ERAD 2012 - THE SEVENTH EUROPEAN CONFERENCE ON RADAR IN METEOROLOGY AND HYDROLOGY Design and test of an X-band optimal rain rate estimator in the frame of the RHYTMME Project

The RHYTMME project (Risques HYdro-météorologiques en Territoires de Montagnes et MEditerranéens) was developed in order to establish a platform of services allowing a better management of hydrometeorological risks in the French southern Alps, which are prone to intense precipitation events and flash floods. In the frame of the RHYTMME Project, Météo France is deploying a dense network of 4 polarimetric X-band radars over the period 2010 – 2013. The first radar (Mont Maurel) is installed and operating since May 2010. The data from another radar (Mont Vial), owned by CNRS and operated by Novimet, is also used. The objective is to generate high-quality networked products, including quantitative precipitation estimation (QPE) mosaics, appropriate for integration into automatic hydrometeorological alerting systems. Radar QPE in a mountainous environment is rather challenging due to issues such as occultation, partial beam blocking (PBB), partial beam filling, etc. Moreover, although X-band radars have the advantage of smaller sensitivity to ground clutter and reduced size and cost, they also suffer from much more attenuation than radar at longer wavelengths. This paper investigates the optimal QPE algorithm to operate in such an environment. Several polarimetric QPE algorithms have been tested such as KDP -based algorithms, conventional Z-R algorithms (with and without attenuation correction), and Testud et al. (2000) ZPHI®. The differential reflectivity (ZDR) has not been used in the tests because its current stability and accuracy is considered insufficient. In a first stage, only single tilt data has been used in order to simplify the evaluation. Section 2 of this paper describes the operational RHYTMME radar data processing chain. Section 3 describes the evaluation methodology followed. Section 4 discusses the results. The sensitivity of the ΦDP filtering and the KDP estimation have been thoroughly evaluated as well as the impact of the attenuation correction. Finally, conclusions are drawn in section 5.