Initial validation of the global precipitation climatology project monthlyrainfall over the United States

The Global Precipitation Climatology Project (GPCP) established a multiyear global dataset of satellite-based estimates of monthly rainfall accumulati ons averaged over a grid of 2.5 degrees x 2.5 degrees geographical boxes. T his paper describes an attempt to quantify the error variance of these esti mates at selected reference sites. Fourteen reference sites were selected o ver the United Stares at the GPCP grid locations where high-density rain ga uge network and high-quality data are available. A rigorous methodology for estimation of the error statistics of the reference sites was applied. A m ethod of separating the reference error variance from the observed mean squ are difference between the reference and the GPCP products was proposed and discussed. As a result, estimates of the error variance of the GPCP produc ts were obtained. Two kinds of GPCP products were evaluated: 1) satellite-o nly products, and 2) merged products that incorporate some rain gauge data that were available to the project. The error analysis results show that th e merged product is characterized by smaller errors, both in terms of bias as well as the random component. The bias is, on average, 0.88 for the merg ed product and 0.70 for the satellite-only product. The average random comp onent is 21% for the merged product and 79% for the satellite-only product. The random error is worse in the winter than in the summer. The error esti mates agree well with their counterparts produced by the GPCP.