An analysis of significant WSR-88D precipitation underestimation over the Palmer Divide: 28 April to 1 May 1999 heavy rain event

From 28 April to 1 May 1999 a heavy rain event occurred over southeast Colorado resulting in significant flooding along Fountain Creek and a 25 to 40 year flood on the Arkansas River at La Junta, CO. A swath of 150 to 310 mm of rain fell in the vicinity of Colorado Springs along the south slopes of the Palmer Divide. The KPUX (Pueblo, CO) WSR-88D storm total precipitation product estimated less than 7 mm of rain over this region, which represents only 2% to 5% of the observed precipitation. This study documents the role the KPUX WSR-88D terrain based hybrid scan played in significantly underestimating rainfall over and near the Palmer Divide.

Much of the precipitation which fell in the vicinity of Colorado Springs resulted from shallow orographic processes, with radar echoes decreasing substantially in intensity between the 0.5 degree and 1.5 degree elevation scans. This shallow storm structure over gently sloping terrain was not captured well by the terrain based hybrid scan. The terrain based hybrid scan is a site specific file employed at each WSR-88D to determine which of the 4 lowest reflectivity slices will be utilized by the precipitation processing subsystem given the unique topography surrounding the radar. The objective is to utilize reflectivity data from the elevation slice whose beam centerline is closest to 1 m above radar level (ARL), as long as it is not blocked by an obstruction in a closer range bin and the bottom of the beam clears the terrain by 500 feet. The KPUX WSR-88D sits at an elevation 250 to 400 m below the Palmer Divide, and the lower one third of the 0.5 degree elevation slice is blocked by the terrain. The terrain based hybrid scan mandates use of reflectivity data from the 1.5 degree elevation slice over the Palmer Divide, in order to meet the terrain clearance requirement. In this shallow orographic precipitation event, the 1.5 degree slice overshot the intense echoes appearing at 0.5 degrees, resulting in significant precipitation underestimation.

To assess the extent of precipitation underestimation resulting from the terrain based hybrid scan, the KPUX radar data were processed through WATADS using the hybrid scan file from the Brownsville, TX WSR-88D (KBRO), where little to no terrain features impinge on the lowest elevation slice. Event total precipitation estimates from this post processing yielded between 100 to 150 mm along and just south of the Palmer Divide. While this still resulted in a 50% underestimation, significant improvement resulted when the algorithm was allowed to utilize the lowest tilt. In this case, the 500 foot terrain clearance requirement of the terrain based hybrid scan proved too restrictive to obtain reasonable precipitation estimates in the shallow orographic precipitation which occurred over the gently sloping terrain. It is likely that the Z-R relationship and other below beam sampling issues also contributed to the underestimation, and these aspects will also be investigated.