Mesure de la réfractivité atmosphérique par radar météorologique : comparaison avec un réseau de capteurs au sol.

International audienceLes radars météorologiques peuvent mesurer les changements de l'indice de réfraction de l'air dans les basses couches de l'atmosphère (Fabry et al., 1997, Fabry 2004). En utilisant les changements de phase provenant de cibles fixes aux alentours du radar, cette mesure permet d'obtenir une mesure de la réfractivité atmosphérique. Celle-ci dépend de la pression, la température et l'humidité. Les échos exploitables proviennent en général des cibles fixes telles que des châteaux d'eau, des tours ou des pylônes électriques. Pendant la campagne HyMeX (Hydrological cycle in Mediterranean expriment), cette mesure a été implémentée avec succès sur les radars bande S du réseau opérationnel de Météo-France. Afin de mieux comprendre les sources d'erreur autour de cette mesure, en particulier lorsque l'on monte en fréquence, Besson et al. 2012 a mené une simulation à partir des données de stations météorologiques automatiques. Cela a permis de mettre en avant une plus forte variabilité du signal l'été et en fin d'après-midi, quand la réfractivité est très sensible aux changements d'humidité. Cette simulation a ensuite été confirmée par des mesures radar. Est-il alors possible d'obtenir une information sur la turbulence à partir de cette mesure ? Pour échantillonner la variabilité spatiale et temporelle de la réfractivité, une analyse a été menée sur un an de données provenant du radar en bande C de Trappes, conjointement à une comparaison avec les stations automatiques alentours. L'étude présentée ici a permis de montrer qu'un lien qualitatif et quantitatif peut être établi entre les variabilités de la réfractivité mesurée par radar ou par les stations automatiques, qui sont liées à la turbulence atmosphérique de basses couches

BASTA : a 95GHz FM-­‐CW Cloud radar

International audienceGround-based continuous observation of non-precipitating clouds and fo

The catastrophic flash-flood event of 8–9 September 2002 in the Gard region, France: a first case study for the Cévennes–Vivarais Mediterranean Hydrometeorological Observatory

The 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

Solutions for Improving the Radar Refractivity Measurement by Taking Operational Constraints into Account

International audienceAtmospheric refractivity depends on meteorological parameters such as temperature, water vapour pressure and air pressure and can be measured using a weather radar. This could be useful for convection prediction through the assimilation by numerical forecasting models in the boundary layer, particularly in pre-storm conditions. However, this measurement is highly sensitive to phase ambiguities, induced by signal undersampling during rapid atmospheric fluctuations due to strong turbulent fluxes in the boundary layer, or during extreme events. The refractivity measurement has been recently implemented on some radars of the French ARAMIS network, which is composed of three different frequencies (S-, C-, X-band). In view of operational applications, investigations are performed to improve the measurement and limit the phase ambiguity rate. The first recommendation is to decrease the time interval between two measurements, to increase the antenna speed rotation or by the use of higher elevation angle. These methods lead to decrease the sensitivity of the refractivity to the phase aliasing. The second recommendation is to improve the information from ground target, furthermore by combining the two polarization radar returns and by using shorter pulse width. These two different approaches, based on the radar capacity and the description of the target, are complementary and noticeably improve the quality of the refractivity retrieval

Operational Multiple-Doppler Wind Retrieval Inferred from Long-Range Radial Velocity Measurements

International audienceThe recent deployment of an innovative triple pulse rise time (PRT) scheme within the French operational radar network allows for the simultaneous collection of reflectivity and radial velocity measurements up to a range of 250 km with no ambiguity. This achievement brings new perspectives in terms of operational exploitation of Doppler measurements including the capability to consistently perform multiple- Doppler wind synthesis in a fully operational framework. Using real and simulated Doppler observations, the authors show that the 3D wind fields retrieved in that framework can definitely be relied upon to achieve a consistent and detailed mapping of the airflow structure in various precipitation regimes despite radar baselines averaging _180 km and very limited scanning strategies. This achievement could be easily transposed to other operational networks and represents a remarkable opportunity to add further value to operational Doppler velocity measurements

La réfractivité radar : vers une cartographie de l'humidité en très basse couche de l'atmosphère

International audienceLes radars météorologiques ont été initialement conçus pour détecter et quantifier les précipitations. Une nouvelle technique a été proposée à la fin des années 1990 pour mesurer le changement de l'indice de réfraction de l'air dans les basses couches de l'atmosphère en exploitant la phase du signal provenant de cibles fixes situées autour du radar. Cette mesure donne des informations sur l'indice de réfraction de l'air, lui-même combinaison de la pression, de la température et de l'humidité, et présente donc un intérêt météorologique, aussi bien pour la prévision numérique que pour les études de processus

An Improved M-PRT Technique for Spectral Analysis of Weather Radar Observations

International audienceThe exploitation of Doppler radars for weather observations is strongly constrained by the well-known range-velocity dilemma. To overcome the range and velocity ambiguities, dual and triple staggered pulse-repetition time (PRT) techniques are commonly used in Doppler radar systems. Today, a triple-PRT (3-PRT) scheme is operational in France. These techniques imply nonuniform sampling of the weather signal, inducing multiple replicas in the Doppler spectrum. The situation is particularly complicated for short-wavelength radars, where larger extension factors of the unambiguous Nyquist interval are needed. To overcome these difficulties, a novel technique called OptM-PRT is proposed. It mainly consists in optimizing the transmission scheme based on multiple pulse repetition time, so that the corresponding autocorrelation function is well filled. The Doppler spectrum is therefore reconstructed with much less ambiguities, from the computation of the autocorrelation function of radar signal and its Fourier transform. Considering both 3-PRT and Opt9-PRT schemes, the magnitude and Doppler velocity of radar returns in rain are simulated for different spectral widths, with and without elimination of the spectral lines of ground clutter. When the ground clutter is filtered out, the 3-PRT is found to better reproduce the Doppler velocity, whereas the Opt9-PRT better restitutes the magnitude of the signal. In the presence of noise, the Opt9-PRT scheme produces the best result for both the magnitude and velocity. The 3- and Opt9-PRT techniques have been applied to the C-band Doppler radar operating in Bourges, France. The experimental results show that Opt9-PRT efficiently reconstructs the Doppler spectrum of rain echoes

Links Between Weather Phenomena and Characteristics of Refractivity Measured by Precipitation Radar

International audienceRefractivity depends on meteorological parameters such as temperature and water vapour pressure and can be measured using a weather radar. A realistic atmospheric simulation from the Meso-NH numerical model is used in order to describe and establish the relation between refractivity and the dynamic and thermodynamic phenomena responsible for the development and propagation of convection. These investigations lead to discussion of the complementarity between the refractivity and the convective available potential energy. The relation observed between the refractivity signal and the meteorological parameters calls the refractivity measurement into question, since it is based on phase differentiation with time and space and can be degraded by phase aliasing problems. These aliasing problems increase with the radar frequency (perceptible in the S-band, serious in the C-band, and more serious in the X-band) and also with the integration range and sampling time. Thus, a statistical approach permits us to simulate the possibility of measuring the refractivity with operational radar during convective events. A typical case in the south-east region of France is selected to simulate measurements by radar (S-band, C-band, X-band) in convective systems, in order to evaluate the measurement feasibility, particularly in terms of phase ambiguity, related to temporal and spatial sampling, of a future implementation of the refractivity measurement over the French operational radar network. This numerical statistical approach is completed with a similar study using in-situ measurements performed at the Trappes station. The seasonal and diurnal dependencies of aliasing are investigated, leading to clarification of the impact of the turbulent fluxes on the refractivity measurement

Errors caused by long-term drifts of magnetron frequencies for refractivity measurement with a radar: Theoretical formulation and initial validation

International audienceRefractivity measurements in the boundary layer by precipitation radar could be useful for convection prediction. Until now such measurements have only been performed by coherent radars but European weather radars are mostly equipped with non-coherent magnetron transmitters, for which the phase and frequency may vary. In this paper, we give an analytical expression of the refractivity measurement by a non-coherent drifting-frequency magnetron radar and we validate it by comparing with in-situ measurements. The main conclusion is that, provided the necessary corrections are applied, the measurement can be successfully performed with a non-coherent radar. The correction factor mainly depends on the local-oscillator frequency variation, which is known perfectly. A second-order error, proportional to the transmitted frequency variation, can be neglected as long as this change remains small

An Observation Operator for Radar Refractivity Change: Comparison of Observations and Convective-Scale Simulations

International audienceWeather radar refractivity depends on low-level moisture, temperature, and pressure and is available at high space-time resolutions over large areas. It is of definite meteorological interest for assimilation, verification, and process-study purposes. In this study, the path-averaged refractivity change is simulated from the Arome cloud-resolving atmospheric system analyses and compared with corresponding radar observations over a 35-day period with various meteorological conditions. For that, a novel post-processing procedure is applied to radar data to improve its quality. Also, an observation operator is developed that ingests Arome analyses and simulates a 3-h path-averaged refractivity change. A sensitivity study shows that simulated path-averaged refractivity change is immune to the modelling of the beam height as long as it remains below approximately 60 m above the ground. Comparisons show overall consistency between observed and simulated path-averaged refractivity change, with discrepancies at times that suggest an improvement in analyses once radar refractivity change observations are assimilated. Finally, errors introduced when retrieving local refractivity from path-averaged refractivity are estimated and it is found for our dataset that such retrievals halve the range of usable observations