Improvement of NEXRAD Rainfall Data for Central and South Florida

The South Florida Water Management District (District) is responsible for managing water resources in 16-counties over a 46,439-square kilometer (17,930 square-mile) area. The area extends from Orlando to Key West and from the Gulf Coast to the Atlantic Ocean and contains the country's second largest lake — Lake Okeechobee and the world famous Everglades wetlands. The District operates approximately 3,000 kilometers (~1,800 miles) of canals and over 500 water control structures. Near-real-time NEXRAD rainfall data and rain gage network is used to manage water resources in South Florida. The District uses a network of approximately 287 active rain gage stations that cover the more populated and environmentally sensitive areas. Five NEXRAD (Next Generation Weather Radar) sites operated by the National Weather Service cover the region. In conjunction with three of the other five water management districts in Florida, the District has acquired processed radar data from OneRain (formerly NEXRAIN Corporation) since July 2002. The 15-minute radar rainfall data was derived from the 2-km x 2-km high-resolution precipitation product, which was produced from NWS Level 3 — NEXRAD reflectivity. To achieve improved accuracy, gage-adjusted radar rainfall data were derived. This paper provides details on improvements that were made to existing radar rainfall data processed and provided for the District. A bias correction methodology was identified to improve data quality and accuracy in the existing radar rainfall data. The method was applied to existing rainfall data and its performance evaluated and assessed. The improvement obtained through reprocessing the existing radar rainfall data was summarized through data comparison of annual totals and by comparison to gage accumulations over the three watershed areas. In addition, Level 2 NEXRAD reflectivity data (also 2-km x 2-km)from surrounding radars were processed to estimate radar rainfall data using a standard Z-R relationship. During a validation event, the radar rainfall data derived from Level 2 produced better agreement and more accurate rainfall than either the existing radar rainfall product or the reprocessed data. The NEXRAD data quality improvement process performed in this study increased the amount of rainfall through application of the spatially variable bias correction, produced more consistent results through verification at control gages using statistical performance measures.