On some aspects of the predictability of the 18 August 2022 derecho using a 910-members convective scale ensemble prediction system

On 18 August 2022, Europe was hit by an extreme convective phenomenon. In the second half of the night, a derecho formed in the Mediterranean and successively hit several countries such as Spain, France, Italy, Slovenia, Austria and the Czech Republic. This exceptional meteorological event, with wind gusts reaching a maximum of 62.2 m/s, caused extensive damage in its path and led to numerous fatalties in several countries with the highest death tolls reported in Corsica and in the Alps.

The aim of our work is to better understand and document various aspects of the predictability of such extreme phenomena. Using the operational convective ensemble forecasting system of Météo-France AROME-EPS, we built a large ensemble of hundreds of forecasts. The initial states of the forecasts come from the 26 (25 perturbed + 1 unperturbed) analyses of the operational AROME ensemble of data assimilations and the 35 (34 perturbed + 1 unperturbed) lateral boundary conditions come from the operational global ensemble prediction system of Météo-France, leading to a 910-members convective-scale forecast ensemble. The automatic bow echo detection tool developed by Mounier et al (2022) has been used to examine several aspects of the predictability of the derecho. First, we evaluated whether using hundreds of members in an ensemble prediction system improves our ability to anticipate the occurrence of the derecho. We examined several aspects of the predictability of the phenomenon, such as its trajectory, chronology and intensity. We also assessed the ability of this extended ensemble to produce more consistent probabilistic forecasts over time than the operational ensemble forecasting system. Finally, we examined the sensitivity of the forecasts ability to predict such an event to their initial states and to their coupling members.