Development of a WSR-88D based Snow Accumulation Algorithm for quantitative precipitation estimates over southwestern Oregon

The Bureau of Reclamation (Reclamation) is working on a water resources project in southwestern Oregon that uses WSR-88D based Quantitative Precipitation Estimates (QPE) over watersheds draining into reservoirs. Such QPEs are being used with Internet decision support tools to improve water management efficiency. The accuracy of the WSR-88D Stage III products over southwestern Oregon based on the standard Z_e=300R^1.4 relationship with no range correction is not sufficient for Reclamation's operational needs. This result is due to the mountainous terrain, the location of the Medford WSR-88D at a high elevation location from which it views ice particles during winter months, and other factors.

The Reclamation-developed Snow Accumulation Algorithm (SAA) for the WSR-88D was adapted to use NEXRAD Information Dissemination System (NIDS) reflectivity data with 4 or 5 dBZ resolution. When considering the Z_e=aS^b relationship for dry snow, it was determined that b=2.0 appeared appropriate for several locations, and a change in the b exponent of ± 0.2 has little practical significance. Reclamation has been testing this SAA for operational use with the Medford, OR, WSR-88D (KMAX) system. Because of the high KMAX elevation (7,546 ft), a value of 100 was selected for the a coefficient and the b exponent was set to 2.0 in the SAA. The resulting relationship was Z_e=100R^2.0.

Criteria used in the selection of precipitation days for this study were: (1) freezing level height indicated by Medford soundings must be below the Medford WSR-88D elevation, so that the lowest 0.5° radar beam could be assumed to illuminate dry snow; (2) the 24-hr precipitation accumulation for the day ending at 2400 local time for 2 gages located near Medford (about 35 km from KMAX) must be equal to or greater than 0.10 in; and (3) the KMAX NIDS base reflectivity volume scan data for the 24-hr local time day must be complete. There were 32 days during the November 1998 - April 1999 period that met these criteria.

Medford soundings made during the 32 days when precipitation was being recorded at the Medford Airport had an average freezing level height of 5000 ft, and an estimated average cloud top of 15,800 ft. The WSR-88D data indicated that the precipitation was from widespread storm systems. Beyond 50 km range from KMAX a significant portion of the precipitation was below the radar beam. Consequently, a range correction factor of 1.00000 - 0.00500*r + 0.0001428*r² if r > 50 km (r is the range from KMAX), was selected and tested with the Z_e=100R^2.0 relationship as part of the SAA for southwestern Oregon.

This study showed that the SAA of Z_e=100R^2.0 using a 4 dBZ minimum value to calculate precipitation, significantly improved the precipitation estimates over the standard Z_e=300R^1.4 with a 22 dBZ minimum value. The correlation between daily radar and gage estimates was 0.75, and the radar/gage ratios for the 32 storm day totals for both Medford gages were 103 percent. Adding a range correction factor beyond 50 km to compensate for precipitation returns below the radar beam provided improved precipitation estimates at far range. Reclamation is exploring using gage measured 24-hr precipitation accumulations and the gridded climatological precipitation pattern from the PRISM model, as bias adjustments to the daily radar precipitation estimates. Results and findings will be included in the paper.