Normal versus anomalous thunderstorms, a comparison of electrical cells properties observed with the SAETTA LMA over the Corsican island

The north-western Mediterranean basin often experiences thunderstorms with heavy precipitation and intense lightning activity causing damages to this densely populated area. This study is conducted within the framework of the EXAEDRE (EXploiting new Atmospheric Electricity Data for Research and the Environment) project that aims to better monitor the thunderstorms in this area through a better understanding of the physical processes that drive the dynamics, the microphysics and the electrical activity of the convective systems. These thunderstorms can exhibit distinct vertical charge structures (normal and anomalous) that produce lightning flashes with different properties. The goal of this study is to compare these characteristics (CG production, flash polarity...) according to both charge structures as measured in Corsica.

The study evaluates the properties of both types of Corsican storms at the electrical cell scale. Hence, observations of the LMA (Lightning Mapping Array) SAETTA network, deployed in Corsica, are used to document in 3D the total lightning activity. Complementary 2D lightning observations recorded by the French LLS (Lightning Locating System) METEORAGE are also used. We also add Météo France weather radar data to document the cumulative rainfall associated to each electrical cell. A clustering algorithm is applied on the lightning data to identify and track the cells. Then we extract lightning and radar data for each cell to document the evolution of several lightning-related parameters during their lifetime. We also apply a recently published method to automatically infer the vertical structure of the electrical charge regions within each cell. These algorithms allow us to create a database of hundreds of electrical cells in Corsica for the period of study (June – October 2018).

We first introduce the different observations and methodologies applied here. Then we present the geographical and temporal distribution of the normal and anomalous cells over the study period. Finally we compare the electrical properties associated to these different vertical charge structure configuration. Overall, anomalous cells represented around 15% of the cells population in Corsica over the study period. Anomalous storms produced less lightning jumps per cell but produced more CGs relative to the total number of flashes per cell. We also show that anomalous cells tend to form shorter flashes. The relationship between number of CGs and cumulative rainfall in Corsica for both charge structure is linear and in accordance with previous results.