Quantitative Precipitation Estimation in the French Alps with a dense network of polarimetric X-band radars

Météo France is currently involved, with several other partners, in a project called RHYTMME (Risques Hydro-météorologiques en Territoires de Montagnes et Méditerranéens), the objective of which is to establish a platform of services allowing a better management of hydrometeorological risks in the French southern Alps, a region prone to intense precipitation events and flash floods. Mountains are known to significantly reduce the performance of traditional weather radar networks because of increased ground clutter, partial beam blockage, measurements made in the bright band or in the snow region, … A major component of the RHYTMME Project is the deployment of a dense network of 4 polarimetric X-band over the period 2010 – 2013 and the generation of high-quality networked products (including QPE mosaics) suitable for integration into automatic hydrometeorological alerting systems. There are currently two X-band radars operating and the radar production platform is being finalized for real-time operations starting in June 2011. All raw (PPIs of high-resolution polar 0.5°x240m Z_H, Z_DR, Φ_DP, ρ_HV, V_R, σ, refractivity) data is concentrated in real-time in Toulouse, stored on a server and available for either real-time or off-line processing. The first step in the processing is the processing of the raw X-band polarimetric variables. This has been adapted from an in-house developed S and C-band radar processing chain. All modules had to be adapted to X-band, especially the echo type classification, the attenuation correction and the rain rate estimation since those three are strongly dependent upon the radar frequency. The polarimetric chain outputs are Z_DR and Z_H (with and without attenuation correction), co-polar correlation coefficient ρ_HV, offset corrected and filtered (using a running median filter of about 6 km) Φ_DP, estimated K_DP, texture of Z_DR, the estimated path-integrated attenuation and differential attenuation, σ_Z (pulse-to-pulse fluctuation of the reflectivity) and the echo type classification in polar coordinates. The corrected Z_H, the echo type and the Path Integrated Attenuation (PIA) are also provided, in Cartesian coordinates. The attenuation correction is at this stage performed using a simple static linear relationship between the path-integrated attenuation (and differential attenuation) and Φ_DP. The next step is to combine the processed polarimetric information available on PPIs to generate the best surface estimation of the 1 km², 5' rainfall accumulation. Several polarimetric rain rate estimation algorithms have been tested such as K_DP-based algorithms, conventional Z-R algorithms (with and without attenuation correction and with and without rain-gauge adjustment), Illingworth and Thompson (2005) integrated ZZDR technique and also Testud et al. (2000) ZPHI. The hourly rainfall accumulation Cartesian maps obtained by the QPE algorithms are compared against hourly rain gauges. The quality of the algorithms is evaluated using the normalized bias and the correlation between the rain gauge and the radar rainfall accumulation. A benchmark to beat is the QPE of a conventional S-band radar (Collobrières), the closest operational radar to the area of interest. All the results are stratified according to thresholds on the rain gauge hourly accumulations. The sensitivity of the various algorithms to 1) partial beam blocking, 2) remaining ground-clutter, 3) calibration errors (on Z_H and Z_DR), 4) the presence of attenuation, … will be assessed. Special emphasis will also be laid on implementing intelligent compositing rules between X-band radars, allowing for a mitigation of attenuation effects and optimal exploitation of the network. Maps of minimum detectable signal will also be produced and used in the compositing rules in order to cope with situations of severe attenuation / extinction leading complete misses of precipitation detection by one particular radar. Finally, the impact of wet radome attenuation will be investigated, as among the two already-installed radars one has a radome and the other hasn't.