Six years of electrified convection over the island of Corsica monitored by SAETTA: General trends and anomalously electrified thunderstorms during African dust south flow events
Résumé
By means of the SAETTA 3D lightning mapping array, the total lightning activity has been detected in a 240 km × 240 km square domain centred on the island of Corsica located in the West Mediterranean basin, and characterized by a maritime and mountainous environment, with a complex and relatively high relief. The study period covers the months from April to December of the 6 years from 2014 to 2019. Observations are reported with a horizontal resolution of 1 km and a vertical resolution of 0.1 km in terms of density of VHF sources emitted by lightning, and of number of lightning day, in plane- and vertical- projections. Vertical distributions of VHF sources are also provided monthly for the whole period and over the full domain. These 3D long term observations show that the number of lightning days is more important on the main relief of the island. The density of VHF sources exhibits a sharp maximum over the confluence area of the 3 main valleys in the center of the northern part of the island, characterized by a recurrent and vertically well developed lightning activity. The period from 11:00 UTC to 14:00 UTC in July, and in a lesser extent in June and August, is at the origin of this density maximum. This behavior is thus due to a diurnal convection. The whole lightning activity is characterized by 2 maxima in June and September. The first one is linked with the diurnal convection in phase with the maximum elevation of the Sun. The second one must be due to usual large-scale organized thunderstorm events of the fall season. The vertical distribution of VHF sources exhibits an increase in the number of VHF sources from April to August at all altitudes, as well as an increase in the altitude of the main upper peak (from 5.8 km to 10 km) and of the secondary lower peak (from 4.2 km to 6.7 km) of this distribution. From August to December, the opposite evolution appears, except that October is characterized by a strong anomaly with a single intense lower peak at 5.5 km, apparently due to 2 strong events corresponding to anomalously electrified thunderstorms. Several of these anomalously electrified thunderstorms (negative dipoles) are reported. They are all characterized by the movement of cloud cells spatially small and relatively undeveloped vertically propagating from southwest to northeast and associated with a strong transport of desert dust from the African continent. The analysis of the meteorological environment of some of the studied events allows concluding that in each of the studied cases elevated convection was triggered, above the relatively dry atmospheric boundary layer characterized by a strong convective inhibition. The low cloud liquid water content which is a key parameter of the non-inductive charging process is probably a good candidate to explain the anomalous electrification of these events.