Dynamics of Orographic Precipitation Associated with the Passage of the Tropical Cyclone Batsirai Nearby Reunion Island

When a tropical cyclone moves close to a mountainous island, the precipitation induced by the cyclone may be enhanced by the local topography increasing the risk of floods and subsequent landslides in the mountains or their vicinity. Although the research on orographic precipitation events is well established, detailed knowledge of the complex processes associated with orographically enhanced cyclonic precipitation is still limited, especially at small spatial and temporal scales. Reunion island is located in the tropical South West Indian Ocean and is marked by steep orographic features.

In order to study the dynamics of orographic precipitation associated with tropical cyclones and intense precipitation, the Laboratory for Atmosphere and Cyclone studies (LACy) deployed an X-band radar in the southern part of La Reunion Island in the framework of the ESPOIRS project. Hence, the X band radar was able to monitor the passage of the tropical cyclone Batsirai between 1^st and 4^th February 2022.

The analysis the radar observations showed that the occurrence of high-intensity precipitation was significantly influenced by the local topography, the position and displacement of the cyclone, and the associated winds. Further comparison of observed reflectivity with 2 Km MESO-NH model wind and mass fields revealed that the strong moisture-laden winds from the tropical cyclone Batsirai impinged on the steep slope of Piton de La Fournaise volcano, inducing strong upward vertical motion and moisture convergence in the lee side of the mountain. These results highlight the role of complex topography in enhancing the precipitation from a tropical cyclone.