A GIS-based methodology for the assessment of weather radar beam blockage in mountainous regions: two examples from the US NEXRAD network

The US National Weather Service (NWS) has installed a large network of weather Surveillance radars (WSR-88D) that provide precipitation maps for the United States. Many of these radars operate in mountainous regions and consequently suffer from beam blockage caused by terrain obstacles. The authors present a methodology for assessing the severity of the beam blockage and outline its implications for radar-derived precipitation estimates. The methodology involves the calculation of two-dimensional maps of power loss using a digital elevation model (DEM)-based algorithm of beam propagation for different radar antenna elevation angles. Using a large sample of actual radar data, the authors compare the simulated beam blockage results with the probability of detection of radar reflectivity above a certain threshold. The authors also compare their results with similar but coarser resolution blockage maps developed by the NWS and used in the NEXRAD system. For visualization, ArcGIS software is used to illustrate the results and offer a physical interpretation of the analyses. The study involves two NEXRAD sites: KRLX in Charleston, West Virginia and KEMX in Tucson, Arizona. The KRLX site does not suffer from significant blockage except for a single narrow sector. By contrast, the KEMX site contains several areas of blockage. The authors conclude that DEM-based prediction of radar beam occultation is a viable tool, as indicated by the good agreement of the calculated patterns of power loss with the actual long-term radar data.