PACJET-2003: Examining Gaps in NEXRAD Coastal Coverage

Contact: Paul Neiman

The WSR-88D (NEXRAD) radar is a marvelous cornerstone of the NWS modernization program. However, a University of Washington study (Westrick et al. 1999) pointed out that areal coverage of storm surveillance by the NEXRAD network is seriously limited in some parts of the Nation. The problem is most severe in the western states where topographic blockage of the lower NEXRAD scans combines with long distances between radars and siting on mountain tops to provide particularly poor low-altitude coverage (see also Reynolds 1995). The result is that crucial lower portions of storms below the melting level are missed and an unknown number of shallow storms may entirely escape detection in these regions of compromised coverage. Storm forecasts and the use of radar data as input to hydrological runoff predictions are crippled by this handicap.

The Pacjet-2003 study of land-falling winter storms on the West Coast has concentrated some of its instrumentation in one of these NEXRAD "black hole" regions between Point Reyes and Point Arena, California. The nearby coastal mountains are the headwater source of frequent flooding on the Russian River and other watersheds. Pacjet and Caljet field experiments in earlier winters provided strong evidence that a substantial portion of the total winter rain accumulation in this area falls from shallow storms that may lie beneath the scans of the nearest NEXRAD units at San Francisco, Sacramento, and Eureka (White et al. 2003). The 0.5-degree-tilt sweeps from these radars, which are the lowest allowed, are more than 3 km above sea level over the coastline in this area, providing ample room for shallow but drenching storms to strike beneath the scan coverage. Pacjet-2003 conducts detailed measurements of the region's storms using a number of remote sensors on and near the coastline, including a polarimetric X-band radar operated by NOAA/ETL at Fort Ross State Historic Park. The attached website shows images from the ETL radar and NEXRAD during a half-inch rainstorm on January 14, 2003, illustrates a modest example of one such shallow rain event. When the ETL radar crew showed this information to the local Park Ranger, Bill Walton, an avid weather buff, he exclaimed, "That explains a lot. I've often surfed the Web and wondered why the NWS radar images indicate no precipitation here, while I'm getting thoroughly soaked!" The Pacjet studies (http://www.etl.noaa.gov/programs/2003/pacjet/) point the way for a solution to this problem in terms of special, strategically-sited "gap-filler" radars such as the X-band system.

References:

• Reynolds, D.W., 1995: Warm rain process and WSR-88D. Western Region Tech. Attachment 95?08, 7pp. [Available from NWS Western Region Headquarters, Federal Building, Salt Lake City, UT 84138.]
• Westrick, K.J., C.F. Mass, and B.A. Colle, 1999: The limitations of the WSR-88D radar network for quantitative precipitation measurement over the coastal western United States. Bull. Amer. Meteor. Soc., 80, 2289-2298.
• White, A.B., P.J. Neiman, F.M. Ralph, D.E. Kingsmill, and P.O.G. Persson, 2003: Coastal orographic rainfall processes observed by radar during the California Land-falling Jets Experiment. J. Hydrometeor., 4, in press.