Linking torrential flood event occurrence to weather-type conditional driving atmospheric conditions — The case of the Northern French Alps

Grenoble Metropolitan area, Northern French Alps. The Alpine region is strongly affected by torrential floods, sometimes leading to severe negative impacts on society, economy, and the environment. Understanding such natural hazards and their drivers is essential to mitigate related risks. In this article, we investigate the atmospheric conditions driving torrential events in the Grenoble Metropolitan area in the Northern French Alps. Since no long- and resolved-enough hydrometeorological data are available in the region, our study is based on the dates of damaging torrential events since 1950 that are retrieved from operational reports. An originality of our analysis is that it is done conditionally to the main types of generating atmospheric circulation derived from Lamb weather classes. The results show the added value of considering weather-type conditional atmospheric situations, since different situations prove to be discriminant depending on the weather type. Furthermore, by visual inspection of the recurrent field patterns of the discriminant atmospheric variables, we show a variety of dynamic/thermodynamic controls through seasons, with large-scale atmospheric situations with a clear dynamic signature triggering torrential events in cold seasons and more mixed signatures between dynamic and thermodynamic in warm seasons. Finally, we show the wide atmospheric signature for such events despite their small scale. • Atmospheric conditions driving torrential floods in an Alpine Metropolitan area since 1950. • Added-value of considering weather-type conditional atmospheric conditions. • A variety of dynamic/thermodynamic controls through seasons. • Wide atmospheric signature of torrential events despite their small scale. • Generic and easily applied methodology that uses only the dates of torrential floods.