Re-calibrating the operational Hydro-Estimator satellite precipitation algorithm

The Hydro-Estimator (H-E) has been the operational satellite precipitation algorithm of the National Environmental Satellite, Data, and Information Service (NESDIS) since the fall of 2002. The H-E replaced the original Auto-Estimator (A-E) because the latter frequently classified cold cirrus anvils as rain clouds and thus significantly overestimated the spatial extent of heavy precipitation. This problem was initially corrected in the A-E by using radar data to identify non-raining clouds; however, such a correction could not be used to improve the A-E in regions outside the radar coverage umbrella where satellite estimates of rainfall were needed most. Consequently, an improved technique for using the infrared window brightness temperature field to screen out cirrus clouds was developed for the H-E, and the skill of the resulting algorithm led to its application in an operational setting.

However, users have continued to express concerns with various aspects of the performance of the H-E. At the root of many of these concerns is the calibration of the relationship between infrared window brightness temperature and rainfall rate. The rain rate curves used in the H-E are based on the A-E calibration curve, which in turn was developed using a relatively small sample of collocated radar and satellite data. In response, a re-calibration of the algorithm is being performed using a more extensive data set covering aprpoximately eight weeks of the 2003 convective season, and rain gauge data are being used to bias-corerct the radar instead of using uncorrected radar as was done with the original A-E. The re-calibrated algorithm and the impacts on performance as assessed using parallel runs will be presented.