Application of Pre-NEXRAD Reflectivity Data to Hourly Precipitation Analyses

Abstract

Multidecadal records of hourly precipitation estimates are needed to provide forcings for the simulation of subdaily processes in hydrologic models. Existing climatological datasets, such as the North American Land Data Assimilation System version 2 (NLDAS2), determine hourly rainfall estimates via the temporal downscaling (disaggregation) of far-more-plentiful daily rain gauge reports. The National Weather Service’s (NWS) National Water Center (NWC) compared NLDAS2 hourly precipitation estimates in the period since 1996, when Next Generation Weather Radar (NEXRAD) Stage-II hourly data became available for use in achieving the disaggregation, with analogous estimates from before 1996, which were disaggregated without the use of radar data. For 20 independently selected days/cases with substantial precipitation during each of the two periods, the post-1996 NLDAS2 hourly amounts were found to be far more highly correlated with cooperative (COOP) rain gauge reports, used for verification, than were the pre-1996 ones. The possibility is then explored that an earlier radar dataset – manually digitized radar (MDR) reports collected from the individual Weather Service Radar-1957 (WSR-57) and WSR-74 radars during the period 1978–1994 – could provide similar benefit in the disaggregation process during the pre-NEXRAD era. In comparisons against COOP hourly reports for 37 warm-season days in the southeast United States and 12 cool-season days in California, it is found that the MDR-disaggregated estimates provided significant statistical improvement over the original, NLDAS2 hourly estimates. Finally, Conterminous United States (CONUS)-wide maps were prepared for each of the four seasons, providing the ratio of hours with measurable precipitation by the MDR methodology against colocated counts of raining hours from surface observation reports. These maps can be used as masks to determine where MDR data would be effective for use in generating any future, hourly precipitation analyses-of-record (AOR), during the 1994-and-prior period.