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

Abstract

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

BASTA: A 95-GHz FMCW Doppler Radar for Cloud and Fog Studies

“…BASTA was one of the radars used during a recent campaign that took place in France at SIRTA in summer 2014. The radar was pointing horizontally toward four calibrated targets and measured the refractivity variations during two months with a sampling rate of 0.25 s. Several instruments allowed comparison between radar refractivity measured by BASTA and in situ measurements (using Besson et al 2012).…”

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

“…Tout d'abord, ces erreurs dépendent des caractéristiques du radar et des paramèt-res de la mesure : à partir d'une simulation utilisant des jeux de données indépendants, Besson et al (2012) ont donné une formulation du taux d'ambiguïté en fonction de la fréquence d'émis-sion du radar, de la distance d'intégration dr et de l'intervalle de temps entre deux échantillons successifs dt.…”

“…Enfin, Heinselman et al (2009) ont montré que les champs de réfractivité constituent une information complémentaire pour les prévisionnistes, utile pour analyser l'environnement proche de la surface et améliorer leur connaissance des masses d'air humide. À partir d'une simulation Méso-NH d'un cas convectif typique du sud de la France, Besson et al (2012) ont pu identifier la présence d'une signature en réfractivité particulière à l'avant de la convection, résultat de l'effet du courant de densité généré par les précipitations. L'intérêt est évident pour localiser les systèmes convectifs et prévoir leurs évolutions.…”

Improving Radar Refractivity Retrieval by Considering the Change in the Refractivity Profile and the Varying Altitudes of Ground Targets

“…Comparisons between refractivity measured by the radar and other instruments in the boundary layer show high correlations in time and space (Fabry et al 1997;Weckwerth et al 2005;Bodine et al 2011). Since hightemporal-(about 5-10 min) and high-spatial-(4 km by 4 km in the horizontal after smoothing) resolution refractivity retrievals have been obtained, many studies have demonstrated the potential utility of refractivity maps for studying near-surface moisture variation associated with a variety of weather phenomena, such as convection initiation, convection evolution, and characteristics of the boundary layer (Weckwerth et al 2005;Fabry 2006;Buban et al 2007;Chen et al 2007;Roberts et al 2008;Koch et al 2008;Besson et al 2012;Nicol et al 2014). Refractivity maps not only provide smallscale moisture variability particularly in those areas without a dense mesonet but also show boundaries prior to the fine line of traditional reflectivity (Weckwerth et al 2005;Heinselman et al 2009;Wakimoto and Murphey 2010;Bodine et al 2011).…”

“…Although the quality of the retrieval has been discussed from different aspects and improved in the last decade (Fabry 2004;Park and Fabry 2010;Besson et al 2012;Parent du Chatelet et al 2012;Caumont et al 2013;Nicol et al 2013;Nicol and Illingworth 2013), the unsolved problem associated with the vertical gradient of refractivity (dN/dh) and uneven target heights (H T ) remains challenging. A consequence of these issues is the difficulties of assigning a height to the retrieved refractivity fields and the prominent diurnal periodicity of the refractivity difference between in situ observations and the radar estimations (Fabry 2004;Weckwerth et al 2005;Bodine et al 2011).…”