# How Saharan dust and elevated convection can produce negative dipole charge structure ?

Gabriel Hausknost, Sylvain Coquillat, Dominique Lambert, Stéphane

Pédeboy, Pierre De Guibert, Eric Gonneau

► To cite this version:

Gabriel Hausknost, Sylvain Coquillat, Dominique Lambert, Stéphane Pédeboy, Pierre De Guibert, et al.. How Saharan dust and elevated convection can produce negative dipole charge structure?. Atmospheric Research, 2026, 335, pp.108783. 10.1016/j.atmosres.2026.108783. hal-05528215

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Submitted on 26 Feb 2026

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# How Saharan dust and elevated convection can produce negative dipole charge structure ?

Gabriel Hausknost<sup>a, ∗</sup>, Sylvain Coquillat<sup>a</sup>, Dominique Lambert<sup>a, †</sup>, Stéphane Pédeboy<sup>b</sup>, Pierre de Guibert<sup>a</sup>, Eric Gonneau<sup>a</sup>

## ARTICLE INFO

Keywords:

Lightning flash

Negative dipole charge structure

Mediterranean thunderstorms

## ABSTRACT

The LMA network SAETTA has observed thunderstorms from 2014 to 2023. During this period, 54 thunderstorm days with negative dipole charge structures were observed during African dust flow events over the Corsica island, mainly during the warm season. The spatial and temporal distribution of thunderstorm days provided by SAETTA data shows nocturnal events with negative dipole charge structure, without preferential regions (even on land and on sea), while classic thunderstorms over Corsica with normal tripole charge structure occur mainly over mountains in the daytime. In the case of negative dipole events, METEORAGE data show more intense and frequent +CG lightning flashes than classical thunderstorms, and 89.4% of -IC lightning flashes. Furthermore, ERA-5, reanalysis data and radiosonde data exhibit dry and warm low layers, with a temperature inversion near the surface, forcing the triggering of elevated convection. This observation could be a consequence of the radiative impact of Saharan dust. This study shows no evidence for the microphysical impact of aerosols, i.e. the multiplication of ice crystals despite supercooled water droplets. Meteorological data also exhibit intense wind speeds associated with negative dipole thunderstorms.

## 1. Introduction

The Mediterranean Sea is a unique area located between two very different climates (a temperate one at its North and a desert one at its South), surrounded by mountains, regularly affected by marine heatwaves and heavy rainfall events and behaving like a hotspot for global warming (Lionello, 2012). Due to this specificity, outstanding measurement campaigns have been carried out in this area, such as the Hydrological cycle in the Mediterranean Experiment (HyMeX) project (Ducrocq et al., 2014). Within the overall framework of this campaign, Corsica was considered as : (i) an observation point located upstream of phenomena affecting the mainland, (ii) a host site for measurement campaigns, some of which have become permanent observatories, and (iii) an ideal natural laboratory for investigating Mediterranean atmospheric phenomena over a mountainous island, such as thunderstorms (Lambert et al., 2011). In this context and in frame of the Plateforme Corse d'Observations Atmosphérique (PCOA, https://corsicaobs-mip.fr/), a Lightning Mapping Array (LMA) called Suivi de l'Activité Electrique Tridimensionnelle Totale de l'Atmosphère (SAETTA) was deployed in Corsica in 2014 to study lightning flashes and thunderstorms in the Mediterranean region (Coquillat et al., 2019). Among the studies using this LMA, one highlights the presence of

anomalous charge structures (i.e. negative dipoles, consisting of an upper negative charge over a lower positive charge) during African dust flow over Corsica (Coquillat et al., 2022). They separated these negative dipoles into 2 types: rare strong convective events, and numerous fast-moving and small thunderstorms. This study is focused on fast-moving and small thunderstorms.

The physical process behind the electrification of thunderstorms is the non-inductive charging mechanism (without a pre-existing electric field). This mechanism is based on the exchange of electrons during the collisions between ice particles and graupel in the presence of supercooled water droplets (Saunders, 2008). In a typical Liquid Water Content (LWC) range, collisions bring a negative (positive) charge to the graupel and a positive (negative) charge to ice particles for temperatures below (above) approximately -10 °C. Due to the sedimentation of the graupel and the upward advection by updrafts of ice crystals, positively charged ice particles are found at the top of the thundercloud, negatively charged ice crystals and negatively charged graupel are found at the middle of the thundercloud, and positively charged graupel are found at the bottom of the thundercloud. This separation of charges, which is usually observed in updraft areas, is

E-mail address: gabriel.hausknost@utoulouse.fr (G. Hausknost).

<footnote><sup>a</sup> Univ Toulouse, CNRS, IRD, LAERO, 14 avenue Edouard Belin, Toulouse, 31400, France</footnote>

<footnote><sup>b</sup> Météorage, Pau, 64000, France</footnote>

<footnote>* Corresponding author.</footnote>
defined as a normal tripole (Stolzenburg et al., 1998; Bruning et al., 2014; Fuchs et al., 2015). This kind of tripole charge structure can produce two types of intra-cloud (IC) lightning flashes: positive for the upper positive dipole, and negative for the lower negative dipole (Fuchs et al., 2015). However, some observations show a more complex charge structure, particularly in downdraft areas, supercell thunderstorms, or mesoscale convective systems (Stolzenburg et al., 1998; MacGorman et al., 2008).

Laboratory studies show a positive charge on the graupel regardless of temperature when the LWC is very low or very high (Takahashi, 1978; Jayaratne et al., 1983; Pereyra et al., 2000; Saunders et al., 2006; Emersic and Saunders, 2010). These laboratory studies explain the anomalous charge structures observed in situ, for example in the Great Plains or the southeastern United States (Lang et al., 2004; Rust et al., 2005; Tessendorf et al., 2007; MacGorman et al., 2008; Stough and Carey, 2020; Stough et al., 2021). However, other laboratory studies (Saunders et al., 1991; Saunders and Peck, 1998) have shown different behaviours of the graupel charging depending on temperature and LWC. They have highlighted a negative charging of the graupel with low LWC. In the present study, we use the Takahashi's representation, as results from more recent laboratory studies (Saunders et al., 2006; Emersic and Saunders, 2010) are closer to those of Takahashi (1978) than to those of Saunders and Peck (1998), who used a different experimental protocol from the other studies (see Saunders (2008) for more details on the experimental protocol for each experiment). In addition, some recent studies used the same representation based on the same assumption (Pawar et al., 2017; Medina et al., 2022b; Barthe et al., 2025). Some supercells in the United States exhibit anomalous charge structures. These structures appear to be related to larger and wider updrafts, particularly in the mixed phase of the cloud, which can produce larger amounts of ice crystals in its upper part and disrupt the LWC within the cloud as well as its electrical structure (Stough et al., 2021). A high cloud base may also be a condition for the formation of anomalous charge structures. A high cloud base reduces the residence time of raindrops in the warm phase of the cloud, which will grow less through coalescence and collision, thus increasing the number of small droplets carried into the mixed phase by updrafts. The more small droplets there are in the mixed phase, the higher the LWC will be, and the more graupel will be positively charged, inducing anomalous charge structures (Williams, 2006; Medina et al., 2022a). Thunderstorms over the Qinghai-Tibet Plateau exhibited negative dipole charge structures during the initial phase of development (Li et al., 2017). This anomalous structure is the result of relatively high temperatures in the mixed phase. At the end of thunderstorm's life, interaction with another storm cell caused a change in its electrical structure. Other anomalous charge structures have been observed in the same region (Qie et al., 2005; Liu et al., 2024; Wei et al., 2025). In Argentina, some thunderstorms with low-altitude cloud bases have been observed with a negative dipole charge structure (Medina et al., 2022b). These anomalous thunderstorms appear to be related to a significant warm phase of the cloud, increasing the residence time of raindrops and thus reducing the number of droplets in the mixed phase, resulting in a low LWC and a negative dipole charge structure.

Another study conducted in India shows the correlation between aerosols and charge structure, with thunderstorms exhibiting negative dipoles with high Aerosol Optical Thickness (AOT) and dry low layers (Pawar et al., 2017). Despite the hypothesis explained in Williams (2006) (see previous section), a credible assumption is that desert dust brings more Solid Condensation Nuclei (hereafter SCN, also called Ice Nuclei Particles (INP) or Ice Formation Nuclei (IFN) in the specialized literature) which leads to fewer droplets, lower LWC, and thus potentially a negative dipole charge structure. Three phenomena may explain this point. Firstly, the more INPs there are in the atmosphere, the more they will collide with supercooled water droplets, and the more the number of ice crystals will increase at the detriment of the droplets (Heterogeneous nucleation by contact, see Kanji et al.,

2017). Secondly, the more ice crystals there are in the cloud due to the heterogeneous nucleation, the more water droplets riming on ice crystals and the lower the LWC will be (Vongpaseut and Barthe, 2025). Thirdly, the saturation vapour pressure is higher above ice than above liquid water, which favours the growth of ice crystals at the detriment of supercooled water droplets (Bergeron effect). Sun et al. (2023) and Gangane et al. (2023) observed a high proportion of positive cloud-to-ground lightning flashes associated with thunderstorms in the presence of dust, probably linked to an anomalous charge structure. A more recent study conducted in India also shows a higher proportion of +CG lightning flashes in the context of dust aerosol, probably caused by an increase in the amount of ice crystals leading to an anomalous charge structure (Gangane et al., 2025). Barthe et al. (2025) simulated a negative dipole charge structure with high AOT in large-scale organized systems in Corsica. They show that the anomalous charge structure is linked to low LWC, and they assumed the effect of Saharan dust on SCN multiplication. Another hypothesis concerning the effect of Saharan dust on the anomalous electrification of small and fast-moving thunderstorms is the radiative effect. Indeed, dust aerosol can cool the surface by backscattering solar radiation and warm the base of the free atmosphere by absorbing radiation (Chen et al., 2010; Francis et al., 2022), triggering elevated convection. The lack of water vapour in the free atmosphere involved in elevated convection induces low LWC and therefore anomalous charge structure (Coquillat et al., 2022). An idealized study using MESO-NH explores the influence of Liquid Condensation Nuclei (hereafter LCN, also called Cloud Condensation Nuclei (CCN) in the specialized literature) and SCN on the charge structure of thunderstorms (Vongpaseut and Barthe, 2025). On the one hand, the charge structure of the cloud always corresponds to a negative dipole when there is a high amount of SCN, regardless of the concentration of LCN, which is consistent with Barthe et al. (2025). On the other hand, the charge structure of the cloud always corresponds to a negative dipole when there is a low amount of LCN, regardless of the concentration of SCN. In both cases, the cloud exhibits an increase in ice crystals at the expense of water droplets. The main sources of LCN are near the surface (pollution, vegetation, etc.), so the concentration of LCNs in the boundary layer is generally higher than in the free atmosphere (Tornow et al., 2022). As the negative dipoles over Corsica are probably elevated convection, the low LCN concentration could explain the negative dipole charge structure.

Elevated convection is defined as convection triggered above the boundary layer, within the free atmosphere (Berry et al., 1945). Elevated convection is widely recognized, both from operational meteorological and research perspective, as one of the least predictable and mainly nocturnal convective regimes (Flack et al., 2023). In the United States, a measurement campaign was conducted to study elevated convection over the Great Plains, named the Plains Elevated Convection at Night Field Project (PECAN) (Geerts et al., 2017). The study shows that elevated convection is triggered by gravity waves produced by downdrafts from other thunderstorms that encounter stable low levels. Generally, this convection occurs near the primary thunderstorm. However, over the Great Plains, a case on 3-4 July 2015 shows an isolated elevated convection thunderstorm likely triggered by gravity waves, without primary thunderstorms nearby Wilson et al. (2018).

The aim of the present study is therefore to highlight the specific meteorological and electrical characteristics associated with this elevated convection with a negative dipole charge structure in Corsica and to discuss the various hypotheses. The present paper is organized as follows: Section 2 describes the methodology; Section 3 presents the results, and Section 4 is devoted to discussion and conclusions.

## 2. Methodology

### 2.1. SAETTA data

Operational since 2014, the 3D lightning flash imager based on 12 LMA stations (see their positions in Fig. 1 in green circles), designed
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Satellite image of Corsica and its surroundings, showing the radiosonde launch site (cyan triangle), the SAETTA station (green circle) and the study area (red square; 7.6385–10.5585°E, 41.2564–43.4148°N).</figcaption></figure>

by New Mexico Tech in the United states, uses Very High Frequency (VHF) waves (60–66 MHz) emitted by lightning flashes to map them with high temporal resolution, around 40 ns in each 80 µs window. Its performances are described by Coquillat et al. (2019). The study area is limited to a 240 km square centred on Corsica, due to the blind area caused by the roundness of the Earth and due to location accuracy and detection efficiency, which decrease with distance (Boccippio et al., 2001; Thomas et al., 2001; Koshak et al., 2004; Thomas et al., 2004; Fuchs et al., 2016; Chmielewski and Bruning, 2016).

LMA networks are useful for determining the polarity of lightning flashes and their parent thunderclouds. The negative leader, marked by its discontinuous propagation, creates rapid fluctuations in electrical current over time, resulting in the generation of powerful VHF emissions. In contrast, the positive leader propagates continuously, with weaker VHF sources emitted along its path. As negative (positive) leaders propagate in positive (negative) charge regions, identifying the polarity of the leaders allows the charge structure of the cloud to be deduced.

The present study uses all SAETTA data from 2014 to 2021, as well as a few days of negative dipoles in 2022 and 2023 in order to obtain a better sample of these events. 54 days of negative dipole thunderstorms and 41 days with Saharan dust and normal tripoles were selected over the whole period. The 775 days of SAETTA data were also used to highlight differences between negative dipole thunderstorms and classical thunderstorms. Quicklooks are available on this website: https://saetta.aeris-data.fr/. We only considered VHF sources with a

minimum of 7 stations participating in the solutions and the goodness of fit given by $\chi^2 < 0.5$ (Thomas et al., 2004; Coquillat et al., 2019).

SAETTA data can be used to plot 2D maps of thunderstorm days with a spatial resolution of 0,1° × 0,1°, temporal distribution of thunderstorm days with a temporal resolution of 10 min and vertical profiles of VHF sources with a resolution of 100 m. On the 2D map, a thunderstorm day is defined by at least 5 VHF sources in a pixel of 0,1° × 0,1° (Coquillat et al., 2022). In the temporal repartition, a thunderstorm day is defined by at least 50 sources across the entire study area in 10 min. This threshold was chosen by determining the minimum number of VHF sources emitted by one lightning flash. A day with an active thunderstorm with small horizontal dimensions was studied to determine the threshold. By counting the number of VHF sources for each lightning flash, a statistical study was carried out. This study shows that 95% of all lightning flashes emit more than 50 VHF sources. We therefore considered that if 50 VHF sources or more are detected throughout the study area in 10 min, this can be considered a thunderstorm day. These thresholds make it possible to separate out noise from lightning flash data.

## 2.2. Radiosonde data

Radiosonde (RS) data are primary data used for the study and forecasting of meteorological phenomena. National weather institutes launch radiosondes into the atmosphere at 0000 UTC and 1200 UTC all
days (Météo-France). These radiosondes are used in this study to obtain information about the atmosphere in a vertical description.

For each day with a negative dipole, an accurate weather analysis is carried out in order to choose the radiosonde that probes the stormy atmosphere at the most suitable location and time. Three sites are used depending on weather conditions: Ajaccio, Pratica di Mare (near Rome), and Decimomannu (in southern Sardinia). Their locations are indicated in Fig. 1 (blue triangle). 44 radiosondes are used in the study, 29 launched at 0000 UTC and 15 launched at 1200 UTC. The average of all radiosondes associated with negative dipoles is shown with the associated standard deviation. There are fewer radiosondes than thunderstorm days for two reasons: some radiosondes at 0000 UTC are used for two consecutive days of thunderstorms, and some days do not have radiosonde data. In some cases, the first data points, close to the ground, are removed due to an overvaluation of the temperature caused by the launch system. All of this data has been graphically represented and studied with MetPy (May et al., 2022).

Three other averaged radiosondes were computed for the following situations: all storm days in the period 2014–2021 (636 RS), days with Saharan dust and normal tripoles (40 RS), and days with Saharan dust and without thunderstorms (331 RS). The averaged radiosondes of these three situations are made from radiosondes launched at Ajaccio at 1200 UTC.

## 2.3. Aerosol data

Aerosol data are necessary for studying electrical processes and cloud formation, with significant microphysical feedback between these processes. Aerosol data used are the AOT at 550 nm of the Goddard Earth Observing System data (GOES-5) from the Global Modeling and Assimilation Office (GMAO, Rienecker et al., 2008) and the Copernicus Atmosphere Monitoring System (CAMS) visualized and extracted through the Earth Nullschool website (https://earth nullschool.net/). The time, latitude and longitude are chosen based on the time and location of the radiosonde launch, i.e. above Ajaccio (41.92° N 8.73°E), at 0000 UTC or 1200 UTC. Data is not available before 2016, so the study of aerosol interactions is limited to the period 2016–2021. These data were compared with the MODIS satellite images to confirm that the high optical thicknesses were indeed related to African dust flows. A day with Saharan dust is considered to be when the AOT exceeds 0.1.

## 2.4. ERA-5 reanalysis

ERA-5 is the fifth generation of the European Center for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis of the global climate, covering the period from January 1940 to the present day. It is produced by the Copernicus Climate Change Service (C3S) at ECMWF. As in Sections 2.1 and 2.2, data are averaged (Scheffknecht et al., 2017) for different situations: days with negative dipole (55 events), all thunderstorm days at Corsica in the period 2014–2021 (772 events), days with Saharan dust and normal tripole (46 events) and days with Saharan dust and without thunderstorm (365 events). These data are chosen at 0000 UTC or 1200 UTC depending on the proximity of the peak electrical activity of thunderstorms, or the proximity of an AOT ≥ 0.1 for days without thunderstorms. This data enables accurate studies to be carried out on the weather context of each situation.

## 2.5. Lightning flash detection network METEORAGE

Météorage, a subsidiary of Météo France, has been operating the METEORAGE Lightning Locating System (LLS) since 1987. This system collects real-time lightning flash data across Western Europe using Vaisala's latest lightning detection technology, namely LS7002 sensors, operating in Low Frequency (LF) connected to the Total Lightning Processor (TLP). METEORAGE locates IC pulses and cloud-to-ground

<table><caption>**Table 1**
Number of stormy days as a function of dust conditions.</caption><thead><tr><th></th><th>Stormy days</th><th>Negative dipole days</th><th>Normal tripole days</th><th>No stormy days</th></tr></thead><tbody><tr><td>Dust</td><td>95</td><td>54</td><td>41</td><td>263</td></tr><tr><td>No-dust</td><td>680</td><td>0</td><td>680</td><td>1382</td></tr><tr><td>Total</td><td>775</td><td>54</td><td>721</td><td>1645</td></tr></tbody></table>

(CG) return strokes which are grouped into flashes according to spatio-temporal criteria (Cummins et al., 1998). In addition to the position, the system provides the time of occurrence, peak current, and polarity for each discharge, supplemented by quality parameters. These parameters are the confidence ellipse evaluating the theoretical location error and the CHI² (χ²) representing the deviation between the measurements of the sensors participating in the solution. Lightning flash data with a χ² > 0.5 or with a 1/2 major axis (SMA) of the confidence ellipse > 1 km are discarded from the dataset. These data, with SAETTA data, allow a multi frequency study of negative dipoles.

# 3. Results

## 3.1. Aerosol data

Firstly, throughout the manuscript, the study is divided into four different situations: negative dipole (with dust) days, normal tripole with dust days, no stormy days with dust and finally stormy days without dust. In order to provide a better representation of each sample and its frequency, Table 1 shows the distribution of days according to thunderstorm frequency and occurrence of dust between 2014 and 2021.

First, it is useful to have an overview of the aerosol context in Corsica. The most favourable period for dust presence in Corsica seems to be during the warm season, particularly between April and July (Fig. 2, a), with 32 to 43 days of Saharan dust accumulated over a 6-year period for each month, or 5–7 days per month on average each year. Saharan dust over Corsica is directly linked to dust concentration in North Africa and with Southerly flows (Michelot et al., 2015). From October to February, aerosol content is at its lowest level in North Africa. From March to May, this content increases, persists until August and decreases in September (Parker et al., 2020). The distribution of dust days in Corsica corresponds to this point. Negative dipoles occur mainly between June and July, with approximately 50% of all negative dipole days occurring during these months. Normal tripoles with Saharan dust are more evenly distributed between April and November.

Coquillat et al. (2022) studied AOT as a function of wind direction for stormy days. Using the same approach, Fig. 2.b and c show AOT as a function of wind speed and direction for dusty days. Negative dipoles (blue dots) generally correspond to higher AOT than normal tripoles (red dots). This result is consistent with the impact – radiative or microphysical – of Saharan dust on the electrification of negative dipoles. Indeed, a higher AOT corresponds to a higher concentration of Saharan dust, and therefore to better interaction between solar radiation and the dust layer for the radiative impact, and to an increase of SCN inducing a multiplication of ice crystals for the microphysical impact (see introduction). However, there are few events with relatively low AOT (< 0.2), mixed with normal tripole events. For these events, the negative dipole charge structure could not be linked to Saharan dust, despite the presence of southerly flows, as the dust concentration may not be sufficient to have a significant impact on the thunderstorm electrification. Furthermore, negative dipoles have more southerly winds and stronger winds than normal tripoles, which probably allows for less alteration of Saharan dust. For a very high AOT (around > 0.8), the majority of dusty days have negative dipoles, particularly with southern flows. Most normal tripole days have low
<figure><img src="image_5.png" /><figcaption>Fig. 2. (a) Monthly repartition of dusty days (AOT > 0.1). (b) AOT for dusty days as a function of wind direction at 850 hPa in °. (c) AOT for dusty days as a function of wind speed in km/h. In this figure, days without thunderstorms are shown in grey, days with negative dipoles are shown in blue and days with normal tripole are shown. The data covers the period 2016–2021 in Corsica. Wind comes from the Earth visualization tool (https://eath nullschool.net).</figcaption></figure>

AOT and low wind speed. These events are probably not affected by Saharan dust, due to too low concentration of Saharan dust. Finally, there are some dusty days with northern flows. These events correspond to dust advection from a dust reservoir in the North of Corsica island, or incorrect dust detection, with an AOT > 0.1 linked to certain windless days with pollution accumulation.

## 3.2. Meteorological context

### 3.2.1. Thermodynamical data

Thermodynamic processes are essential for determining the structure and intensity of convective systems in thunderstorms development. Radiosondes are commonly used to forecast and study thermodynamic aspects, particularly under conditions of instability. Radiosondes averaged across all events in a given situation are shown in Fig. 3.

Typically, thunderstorms in Corsica (Fig. 3.d) occur due to warm and wet low layers (between 1000 and 800 hPa) and relatively drier and cooler air aloft, which leads to atmospheric destabilization and formation of thunderstorms. Radiosondes from normal tripole events with Saharan dust (Fig. 3.b) are very similar to those from classic thunderstorms in Corsica, indicating that these thunderstorms may have similar behaviour and characteristics to classic thunderstorms.

Negative dipole radiosondes (Fig. 3.a) are clearly different from those of classic thunderstorms in Corsica: just above the surface, these situations present a warm and dry layer (between 950 and 700 hPa), causing a temperature inversion in the low layers that stabilizes them. These data therefore suggest a strong Surface Based Convection Inhibition (SBCIN), with a median SBCIN of –350 J/kg in these cases (not shown). These warm layers, combined with humidification around 600 hPa and relatively cool air at altitude, induce destabilization of the free atmosphere, allowing elevated convection to be trigger in these layers. Finally, there appears to be mixing between 800 and 600 hPa, as indicated by a temperature curve close to the dry adiabatic line. This mixing may be an indicator of elevated convection.

No-stormy days with Saharan dust (Fig. 3.c) have a relatively warm and dry profile across all atmospheric columns, with a temperature inversion near the surface, preventing the formation of thunderstorms, possibly due to the backscattering of solar radiations by Saharan dust combined with anticyclonic subsidence.

Finally, analysis of the radiosondes shows that the formation of negative dipoles is closely linked to the presence of warm and dry low layers. The origin of these low layers could be explained by the radiative impact of desert aerosols. This statement could be unlikely regarding the temperature profile for normal tripole in dusty conditions
<figure><img src="image_6.png" /><figcaption>Fig. 3. Radiosonde (RS) data, average in curve and standard deviation in colour range, with temperature (red), temperature of wet bulb (blue) and dew point temperature (cyan) for four situations: days with negative dipoles (a), days with normal tripoles and Saharan dust (b), non-stormy days with Saharan dust (c), and all days with thunderstorms in Corsica in the period 2014–2021 (d).</figcaption></figure>

(Fig. 3.b), in which dry and warm low layers do not appear. However, a more detailed analysis of Fig. 2.b,c shows that the AOT is significantly lower in most tripole events, so the radiative impact should not be large enough to alter the temperature profile. Liu et al. (2024) and Wei et al. (2025) studied thunderstorms with an anomalous charge structure over the Tibetan Plateau. Liu et al. (2024) observed moist low layers, while negative dipoles over Corsica present an environment with warm and dry low layers. However, in the study by Wei et al. (2025), the anomalous charge structure is triggered in an environment with dry low layers, as in the case of negative dipoles over Corsica. Thus, both cases suggest that dry low layers may be conducive to the development of an anomalous charge structure.

### 3.2.2. Dynamical data

In addition to thermodynamic processes, studying wind profiles across the troposphere is essential to understanding the movement and organization of thunderstorm cells. Two types of data are used to study wind: radiosonde data, which display wind speed and direction with hodographs in Fig. 4 and wind speed in Fig. 5a, and ERA-5 reanalysis data, which show an averaged 2D map of wind speed and direction for negative dipole days at 850 hPa in Fig. 5b.

All Corsican thunderstorms (Fig. 4.d) have mainly westerly winds at all altitudes, with wind speed increasing with height. Normal tripoles with dust (Fig. 4.b) and no stormy days with dust (Fig. 4.c) appear to have the same wind speed, but with south-westerly wind to advect Saharan dust to Corsica. Negative dipole days (Fig. 4.a) seem to have stronger and more southerly winds than other situations at all altitudes, particularly around 1000 m with southerly winds of around 20 km/h, which probably advect a better and less altered amount of Saharan dust. This result is consistent with Section 3.1. Moreover, the clockwise wind veering with altitude is an indicator of warm air advection from a warm region.

Fig. 5.a shows the average wind speeds as a function of altitude for the four situations. This graph shows stronger winds at all levels in the case of negative dipoles (green curve) than in the other cases, particularly between 2000 and 6000 m altitude, where other curves are outside the standard deviation of negative dipole days (green range), consistent with Fig. 4. Fig. 5.b shows the wind speed and direction for negative dipoles at 300 hPa. The values are consistent with Fig. 5.a, around 90 km/h, an important value since it is an average. Corsica is located in the jet stream, near updraft areas (red outlines in Fig. 5.b), which can probably contribute to the triggering of thunderstorms.
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Averaged hodographs for 4 different situations in the period 2014–2021: days with negative dipoles (a), days with normal tripoles and Saharan dust (b), non-stormy days with Saharan dust (c), and all days with thunderstorms in Corsica (d). Colour line shows wind speed and direction of the average, and grey lines show each situation. Wind speed is represented by the distance between the curve and the centre of the plot, and the value is represented by the grey circles plotted every 25 km/h. If the curve follows the north array, this means that the wind is coming from the south and blowing northward.</figcaption></figure>

Finally, since negative dipoles feed on the free atmosphere rather than surface air, their movement is not coupled to surface processes. They are therefore subject only to mid-level and high-level winds, which, as observed, are generally strong and even stronger in the case of negative dipoles. These two phenomena explain the high travel speed of negative dipole cells.

### 3.3. Electric trend

#### 3.3.1. SAETTA data

SAETTA data are presented in terms of spatial and temporal distributions of stormy days and in terms of vertical profiles of VHF sources. Fig. 6 displays SAETTA data for three situations: negative dipoles, normal tripoles during dusty days, and all stormy days in Corsica. All thunderstorms in Corsica (Fig. 6c) have a maximum number of stormy days generally above mountains, particularly in the north-central part of the island. This type of convection is typical of diurnal convection, as shown in Fig. 6f, with a maximum occurrence around 1300 UTC. Normal tripole days have almost the same spatial and temporal distribution (Fig. 6b, e), with a lower frequency on the west side of Corsica, probably due to south-westerly winds pushing orographic thunderstorms eastward, and also a lower frequency near the Gulf of Genoa.

However, negative dipoles behave very differently, with a relatively homogeneous spatial representation of stormy days (Fig. 6a). Nevertheless, there is a relative minimum in the frequency of thunderstorms in the northern quarter of the island. This minimum is probably due

to the passage of stormy air masses over the Corsican mountains: the warm and dry low layer is forced to rise to higher altitudes, where it interacts with thunderstorms above, drying them out and causing it to dissipate. Fig. 6d shows another unusual characteristic of negative dipole events: thunderstorms seem to occur more at night than during the day, with a peak in thunderstorm days after midnight. This representation shows that negative dipoles do not need daytime heating to form, which is a recurring feature of elevated convection. This atypical behaviour may be related to gravity waves. Indeed, gravity waves can form when an outflow enters stable lower layers, which are then used as waveguides. Some air particles can rise to altitudes on the wave crests, triggering convection. Gravity waves spreading across the boundary layer form thunderstorms known as elevated convection (Geerts et al., 2017). Isolated elevated convection thunderstorms can also form due to gravity waves in the presence of pre-existing mesoscale uplift (Wilson et al., 2018). Section 3.2.2 shows large-scale uplift, providing a favourable context for the triggering of elevated convection by gravity waves. These gravity waves may be triggered by previous thunderstorms upstream of Corsica, or by a crossing mountain range (Corsican relief, Atlas, Pyrenees, etc.). For example, a Meteosat Second Generation (MSG) satellite image from 21/07/2018 shows cloud lines over Corsica (Fig. 7) a sign of a probable gravity wave propagating from the Southwest to the Northeast. One of these cloud lines will form a negative dipole in this case.

The vertical profiles of VHF sources are similar in all cases of Corsican thunderstorm days and normal tripoles with Saharan dust (Fig. 6i, h). These profiles are typical of normal tripoles, with a dominant
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** (a) Averaged wind speed over Corsica (curve) as a function of altitude for 4 situations: days with negative dipoles (green), days with normal tripoles and Saharan dust (orange), no-stormy days with Saharan dust (red), and all days with storms in Corsica in the period 2014–2021 (blue). The green colour range represents the standard deviation in the case of negative dipoles. Each curve is a moving average over 5 indices before and after the calculation point in order to remove noise. (b) Average wind speed (colour range) and direction (arrow) for negative dipoles at 300 hPa, with updraft area (red) and downdraft area (blue).</figcaption></figure>

positive pole at the top of the thunderstorm, at about 10 km. As a reminder, positive charges are associated with a high density of VHF sources, because negative leaders spreading in the positive charge emit heavily in the VHF band. For negative dipoles (Fig. 6g), the vertical profile of VHF sources shows a dominant positive pole at the bottom of the thunderstorm, at about 5 km.

### 3.3.2. METEORAGE data

METEORAGE data correspond to the low-frequency emissions from lightning flashes resulting from more intense and vertically developed discharge phases, such as return strokes. They enable the polarity of lightning strikes to be determined and vertical IC lightning strikes to be distinguished from CG lightning strikes. This data complements that from the SAETTA network, which does not provide information on the intensity of discharges and does not enable accurate +CG mapping.

As expected, negative dipoles in Corsica exhibit a majority of negative intra-cloud lightning flashes (-IC consisting of positive upward leaders and negative downward leaders), as illustrated by the low proportion of +IC (11%) in Fig. 8a. This proportion of +IC is significantly and logically lower than that observed by Pédeboy et al. (2018) who

studied lightning flashes throughout France. They found a proportion of up to 53% for +IC/IC (Fig. 8a). The few +IC detected in negative dipoles may have several origins: (i) very temporary electrification in normal tripoles of thunderstorms, (ii) poor detection of short +CG.

As regards cloud-to-ground lightning flashes, the proportion of +CG observed during negative dipole events in Corsica (between the ground and the lower positive pole of thunderstorms) is rather high (19%, see Fig. 8b). It is obviously higher than that observed by Poelman et al. (2016), who studied the characteristics of CG in Europe using the Euclid network. Their climatology shows an average +CG/CG ratio of 15%. Furthermore, as negative dipoles occur mainly during the summer season in Corsica, the 19% of +CG in negative dipoles should be compared more accurately with that observed by Poelman et al. (2016) also in summer (10%, see Fig. 8b). This comparison further confirms the specific characteristics of CG lightning flash in negative dipoles. Although to a lesser extent, Salvador et al. (2021) also observed higher +CG/CG rates in negative dipoles, particularly at the beginning or end of an event. This result is consistent with a lower positive charge. Finally, Sun et al. (2023) and Gangane et al. (2023) found a higher proportion of the +CG/CG ratio in thunderstorms with high AOT environment, which may explain the higher ratio of negative dipole cases in Corsica.

According to Poelman et al. (2016) and Honoré et al. (2014), the median values of peak currents for -CG and +CG for EUCLID (in West Europe), GLD360 (in France), and METEORAGE (in France) are: -15 kA/22 kA, -13,7 kA/11,5 kA, -12,1 kA/12,4 kA. Salvador et al. (2021) (Catalonia) showed a median value for negative dipoles of -13,6 kA/+15,8 kA, they indicated that these structures have the highest current peaks in this region. Negative dipoles in Corsica have median CG peak currents of -9 kA and 25 kA (Fig. 9). -CG lightning flashes appear to be much less intense than in other studies, while +CG lightning flashes appear to be more intense. These differences could be related to different lightning flash detection systems, and different study areas, with a different proportion of mountains and sea that can alter the intensity of lightning flashes. These differences may also be related to a significantly lower positive charge due to the negative dipole charge structure. Salvador et al. (2021) explain the increase in CG power in the case of negative dipoles as being linked to a low flash rate in these structures. They explain that weak convection would tend to favour the appearance of large charge regions, which would cause infrequent but more intense lightning flash. Negative dipoles over Corsica have the same order of magnitude of flashrate as negative dipoles in Salvador et al. (2021), i.e. respectively 2.0 flash.min⁻¹ and 3.0 flash.min⁻¹.

Qie et al. (2005), Liu et al. (2024) and Wei et al. (2025) studied anomalous charge structures and the occurrence of IC/CG lightning flashes over the Tibetan plateau. Comparison between these studies and the present study reveals some similarities. Firstly, thunderstorms over the Tibetan plateau exhibit recurrent -IC flashes interspersed with -CG flashes. This phenomenon is also observed in negative dipoles over Corsica. Secondly, Qie et al. (2005) observed a low lightning flash rate, with an average value of 1 flash.min⁻¹, similar to the negative dipoles in the present study, which have an average flash rate of 2 flash.min⁻¹. However, there is a considerable difference between these studies and the present study: they did not observe +CG lightning flashes while we observed 19% of +CG lightning flashes. This difference could be related to differences in the charge structure: in Corsica, there are only negative dipoles, while on the Tibetan plateau, there are “bottom heavy” charge structures, which can modify the electric field and thus the occurrence of +CG lightning flashes.

## 4. Discussion and conclusion

Throughout the discussion, the Takahashi's (1978) representation is used to explain the negative dipole charge structure hypothesis with low LWC. It should be noted that the Saunders and Peck's (1998)
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Stormy days per 0.1° × 0.1° (a,b,c), stormy days per 10 min (d,e,f) and vertical profiles of VHF sources (g,h,i) for three situations: negative dipoles (a,d,g), normal tripoles during dusty days (b,e,h) and all stormy days at Corsica (c,f,i). The orange range in panel e and f shows the maximal thunderstorm frequency at daytime.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Satellite image from MSG at 10.8 µm the 21/07/2018 at 0600 UTC.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** (a) Proportion of +IC/IC for negative dipoles over Corsica and for all thunderstorms in France according to Pédeboy et al. (2018) (b) Proportion of +CG/CG for negative dipoles over Corsica and for all thunderstorms in West Europe according to Poelman et al. (2016).</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Box plot for negative dipole in Corsica of peak intensity for 4 types of lightning flashes: +CG, -CG, +IC and -IC. Median from negative dipole in continue line and median from non-identify charge structure in dotted line (all thunderstorms in West Europe according to Poelman et al., 2016, all thunderstorms in France according to Honoré et al., 2014).</figcaption></figure>

representation shows different behaviour in the non-inductive charging process of graupel for low LWC.

An important hypothesis concerning the anomalous electrification of thunderstorms is the microphysical impact of Saharan dust (Pawar et al., 2017; Barthe et al., 2025). However, in the present study, no strong correlation can be established between AOT and electrical structure, even though stormy days with an AOT greater than 0.4 are almost systematically associated with negative dipoles, which account for 26% of all negative dipole events. Saharan dust may be more effective as a SCN than as a LCN. Indeed, a study conducted in Tenerife (near Morocco) shows that the Saharan air layer is more effective as a SCN than as a LCN (Boose et al., 2016). Although this study was conducted in a different location, the origin of the dust layer (the Sahara desert) and the distance between the desert and the study area (approximately 500 km for Tenerife, and 1000 km for Corsica) are similar, so the behaviour of desert dust is likely to be similar in the context of the present study. Based on this assessment, the negative dipole charge structure of thunderstorms could be hypothetically explained by the microphysical impact of Saharan dust (see Introduction for more details).

It is assumed that Saharan dust present during negative dipole events will have a radiative impact more or less linked to the formation of these anomalous electrical cloud structures. Such an impact may occur with the backscattering of solar radiation (direct effect), which causes the surface to cool and thus stabilizes the lower layers, making the meteorological context less conducive to the triggering of convection fed by the surface air. As a result, thunderstorms can only occur in free atmosphere (a phenomenon known as upper convection). It can also occur through the absorption of solar radiation by the dust layer, causing local warming (semi-direct effect) which destabilizes the overlying atmosphere and thus also promotes the triggering of elevated convection. However, most of these negative dipole events occur during the night in the Corsican region (Fig. 6d). At night, ground radiation can cause a temperature inversion near the ground, so the temperature inversion observed in Fig. 3 may also be related to this phenomenon. On the other hand, a temperature inversion in the lower layer can also occur in environmental contexts characterized by relatively low optical thickness (around 0.1). Consequently, the concentration of aerosols that drives radiative effects should not be a key parameter, or the radiative impact may not be as effective as expected in triggering elevated convection.

Negative dipoles not associated with strong convective events over Corsica are most often associated with elevated convection, i.e. convection triggered above the atmospheric boundary layer, in the free atmosphere. However, at these levels, the water vapour concentration is relatively low, so clouds fed by such amounts of water vapour logically contain low LWC. This means that the charge acquired by graupel during non-inductive collisions is positive, leading to a negative dipole charge structure. In other words, elevated convection is likely to lead to the formation of thunderclouds with negative dipole structure, if the lower part of the free atmosphere is not supplied with sufficient amount of water vapour. Moreover, in an environment rich in Saharan dust, the microphysical impact of desert dust can further reduce the LWC, leading to a negative dipole charge structure.

In addition, negative dipoles can also result from an excess of SCN (Barthe et al., 2025) or a lack of LCN (Vongpaseut and Barthe, 2025). In both cases, ice crystal production is favoured at the expense of the formation of liquid water droplets, leading to low LWC values and thus to consistently positive electrification of graupel, resulting in a negative dipole structure of the storm cell. In the first case, the excess of SCN could be provided by Saharan dust residing at the bottom of the free atmosphere, which feeds elevated convection. In the second case, the high stability of the lower layers observed in radiosondes corresponding to negative dipole events (see Section 3.2.1) explains the difficulty of LCN to be injected into the free atmosphere and thus feed water droplet formation.

According to Williams (2006) and Medina et al. (2022a), an elevated cloud base appears to lead to a negative dipole charge structure due to a shorter residence time of liquid droplets in the warm phase, inducing high LWC in the mixed phase where the non-inductive charging process occurs, and thus positive electrification of graupel. In the
present study, negative dipoles seem to have high cloud bases, but they
are generated by convection triggered in the free atmosphere, where
the concentration of water vapour is relatively low. Consequently, the
low water vapour concentration that can feed the thundercloud is
incompatible with a high LWC value. Thus, the hypothesis of Williams
(2006) and Medina et al. (2022a) does not seem relevant for the
negative dipoles observed in Corsica.

There is a non-negligible proportion (approximately 26% of all
events) of negative dipole thunderstorms in Corsica with cloud tops
at altitudes below the -40 °C isotherm, the level at which the upper
positive cloud layer is generally found (Fuchs et al., 2015). For these
events, the electrical structure appears as a negative dipole, probably
because the relatively weak vertical development of the cloud does
not allow the formation of an upper positive layer. This might be
comparable to the "lower dipoles" observed by Salvador et al. (2021)
in South Catalonia.

This work has enabled a better characterization of negative dipoles
in Corsica. First of all, negative dipoles appear to be events that occur
mainly during the warm season. Moreover, the study shows that these
thunderstorms only occur in meteorological conditions characterized
by dry and warm low levels and high wind speed. These data confirm
that all negative dipoles that are not part of organized thunderstorms in
Corsica originate from elevated convection. This context also provides
initial clues about the triggering of these thunderstorms, which appear
to be linked to large-scale updrafts at high altitude, associated with
gravity waves in the lower layers. The electrical data revealed the
atypical behaviour of negative dipoles, which form preferentially at
night and away from mountains, contrary to the typical conditions of
classical thunderstorms in Corsica. They are also unique in terms of
lightning flashes, with a higher than normal proportion and intensity
of +CG, and a higher proportion of -IC, which corresponds to a negative
dipole charge structure.

The role of Saharan dust is quite difficult to identify from ob-
servational data. Its radiative role may be linked to the temperature
inversion in the lower layers, but there is no solid causal link on this
point. The anomalous electrification of some of these thunderstorms
seems to be linked to their low cloud top height, which prevents the
accumulation of a sufficient upper positive charge to trigger lightning
flash in the upper part of the thunderstorm.

It would be appropriate to study these negative dipoles using mod-
els in order to confirm or refute each hypothesis. Sensitivity studies
would be the best approach, in particular by varying the Saharan
dust concentration, in order to observe the reaction of the convection.
Modelling can also be a good way to studying the influence of gravity
waves in the triggering of elevated convection, as there are not enough
effective observation methods to study gravity waves in Corsica. Some
thunderstorms change their electrical structure as they move, changing
from negative dipole to normal tripole, or some negative dipole thun-
derstorms coexist alongside normal tripoles. A study of these atypical
cases could be interesting in order to compare the behaviours and the
environments of both charge structures.

CRediT authorship contribution statement

**Gabriel Hausknost:** Writing – original draft, Visualization, Validation, Software, Methodology, Investigation, Formal analysis, Conceptualization. **Sylvain Coquillat:** Writing – review & editing, Validation, Supervision, Resources, Methodology, Investigation, Formal analysis, Conceptualization. **Dominique Lambert:** Writing – review & editing, Validation, Supervision, Methodology, Investigation, Formal analysis, Conceptualization. **Stéphane Pédeboy:** Writing – review & editing, Resources, Data curation. **Pierre de Guibert:** Writing – review & editing, Resources, Data curation. **Eric Gonneau:** Writing – review & editing, Resources, Data curation.

Declaration of competing interest

The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.

Acknowledgements

This work is supported by a French ministerial doctoral grant awarded by the SDU2E Doctoral School. Acknowledgments are addressed to SAETTA's main sponsors: Collectivité Territoriale de Corse through the Fonds Européen de Développement Régional of the European Operational Program 2007–2013 and the Contrat de Plan Etat Région that funded the CORSICA project; Collectivité de Corse through the CORSICA 2017–2019 project; CNRS-INSU through the HyMeX/MISTRALS programme; ANR IODA-MED; CNES through the SOLID project; UT/Observatoire Midi-Pyrénées; and Laboratoire d'Aérologie. We would also like to express our sincere thanks to the many individuals and regional institutions in Corsica (in particular the Conservatoire du Littoral, Qualitair Corse and INRA San Giuliano) who host the 12 stations in the network, who helped us find sites or who provide us with logistical support during field missions. The authors would like to thank Météorage for providing data on lightning flash activity, the French data and service centre AERIS/ICARE, which provided MSG/SEVIRI data for the CTT, the French meteorological service Météo France for the radiosonde data published on its public website, the independent intergovernmental organisation ECMWF for the ERA-5 reanalysis, and the Earth website for GOES and CAMS AOT data. This work was supported by ANR Grant SAPHIR project ANR-21-CE04-0014.

Data availability

Data will be made available on request.

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