Seasonal Variation in Multi-Radar Coverage for WSR-88D Precipitation Estimation in a Mountainous Region

The WSR-88D Precipitation Processing System (PPS) computes estimates of rainfall out to a radius of 230 km from the radar. In computing the rainfall estimates the PPS also attempts to minimize the impact of terrain induced beam blockages by using the lowest unobstructed tilt which, according to beam geometry and digital elevation model (DEM) data, should clear the terrain by at least 500 ft. Even though higher tilts are used in creating a hybrid reflectivity scan for the PPS in an effort to "see" precipitation over and beyond terrain induced blockages, the fact that a higher tilt is used, means that the beam will overshoot the precipitation at a much closer range than if a lower tilt could have been used. Because of this effect, mountainous terrain greatly reduces the effective coverage of the radar for precipitation estimation at some azimuths. In addition, the high altitude at which some radars are located in mountainous areas causes even the lowest tilt to overshoot the precipitation at a closer range than if the radar was located close to sea level further reducing the true coverage. Both of these mountain induced range degradation effects are best illustrated with long term accumulations and gridded frequencies of radar-derived precipitation and are shown here.

Errors induced by the radar beam overshooting the precipitation as a result of mountainous terrain, the depth of the precipitation producing clouds must also be accounted for when estimating the range of coverage for a given radar. While it is difficult to use a radar to estimate the height of the precipitating cloud over the entire domain of the radar in real time, the seasonal variability of the height can be indirectly inferred by examining long term accumulations of rainfall and the frequency of rainfall over the radar coverage domain. Here we use long term radar rainfall climatologies, based on the 1996 - 1999 archive of the Hourly Digital Precipitation Array (DPA), to show the effective radar coverage for both the cool and warm season in the Pacific Northwest.

The National Weather Service (NWS) has been using radar-derived precipitation estimates from multiple radars to create a regional precipitation mosaics for use by River Forecast Centers (RFCs). When creating these mosaics, it its important to use the lowest available radar coverage as opposed to using the mean value in the overlapping regions of the 230 km radius coverage provided by individual radars. Here we use the climatological radar coverage from each individual radar in building a multi-radar mosaic for the Northwest RFC. Warm season and cool season multi-radar coverage maps are shown and compared for Pacific Northwest.