International audienceThe Cévennes–Vivarais Mediterranean Hydrometeorological Observatory (OHM-CV) is a research initiative aimed at improving the understanding and modeling of the Mediterranean intense rain events that frequently result in devastating flash floods in southern France. A primary objective is to bring together the skills of meteorologists and hydrologists, modelers and instrumentalists, researchers and practitioners, to cope with these rather unpredictable events. In line with previously published flash-flood monographs, the present paper aims at documenting the 8–9 September 2002 catastrophic event, which resulted in 24 casualties and an economic damage evaluated at 1.2 billion euros (i.e., about 1 billion U.S. dollars) in the Gard region, France. A description of the synoptic meteorological situation is first given and shows that no particular precursor indicated the imminence of such an extreme event. Then, radar and rain gauge analyses are used to assess the magnitude of the rain event, which was particularly remarkable for its spatial extent with rain amounts greater than 200 mm in 24 h over 5500 km2. The maximum values of 600–700 mm observed locally are among the highest daily records in the region. The preliminary results of the postevent hydrological investigation show that the hydrologic response of the upstream watersheds of the Gard and Vidourle Rivers is consistent with the marked space–time structure of the rain event. It is noteworthy that peak specific discharges were very high over most of the affected areas (5–10 m3 s−1 km−2) and reached locally extraordinary values of more than 20 m3 s−1 km−2. A preliminary analysis indicates contrasting hydrological behaviors that seem to be related to geomorphological factors, notably the influence of karst in part of the region. An overview of the ongoing meteorological and hydrological research projects devoted to this case study within the OHM-CV is finally presented

This paper presents the design and the test of an operational (staggered) triple-PRT (pulse repetition time) scheme that was implemented in July 2004 on the French operational C-band Trappes radar. The three interleaved pulse repetition frequencies (PRFs: PRF1, PRF2, and PRF3) that are used have the particularity to be very low and very close to each other (379, 325, and 303 Hz). The associated individual Nyquist velocities, VN1, VN2, and VN3, range between 4 and 5 m s−1 and the corresponding unambiguous distances are between 495 and 395 km. The ratios VN2/VN1 and VN3/VN1 are, respectively, equal to 6/7 and 4/5. The resulting extended Nyquist velocity is equal to 60 m s−1. One month of data (August 2004) representing more than 10 000 plan position indicators (PPIs) of clear-air, convective, and stratiform rain has been analyzed and histograms of errors have been computed. All histograms are symmetrical and show a well-defined peak centered on zero. The error distributions of the dual-PRT (V12, V13, and V23) and triple-PRT (V123) velocities present secondary peaks that correspond to dealiasing failures. Overall, the dealiasing success rate for V123 is beyond 92%. A simulation “à la Zrnić” of I and Q time series is then used to reproduce the observed error histograms and exhaustively compare dual versus triple PRT for various extended Nyquist velocities. This last step shows that (i) for the same Nyquist velocity, triple PRT systematically overperforms the dual PRT in terms of the dealiasing success rate and (ii) among the couples that yield a Nyquist velocity equal to 60 m s−1, the (4/5; 6/7) couple that is in place on Trappes is optimal.

This paper describes the test of a staggered PRT scheme for the French radar network. The design of the staggered scheme has been done in the following context: 1) the scheme is meant to be implemented on existing C-band and S-band radars of the network, the PRF of which cannot, for technical reasons, be increased above their current values (300–400 Hz for C band and 250–300 Hz for S band); 2) the minimum expected Nyquist velocity should be around 30 m s−1; and 3) the Doppler information will essentially be used to retrieve a VAD wind profile and for data assimilation by operational nonhydrostatic numerical prediction models. An operational C-band radar of the network, located in Trappes, near Paris, has been modified in order to allow Doppler processing and a dual-PRT staggered scheme with two low PRFs (PRF1=310 and PRF2=360 Hz corresponding to a ratio of 6/7) has been implemented. Two months of data (July–August 2003) have been analyzed. The performance of the scheme is assessed qualitatively and quantitatively through the computation of the error structure of the various velocities: velocity at PRF1, velocity at PRF2, and combined velocity. The sensitivity of the error structure on the signal-to-noise ratio is documented. A 7 km × 7 km median filter is introduced to reduce dealiasing errors.

Résumé AbstractOperational wind retrieval using data of the French weather radar network, Aramis