Rain gauges and X-band radar hourly comparison under complex orographic conditions in Reunion Island during the passage of the tropical cyclone Batsirai

Abstract

The South-West Indian Ocean (SWIO) region is prone to heavy rainfall, particularly during the passage of tropical cyclones. Given the topography of SWIO islands, orography plays an important role in the spatial distribution of precipitation. The ESPOIRS project was designed to investigate such dynamics in Reunion Island, Seychelles, and Madagascar using a mobile X-band radar. Reunion Island served as a testbed to evaluate X-band radar retrieved QPE using specific comparisons between several radar approaches and available rain gauges. This is the first study to use an X-band dual-polarization radar in the SWIO region. Our research focuses on the intense tropical cyclone event Batsirai in Reunion Island and shows the effectiveness of dual-polarization radar when compared to single-polarization radar in mitigating reflectivity attenuation. Both the Hitschfeld and Bordan and the philinear algorithms were employed and evaluated for this purpose. As study encountered challenges related to noisy and low-resolution differential phase (ϕdp) data, the pre-processing steps used to extract reliable ϕdp data from the observed measurements are detailed. Furthermore, two precipitation estimators were tested, R(Z) and R(kdp). Results show that R(Z) is sensitive to the attenuation correction method. Additionally, using the extracted ϕdp, empirical model for R(kdp) for Reunion Island is proposed and discussed. This model provided better results compared to the R(Z) estimates for heavy precipitation, which can be explained by the fact that kdp is directly linked to precipitation concentration and does not require attenuation correction. The findings highlight that the accuracy of the radar QPE is strongly influenced by local topography, which in turn governs local rainfall patterns, while the accuracy of QPE also depends on the type of precipitation.