Bollène-2002 Experiment: Radar Quantitative Precipitation Estimation in the Cévennes–Vivarais Region, France

Abstract

The Bollène-2002 Experiment was aimed at developing the use of a radar volume-scanning strategy for conducting radar rainfall estimations in the mountainous regions of France. A developmental radar processing system, called Traitements Régionalisés et Adaptatifs de Données Radar pour l'Hydrologie (Regionalized and Adaptive Radar Data Processing for Hydrological Applications), has been built and several algorithms were specifically produced as part of this project. These algorithms include 1) a clutter identification technique based on the pulse-to-pulse variability of reflectivity Z for noncoherent radar, 2) a coupled procedure for determining a rain partition between convective and widespread rainfall R and the associated normalized vertical profiles of reflectivity, and 3) a method for calculating reflectivity at ground level from reflectivities measured aloft. Several radar processing strategies, including nonadaptive, time-adaptive, and space-time-adaptive variants, have been implemented to assess the performance of these new algorithms. Reference rainfall data were derived from a careful analysis of rain gauge datasets furnished by the Cévennes-Vivarais Mediterranean Hydrometeorological Observatory. The assessment criteria for five intense and long-lasting Mediterranean rain events have proven that good quantitative precipitation estimates can be obtained from radar data alone within 100-km range by using well-sited, well-maintained radar systems and sophisticated, physically based data-processing systems. The basic requirements entail performing accurate electronic calibration and stability verification, determining the radar detection domain, achieving efficient clutter elimination, and capturing the vertical structure(s) of reflectivity for the target event. Radar performance was shown to depend on type of rainfall, with better results obtained with deep convective rain systems (Nash coefficients of roughly 0.90 for point radar-rain gauge comparisons at the event time step), as opposed to shallow convective and frontal rain systems (Nash coefficients in the 0.6-0.8 range). In comparison with time-adaptive strategies, the space-time-adaptive strategy yields a very significant reduction in the radar-rain gauge bias while the level of scatter remains basically unchanged. Because the Z-R relationships have not been optimized in this study, results are attributed to an improved processing of spatial variations in the vertical profile of reflectivity. The two main recommendations for future work consist of adapting the rain separation method for radar network operations and documenting Z-R relationships conditional on rainfall type.

A Dynamical Z-R Relationship for Precipitation Estimation Based on Radar Echo-Top Height Classification

“…e area of interest in the present study is over the middle and lower reaches of Yangtze River (113-119°E, 27.5-31°N), where short-time intense precipitation (rain intensity more than 10 mm·h When estimating the rainfall using the reflectivity of the Doppler radar, it is common to use the echoes on the constant-altitude plan position indicator [28] at 1.5 km from the sea level or echoes at low elevation angles [29]. However, in the area of interest here, there are some blind regions in the radar CAPPI at 2 km due to the uneven distribution and different altitudes of radar stations.…”

Radar rainfall estimation for the post-event analysis of a Slovenian flash-flood case: application of the Mountain Reference Technique at C-band frequency

“…However, the complexity of the radar technology, the variety of uncertainty sources and the variability of precipitation at all scales still make the radar quantitative precipitation estimation (QPE) a very challenging task. This is especially true in mountainous regions (e.g., Joss and Waldvogel, 1990;Andrieu et al, 1997;Germann et al, 2006;Delrieu et al, 2009) due to the impact of the orography on the propagation of the electromagnetic waves (clutter due to the relief and anthropic targets; screening; anomalous propagation). The radar QPE quality depends much on the relative locations of the radar and the rain event, the intervening relief, the radar parameters, the operating protocol and the data processing (Pellarin et al, 2002).…”

“…In Sect. 4, the attenuation correction is placed in the broader context of the radar quantitative precipitation estimation, using the TRADHy radar processing system developed at LTHE (Delrieu et al, 2009; TRADHy stands for Traitements Régionalisés et Adaptatifs de données radar pour l'Hydrologie / regionalized and adaptive radar data processing for hydrological applications). Section 5 provides a series of sensitivity tests and assessments of the radar QPEs with respect to raingauge data.…”

Estimation of Finescale Rainfall Fields Using Broadcast TV Satellite Links and a 4DVAR Assimilation Method

“…This region experiences intense Mediterranean rain events with strong spatial and temporal variabilities, which are amplified by the mountainous topography. A description of the region and its characteristics is provided in Delrieu et al (2009) and Molinié et al (2012). The Ku sensor can record four different satellite-to-Earth microwave links simultaneously.…”

“…(2), we use the finite difference scheme developed by Smolarkiewicz (1983). The rain fields generally have a strong spatial variability, especially in mountainous areas such as the Cévennes-Vivarais region (see, e.g., Delrieu et al 2009). To conserve the strong rainfall gradients when the fields are propagated through time by the numerical advection scheme, a scheme without strong numerical diffusion is needed.…”