Improving Radar-based Precipitation Estimation Through VPR Correction

Our current research work is mainly focused on radar quantitative precipitation estimation (QPE) for varies rainfall types and terrain affected areas. In order to improve the accuracy of radar QPE, our current work can be listed as below:1), Developed an apparent vertical profile of reflectivity (AVPR) correction scheme, and the main function of this scheme is elimination of the radar QPE errors caused by bright band (BB). The correction was based on the radar observed ("apparent") vertical profiles of reflectivity (AVPRs) from volumetric radar data. The AVPR was computed for each single tilt, and only in the bright-band area that was delineated according to radar reflectivity distributions and atmospheric environmental data. A linear model was fitted to the AVPR and then used to correct for BB effects in the observed reflectivity field. The AVPR correction scheme was tested for eight heavy precipitation events from different geographical regions and seasons in the United States. The linear VPR model was found to be representative and stable for various BB structures. High reflectivities associated with BB were correctly reduced in most of the cases and the corrected reflectivity field showed physically continuous distributions. The overestimation errors in radar-derived QPE were largely reduced after the correction, and the corrected radar-derived QPE agreed well with rain gauge observations. The AVPR correction is most effective and robust for flat land radars because of relative uniform spatial distributions of BB. For mountainous radars, the performance of the correction is mixed because of large spatial variations of VPRs caused by underlying topography.2), When a melting layer is near the ground and the bottom part of the bright band cannot be observed by the WSR-88D radar, a VPR correction can not be made directly from radar observations. However, under this situation, bright band is very close to radar site and the radar beam width is very narrow, so the bright band will bring in severe overestimation for radar QPE. In order to reduce the overestimation, a scheme for finding bright bottom was developed based on S-band profiler data. According to these high-resolution precipitation profiler data, an empirical relationship between BB peak and BB bottom is developed. The empirical relationship is combined with the apparent BB peak observed by volume scan radars and the BB bottom is found. Radar QPEs are then corrected based on the estimated BB bottom. It is shown to be effective in reducing the radar QPE overestimation under low bright band situations.3) According to the large physical differences between convective and stratiform precipitation in MCSs, a new separation scheme is developed through using VIL, composite reflectivity, different vertical structures of two rainfall types, and seeded growing method. Meanwhile, a parameterized VPR is derived from stratiform area based on TRMM precipitation data, and apply the parameterized VPR to correct the errors caused by bright band. It is shown that, the performance of the new VPR correction scheme is much better than Zhang and Qi2010(ZQ10), and the corrected radar-derived QPE agreed well with rain gauge observations.4) A new radar QPE VPR correction scheme was developed for cool season precipitation in the complex terrain of northern California. The new scheme included multiple steps:1) identifying and removing nonprecipitation echoes;2) constructing the hybrid scan reflectivity;3) applying VPR corrections to the reflectivity; and4) converting the reflectivity into precipitation rates using adaptive Z-R relationships. Additional procedures were developed to address specific issues in radar rainfall accumulations:1) wind farm contaminations;2) blockage artifacts; and3) discontinuities due to radar overshooting. The new scheme was tested on all precipitation events during the NOAA HMT-Wcst2006experiment. The new hybrid scan approach combining unblocked higher tilts with partially blocked lower tilts was found to mitigate some azimuthal discontinuities in rainfall accumulations. The VPR correction was effective in reducing radar QPE overestimation in the brightband area and the underestimation in the ice region.