig. 8 to draw the structural features of the thunderstorm. The brown region represents the elevation difference between land and ocean areas.

4.1   Lightning properties

The number and frequency of lightning flashes in SCS thunderstorms are lower than those in thunderstorms in South China, but SCS lightning exhibits greater flash properties such as length, footprint, and radiance. Although land thunderstorms have smaller-sized lightning flashes, they exhibit higher flash frequency and greater flash density, compared with SCS thunderstorms (Fig. 11).

The differences in lightning flash properties between land and ocean areas might be attributable to differences in the convective structure and the underlying surfaces. According to the non-inductive charging mechanism (Takahashi, 1978), a lightning discharge process involves particle-scale collisions between rimed ice particles such as graupel and ice crystals in the presence of supercooled liquid water. Therefore, lightning production is associated with a vigorous mixed-phase ice process in a cumulonimbus cloud. Stronger convective intensity means stronger updrafts, more ice particles, and a larger liquid water content in the mixed-phase region, thereby leading to substantial increase in lightning activity. Earlier studies showed that flash properties (i.e., duration, length, and radiance) are proportional to thunderstorm area but inversely correlated with convective intensity. Ding et al. (2023) proposed an inverse relationship between thunderstorm intensity and the average values of lightning flash size and radiance in the Northwest Pacific region, indicating that stronger convective intensity is accompanied by smaller flash size and lower flash radiance. This inverse relationship also exists in land thunderstorms (Calhoun et al., 2014; Zheng and MacGorman, 2016; Zhang et al., 2017; Souza and Bruning, 2021).

The inverse correlation between convective intensity and flash size might be related to the charge distribution patterns that are dominated by varying convective intensities (Bruning and MacGorman, 2013). Land thunderstorms have more unstable dynamical conditions (e.g., wind shear and turbulence) and are more prone to produce high-frequency lightning owing to the relative fragmentation of charge regions within the cloud and the closer proximity of charge regions of different polarities. However, charge regions within land thunderstorms are small and therefore the spatial expansion of lightning channels is generally smaller. In contrast, oceanic thunderstorms exhibit relatively weak convective intensity, with the distribution of charge regions within the cloud being dominated by large horizontal expansion and vertical stratification. In this context, flash frequency in oceanic thunderstorms is relatively low, yet the presence of large charge regions results in notable horizontal extension of lightning channels. Thus, land thunderstorms with enhanced convection are more likely to produce small-scale charge regions, conducive to higher flash frequency and smaller spatial expansion of lightning channels. Conversely, oceanic thunderstorms with relatively weak convection are more likely to sustain larger charge regions and greater spatial expansion of lightning channels, but with lower flash frequency.

Other studies examined the relationship from the perspective of the underlying surface. For example, Chronis (2012) found positive correlation between the intensity of lightning currents and oceanic salinity. Tyahla and López (1994) proposed that the higher conductivity of the oceans accelerates the neutralization of lightning charges, resulting in higher energy output. Additionally, Boccippio et al. (2000) emphasized that forcing conditions are required for the generation of strong convection and lightning over ocean. Lightning originates within convective systems that are induced by meteorological forcing mechanisms (e.g., gust fronts or similar boundary features) over the ocean (Szoke and Zipser, 1986; Pessi and Businger, 2009). Owing to the complex interplay between flash properties and the dynamic and microphysical processes of thunderstorms, the factors that affect flash properties can vary under different thunderstorm conditions.

4.2   Thunderstorm structure

Figure 11 shows that SCS thunderstorms exhibit a larger scale compared with thunderstorms in South China, characterized by greater horizontal expansion, increased vertical development (20-dBZ echo top height), and a larger precipitation area. Thunderstorms in South China are of smaller scale than SCS thunderstorms, but they exhibit stronger intensity characterized by higher deep convection cores and higher strong (> 40-dBZ) echo top.

Thermodynamic parameters such as warm cloud thickness and lifting condensation height can explain the differences in precipitation area between oceanic and land thunderstorms (Stolz et al., 2015). External forcing is essential for initiation of oceanic convection, leading to the development of strong updrafts and subsequently larger horizontal and vertical scales of oceanic thunderstorms (Bang and Zipser, 2016). Compared with oceanic thunderstorms, land thunderstorms have a lower cloud base and higher lifting condensation heights, which reduce the vertical extent of warm rain processes and increase the amount of liquid water that can extend above the freezing level and the mixed-phase region. Consequently, precipitation from land thunderstorms occurs earlier and covers a smaller area (Bang and Zipser, 2016).

In terms of convective intensity, oceanic convective clouds have weaker updrafts because of their larger horizontal scale, which results in rapid dilution of buoyancy through entrainment effects (Lucas et al., 1994). Because of the weak updrafts over ocean areas, substantial amounts of warm rain fall below the freezing level, thereby hindering the development of convection and lightning (Szoke and Zipser, 1986; Pessi and Businger, 2009). Land thunderstorms have higher levels of unstable energy and convective available potential energy owing to uneven surface heating, leading to more robust updrafts and stronger convective intensity (e.g., higher 30- and 40-dBZ echo top heights).

Recent studies attributed the differences between land and ocean convection to aerosol concentrations. The lower concentration of cloud condensation nuclei over the ocean, compared with that over the land, leads to coalescence and growth of water droplets before freezing, resulting in the formation of warm rain that suppresses the development of strong convection (Rosenfeld et al., 2008; May et al., 2011). Previous observational and numerical studies demonstrated the effects of aerosols on the increase in lightning frequency (e.g., Khain et al., 2008; Wang et al., 2011; Yair, 2018). Enhancement of lightning activity is related to increase in the concentration of cloud condensation nuclei. Using the WRF-ELEC model, Sun et al. (2021) revealed that lightning activity is notably enhanced under polluted conditions, especially during the developing and mature stages of thunderstorms. They found that elevated aerosol concentrations increase the number of cloud droplets, enhance latent heat release, and promote updrafts in clouds, and that an increase in the number of ice particles (e.g., graupel) in the upper levels enhances the non-inductive charge transfer and the electrification process. Conversely, in cases where aerosol concentrations are low, it was determined that latent heat release is lower, updrafts are weaker, and smaller concentrations of ice particles are present in the upper levels; consequently, charging rates are lower and lightning frequency is reduced.

Our study revealed that SCS thunderstorms have lower 85-GHz PCTs than land thunderstorms, consistent with findings of previous studies. Toracinta et al. (2002) suggested that if a fixed probability of lightning occurrence is set for both land and ocean, the 85-GHz brightness temperature of an oceanic convective system is 50 K smaller than that over land. A possible reason for the higher brightness temperature of systems over land is the microwave radiation effect of a substantial amount of supercooled water in the mixed-phase region. On the other hand, Bang and Zipser (2015) highlighted that the TRMM TMI might overlook regions of the high ice water content in small-scale land thunderstorms owing to its swath and resolution, resulting in higher 85-GHz PCT values for land thunderstorms, compared with those of oceanic thunderstorms.

Based on our analysis, together with comparison with the results of previous studies, we establish that there is a gradual transition from land to the SCS and then to the open ocean in terms of the properties of thunderstorm scale and the proportion of convective precipitation. For example, the median proportion of convective precipitation gradually decreases from land to ocean, i.e., it is 85% in South China, 77% over the SCS, and 62% in the open Pacific Ocean [results from Bang and Zipser (2015, 2016)]. Integrating the results of our study with those of Bang and Zipser (2015, 2016), in terms of the region of thunderstorm activity, reveals that the SCS is twice as large as the land area (South China/Congo), and the open ocean (Pacific Ocean) is 10 times larger than the land area (Congo), i.e., the area of thunderstorm activity is progressively larger. Our findings suggest that SCS thunderstorms have oceanic properties, but they also exhibit properties that distinguish them from open-ocean thunderstorms, which are attributable to the influence of land.

5.   Conclusions

Based on TRMM LIS lightning data and TRMM RPF cloud and precipitation property data obtained during 1998–2014, this study focused on two specific regions, i.e., South China and the SCS, and conducted comparative statistical analysis of the thunderstorm structure and lightning properties over land and the ocean. The study also explored the impact of thunderstorm structure on lightning properties. The main conclusions are as follows.

(1) The lightning activities and flash properties of thunderstorms over the SCS and in South China exhibit notable differences. Lightning activities in South China are more intense than those over the SCS, while the mean values of flash properties over the SCS are higher than those in South China. Although the average flash frequency and flash density over the SCS are only half those in South China, with similar flash duration, lightning over the SCS exhibits a greater length, larger footprint, and higher radiance compared with that in South China. The correlation between flash size and flash radiance is stronger than that between flash size and flash duration in both regions.

(2) The horizontal scale of thunderstorms over the SCS is larger than that of thunderstorms in South China, with both the precipitation area and the 20-dBZ area approximately twice as large as those in South China. There are obvious seasonal variations in the precipitation area of thunderstorms over the SCS, with notable enhancement in winter. While convective precipitation samples account for 85% of the total precipitation in South China, they constitute only 77% of the total precipitation samples over the SCS.

(3) The vertical development of SCS thunderstorms is higher than that of South China thunderstorms, but the convective intensity of thunderstorms in South China is stronger and the height of the deep convection is much higher. The average height of the 20-dBZ echo top over the SCS is 1.25 km higher than that in South China, whereas the 40-dBZ echo top height is markedly higher in South China thunderstorms. The average 37-GHz PCT is comparable between both regions, but the average 85-GHz PCT is lower over the SCS, indicating a larger content of smaller-scale ice particles within SCS thunderstorms.

(4) A conceptual diagram is provided to illustrate the differences in the structure and lightning properties between land and oceanic thunderstorms. The diagram highlights the differences in terms of horizontal scale, vertical development, convective intensity, flash frequency, and flash properties. The notable distinctions in lightning properties between land and ocean areas are closely related to the thermodynamic processes of thunderstorms, but also to the underlying surface conditions, weather system forcing conditions, and other relevant factors.

(5) Compared with previous studies, this study has identified the distinct characteristics of thunderstorm and lightning properties (e.g., thunderstorm size, ratio of convective cloud precipitation, convective intensity, and the footprint, length, and radiance of lightning) over the SCS, and highlighted the gradual transition of these characteristics from land, to the SCS, and finally to the deep ocean area of the Northwest Pacific Ocean.

Previous studies showed that diurnal propagation of thunderstorm activity exists over the South China coast (Li et al., 2021; Zhu et al., 2024). However, because such diurnal variations are beyond the scope of this study, daytime and nighttime samples were not separated in the database. Therefore, the results obtained in this study are climatological characteristics based on statistical analy-sis. Furthermore, thunderstorm activities in South China and over the SCS are affected by various factors that include the monsoon circulation patterns, thermodynamics, microphysics, solar radiation, and underlying surfaces. Therefore, the specific reasons behind the differences in lightning properties between land and ocean areas require further investigation.