A Heavy Rainfall Winter Event Associated with Cumulus Congestus Clouds over Reunion Island (21°S, 55.5°E)

Located in the southwestern Indian Ocean, Reunion Island is submitted to important precipitation all year round due to both cyclonic systems and local non-cyclonic convective systems. In the night of 29 August 2017 (austral winter), a 10-year return period event occurred in the northern part of the island. This storm, which led to record rainfalls for a month of August - with up to 255 mm of rainfall in 24 hours – was missed by numerical weather prediction models. This event originated from interactions between a tropopause dynamic anomaly and warm air coming from the North of Madagascar, which fostered the development of a convergence line. Associated with a front coming from the west, some cumulus congestus clouds regenerated during the evening on the eastern side of the island. The most intense precipitation coincides with the arrival of the edge of the front in the middle of the night. This study aims at improving the forecast of such events, using high resolution model simulations and available observations. In this study, ground-based observations (radar imagery, meteorological stations, lidar profiles..) and global meteorological analysis are first relied upon to describe the synoptic (dry air layer, stratospheric intrusion) and the local atmospheric environment, along with their interactions. Cloud formation mechanisms as well as convective and precipitation processes associated with the cumulus congestus clouds are then characterized using observations and high resolution numerical simulations with the research model Meso-NH, initialized and coupled with the AROME-OI analysis. Finally, the impact of radar data assimilation on operational analysis and forecasts will be discussed.