An examination of radar and rain gange-derived mean areal precipitation over Georgia watersheds

Compared to conventional rain gauge networks, the Weather Surveillance Rada r-1988 Doppler provides precipitation estimates at enhanced spatial and tem poral resolution that River Forecast Centers can use to improve streamflow forecasts. This study documents differences between radar-derived (stage II I) mean areal precipitation (MAPX) and rain gauge-derived mean areal precip itation (MAP). The area of study is the headwaters of the Flint River basin , specifically the Culloden basin located in central Georgia south of Atlan ta, with a drainage area of 1853 mi(2). The timing of radar installations i n the southeast United Stares provided overlapping data for only 2 yr (Jun 1996-Jul 1998). The MAP and MAPX products being examined were prepared usin g procedures identical to those employed operationally at the National Weat her Service's Southeast River Forecast Center. Results show that the radar (MAPX) underestimates gauge-derived rainfall (M AP) by similar to 38% at the end of the 2-yr period. This underestimate is most pronounced during the winter months of November-April when MAPX undere stimates MAP by similar to 50%. Comparisons during the summer (May-Oct) ind icate that MAPX is similar to MAP. The underestimation of winter rainfall l ikely is due to several factors: the inappropriate combination of radar val ues in areas of overlapping coverage, the radar beam overshooting the tops of stratiform rainfall, an inappropriate Z-R relationship. faulty radar cal ibration, and too few hourly rain gauges to prepare an accurate stage II bi as adjustment factor and quality control the stage III product.