Improvements to the WSR-88D Snow Accumulation Algorithm

The Snow Accumulation Algorithm (SAA) was developed in the late 1990s by the U.S. Bureau of Reclamation for use with the national WSR-88D radar network. This algorithm produces near-real-time snow water equivalent and snow depth estimates, which are available on the Internet. The SAA will be incorporated into the operational WSR-88D software suite, which presently only estimates rainfall. This paper describes several enhancements to the algorithm that should extend its accuracy and applicability.

The current SAA range correction is replaced by a fixed adjustment for the vertical profile of reflectivity that has comparable range effects. This corrects for the usual growth of precipitation below the radar beam and reduces spurious discontinuities from prescribed beam elevation changes. The current SAA applies only to solid precipitation ("dry" snow). New logic uses temperature and relative humidity from numerical model soundings to transform the SAA into a more generic precipitation accumulation algorithm (PAA). The radar data are thereby classified according to areas of solid or liquid precipitation, and flagged according to bright band and virga contamination.

The enhanced PAA is being input to the National Weather Service's Snow Data Assimilation System (SNODAS), a new and sophisticated snow energy and mass balance modeling system. The SNODAS will provide quantitative snowpack and snowmelt data to the NWS's river models. The end result should be more accurate streamflow forecasts for the public and water operations management.