VPR corrections of cool season radar QPE errors in the mountainous area of northern California

Non-uniformity of the vertical profile of reflectivity (VPR) is one of the major error sources for radar quantitative precipitation estimation (QPE) in the cool season, especially for mountainous areas. The error is due to two factors: one is that the radar beam samples too high above the ground and misses the microphysics at lower levels; the other is that the radar beam broadens with range and thus cannot resolve vertical variations of reflectivity structure. These errors have posed a major challenge for radar QPE in the complex terrain of northern California. The current study used precipitation profiler observations obtained in this mountainous area and developed a new VPR correction methodology for scanning radar QPE. The precipitation profiler data were used to determine slopes of a linear VPR model in the ice, bright band, and rain regions, and the slope parameters are derived for different geographical areas. The parameterized VPR is then used to correct for scanning-radar QPE. The new methodology was tested using a heavy rain case that occurred over the period 30 December 2005 to 1 January 2006 in northern California, and was found to provide significant improvements over the operational radar QPE.