Two widely used procedures for optimally combining radar- and gauge-derived rainfall are those of the OneRain Corporation and the National Weather Service (NWS). The NWS procedure, called the Multi-sensor Precipitation Estimator (MPE), produces an hourly product on the 4×4 km Hydrologic Rainfall Analysis Project (HRAP) grid. MPE is used operationally by local NWS offices and NWS River Forecast Centers (RFCs). Florida State University (FSU) has employed the MPE scheme with NWS hourly digital precipitation arrays (DPAs) to create an hourly historical precipitation database for the Florida Department of Environmental Protection (FDEP) for the period 1996-2006. The OneRain procedure is proprietary and has not been described well in the literature. However, it produces a product at 15 min intervals on a 2×2 km Cartesian grid. Florida's Water Management Districts as well as other government agencies and private firms use the OneRain product. Although their methodologies and their temporal and spatial resolutions differ, each dataset is being used to make water management and regulatory decisions. Thus, it is useful to evaluate the two procedures against independent data.

This paper will evaluate the two procedures against daily co-op gauges from the National Climatic Data Center (NCDC) that were not used in creating either the MPE or OneRain products. The area of the South Florida Water Management District is investigated during the 2004-2005 calendar years. Both radar-derived products are summed over 24 h periods based on the daily recording time of each gauge. The OneRain precipitation values then are placed onto the same 4×4 km HRAP grid containing the MPE data. Finally, the 4×4 km MPE and OneRain values are compared with any NCDC gauges located within the HRAP grid cells.

Results of daily precipitation comparisons will be presented for all gauges combined over the two year time period, over individual years, the cold and warm seasons, and over individual months. Individual gauge sites also will be evaluated. Intervals of precipitation are analyzed to see how each scheme handles light, moderate, and heavy rainfall events. Finally, case studies describe how each scheme estimates particular rainfall events, including land-falling tropical cyclones. In summary, this paper will describe which procedure compares best with the NCDC independent gauges, and whether the OneRain and MPE products can be used interchangeably.