TOWARD AN OBJECTIVE ANALYSIS OF RAINFALL RATE COMBINING OBSERVATIONS AND SHORT-TERM FORECAST MODEL ESTIMATES

This study explores the feasibility of performing an objective analysi s of instantaneous rain rate combining satellite estimates (and eventu ally other types of observations) with those from a numerical predicti on model using the method of statistical interpolation. Results demons trate that the quality of the short-term precipitation forecasts servi ng as background field has reached a level that makes such an objectiv e analysis possible. The two main requirements to obtain an accurate a nalysis from available information are a realistic estimate of backgro und field and observation errors and knowledge of the horizontal corre lation of these errors with distance. The importance of specifying the errors for joint model-observation situations is emphasized; it is es pecially important in situations where model and observations are in c onflict. These aspects of the problem are studied using collocated 6-h forecasts with satellite estimates derived from visible and infrared imagery, and ground-truth rainfall data available over Japanese territ ory from the Global Precipitation Climatology Project. Over 90 000 tru th-model-satellite collocations are available at the common scale of 1 30 km x 130 km. An alternative means of establishing the model error c orrelation with distance and azimuth direction from 6- and 18-h foreca st differences valid at the same time yield results that are similar t o those derived from collocations with truth rainfall over large domai ns, but not locally; this result suggests a means of relaxing the assu mption of homogeneity and isotropy of model errors. The sensitivity of the rain rate analysis to different specifications of the satellite t o model error ratios is shown with an example.