An Examination of Radar and Rain Gauge–Derived Mean Areal Precipitation over Georgia Watersheds

Abstract

Compared to conventional rain gauge networks, the Weather Surveillance Radar-1988 Doppler provides precipitation estimates at enhanced spatial and temporal resolution that River Forecast Centers can use to improve streamflow forecasts. This study documents differences between radar-derived (stage III) mean areal precipitation (MAPX) and rain gauge?derived mean areal precipitation (MAP). The area of study is the headwaters of the Flint River basin, specifically the Culloden basin located in central Georgia south of Atlanta, with a drainage area of 1853 mi2. The timing of radar installations in the southeast United States provided overlapping data for only 2 yr (Jun 1996?Jul 1998). The MAP and MAPX products being examined were prepared using procedures identical to those employed operationally at the National Weather Service?s Southeast River Forecast Center. Results show that the radar (MAPX) underestimates gauge-derived rainfall (MAP) by ?38% at the end of the 2-yr period. This underestimate is most pronounced during the winter months of November?April when MAPX underestimates MAP by ?50%. Comparisons during the summer (May?Oct) indicate that MAPX is similar to MAP. The underestimation of winter rainfall likely is due to several factors: the inappropriate combination of radar values in areas of overlapping coverage, the radar beam overshooting the tops of stratiform rainfall, an inappropriate Z?R relationship, faulty radar calibration, and too few hourly rain gauges to prepare an accurate stage II bias adjustment factor and quality control the stage III product.