Rainfall Estimation Using Palarimetric Radar

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Under support from the U.S. Weather Research Program, the Principal Investigators will develop a new method to measure rainfall using polarimetric radar. Recent evidence obtained from polarimetric weather radars suggests that when existing polarimetric methods fail to adequately estimate rainfall, a significant cause is the variability in drop size distribution (DSD). Thus, understanding the impact of DSD on the performance of the polarimetric algorithms for rainfall estimation will be a major thrust of this research effort. The Principal Investigators will exploit this knowledge to develop a novel approach for rainfall estimation. The idea is to use polarimetric radar data to classify various rainfall regimes according to their DSD so that appropriate polarimetric relations can be continuously matched to each rain type. Characterization of a rainfall regime will be derived from various polarimetric variables. The corresponding DSD's will be measured by a state-of-the-art two-dimensional video-disdrometer. The disdrometer will be placed in a pit within an array of closely spaced rain gauges to eliminate wind effects and to acquire independent estimates of rainfall. Disdrometer data will be collected in all types of rainfall found in Oklahoma storms to develop the rain regime classification and to understand the differences among the various regimes.Verification of the "matched" polarimetric rainfall estimates will be done by comparison with rain gauge data from a micronetwork (42 gages at 5 km spacing, operated by the Agricultural Research Service, USDA) in south-central Oklahoma, the Oklahoma Mesonet (one or two gauges in each Oklahoma county), and the NOAA Cooperative Network. The latter two data sets will be employed primarily to assess range effects on the quality of radar rainfall estimation. At long distances non-uniform radar beam filling and vertical stratification of precipitation add to the uncertainty in rainfall estimates. The complement of network rain gauges, the Cimarron polarimetric radar just west of Oklahoma City, three operational WSR-88D radars in and around central Oklahoma, and a rain gauge comparison facility in Norman, provide a unique field laboratory to support the investigation. In addition, the Principal Investigators have a long history of radar - rain gauge comparisons.Improvement in radar rainfall estimation will have a positive impact on the quality of precipitation forecasts. Improved estimates will assist in mitigation of natural disasters due to flooding and lead to better management of water resources by government and commercial organizations.