Application of the Hydro-Estimator rainfall algorithm over Hawaii

The Hydro-Estimator (HE) has been the operational satellite rainfall algorithm of the National Environmental Satellite, Data, and Information Service (NESDIS) since the autumn of 2000, and has been available over the continental United States to National Weather Service (NWS) forecasters via the Advanced Weather Interactive Processing System (AWIPS) since the spring of 2004. The HE uses infrared (IR) window channel (10.7-µm) brightness temperatures as the main basis for discriminating raining from non-raining areas, and also uses data from numerical weather prediction models to provide additional information about moisture availability, subcloud evaporation of precipitation, impact of the thermodynamic profile on cloud heights, and orographic enhancement or reduction of rainfall.

Hawaii is an area where satellite-based rainfall information would be highly useful, given the incomplete radar coverage over the state and the very rapid response time of the streams in that area. However, the HE was developed primarily for application to flash-flood-producing rainfall produced by deep convective clouds. This has greatly limited its applicability to regions like Hawaii, where complex topography and oceanic air masses often combine to produce very heavy rainfall from very warm cloud tops. To make the HE more applicable to this region, a special HE calibration has been developed for Hawaii using data from that region for a variety of meteorological situations. This special calibration will be described, and comparisons will be made between the recalibrated HE, the original HE, and radar and rain gauge data over Hawaii.