Following this approach, since 2004 Météo-France has used a software named SURFILLUM to estimate the beam blocking rate due to the relief for all the operational radars of the French weather radar network (36 radars in 2017, for Mainland France and Overseas Territories). This software has been developed by the LTHE (Laboratoire d'Etudes des Transferts en Hydrologie et Environnement), and presents the interest to integrate the interaction between the radar beam and the ground with a great (an adaptable) angular resolution, taking into account an antenna diagram as a model of angular distribution of the energy into the main beam (Delrieu et al 1995). This software also allows to estimate ground clutter values, and to combine the estimated blocking rate with a VPR model in order to estimate the "hydrologic visibility" as defined by Pellarin et al 2002, and used by Faure et al 2005.
After this simulation of the beam blocking due to the relief, a complementary human analysis allows Météo-France to detect and quantify partial beam blockage due to small obstacles close to the radar (mainly within 1km). This identification is based on the analysis of the azimuthal variation of the radar measurement (converted in cumulated radar rainfall fields), with a 0,5° polar resolution, in order to determine sharp and narrow beam blocking which cannot be due to the relief. A verification with aerial photographs or ground photographs generally allows to identify the target (mainly anthropogenic) responsible for the beam blocking (tree, pylon, building, grove …). This human analysis is time consuming, and must be actualised regularly. So, Météo-France has been interested by the possibility to simulate the impact of these anthropogenic sources of beam blockage by using very high resolution DEMs, regularly refreshed, to describe the immediate environment of the radar. The process use different digital elevation models in function of the distance to the radar, two Digital Terrain Models (DTMs) provided by IGN representing the elevation of the bare ground of the terrain with a spatial resolution of 75m and 25m, i.e. the relief, as well as a sub-meter resolution Digital Surface Model (DSM) which includes the elevation of all objects on the ground like plants and buildings :
- the sub-meter DSM is used for ranges within few km from the radar,
- the 25m DTM is used for ranges from the radar between few km and 50 km,
- the 75m DTM is used for ranges beyond 50 km from the radar (up to 256 km).
This presentation emphasises the interest and contribution of the sub-meter DSMs and presents two examples of results for operational C-band radars operated by Météo-France. The first one concerns a radar located in an urban area, and a 19.5x18km² DSM with 0,5m resolution estimated by IGN from Pléiades satellites stereoscopic data. The second one concerns a radar in a rural setting, close to woods, and a 2.4x2.1km² DSM of 1m resolution derived from airborne Lidar data. The discussion details the part of the results depending on the specific situation of the radars, and clarifies the potential and the limits of this kind of simulations to efficiently take into account small size obstacles near a radar.
References:
Delrieu, G.; Creutin, J.D.; Andrieu, H. , 1995 : Simulation of X-band weather radar mountain returns using a digitized terrain model. Journal of Atmospheric and Oceanic Technology, 12, 1038-1049.
Pellarin, T.; Delrieu, G.; Saulnier, G.M.; Andrieu, H.; Vignal, B.; Creutin, J.D. , 2002 : Hydrologic visibility of weather radar systems operating in mountainous regions: Case study for the Ardèche catchment (France). Journal of Hydrometeorology, 3, 539-555.
Faure, D.; Delrieu, G.; Tabary, P.; Du Chatelet, J. P.; Guimera, M. , 2005 : Application of the hydrologic visibility concept to estimate rainfall measurement quality of two planned weather radars. Atmospheric Research, 77, 232-246.
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