An overview of the Hurricane Imaging Radiometer (HIRAD)

Accurate observations of ocean surface vector winds (OSVW) with high spatial and temporal resolution are critically important to improve both our understanding and predictability of tropical cyclones. As the successful NASA QuikSCAT satellite continues to age beyond its planned life span, many members of the tropical cyclone research and operational community recognize the need to develop new observational technologies and strategies to meet the essential need for OSVW information. This concern has been expressed in both the “Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond” developed by the National Research Council Committee on Earth Science and Applications from Space and the “Interagency Strategic Research Plan for Tropical Cyclone – The Way Ahead” developed by the Joint Action Group for Tropical Cyclone Research (JAG-TCR) sponsored by the Office of the Federal Coordinator for Meteorology.

One innovative technology development which offers the potential for new, unique remotely sensed observations of tropical cyclone OSVW and precipitation is the Hurricane Imaging Radiometer (HIRAD). This new instrument is passive microwave synthetic thinned aperture radiometer under development at the NASA Marshall Space Flight Center that will operate at the C-Band frequencies of 4, 5, 6, and 7 GHz. These frequencies have been successfully demonstrated by the NOAA nadir-staring Stepped Frequency Microwave Radiometer (SFMR) as useful for monitoring tropical cyclone ocean surface wind speeds and rain rates from low altitude reconnaissance aircraft. The HIRAD design incorporates a unique antenna design as well as several technologies that have been successfully demonstrated by the University of Michigan Lightweight Rain Radiometer sponsored by NASA Earth Science Technology Office Instrument Incubator Program.

HIRAD will be a compact, lightweight, low-power instrument with no moving parts that will produce imagery of ocean wind surface wind parameters and rain rate during the strong wind and heavy rain hurricane conditions that hamper the observational capabilities of higher frequency passive microwave radiometers or scatterometers. It will also produce imagery of sea surface temperature under cloudy and lightly precipitating skies eliminating the need for additional thermal infrared imagers. The strategic plan for HIRAD includes a roadmap for ocean surface wind speed and OSVW technology development using flight demonstrations on piloted aircraft, uninhabited aerial vehicle systems, and satellite platforms. The roadmap will include exit opportunities for technology transfer from research to operations based on satisfactory demonstrations. The first aircraft version of HIRAD will be singular polarization sensor designed to observe ocean surface wind speed and rainfall. The second aircraft version of HIRAD will be dual polarization sensor designed to observe OSVW as a prototype for a future satellite sensor.

The core HIRAD collaborative team consists of personnel from the NASA Marshall Space Flight Center, NOAA Hurricane Research Division, University of Central Florida, and University of Michigan. An additional sub-team of HIRAD collaborators, representing the NOAA Atlantic Oceanographic and Meteorological Laboratory (AOML), University of Miami, North Carolina State University, and Naval Research Laboratory are conducting an Observing Systems Simulation Experiment (OSSE) to evaluate the potential impact of HIRAD observations to operational hurricane wind analysis. Results of the OSSE are presented in a separate paper at this conference. These results suggest HIRAD will be a useful augmentation to the suite of satellite and aircraft sensors that are used in field experiments to study tropical cyclone genesis, tracking, intensity change, and precipitation structures. However, the most significant future benefit of HIRAD could be improved hurricane monitoring and forecasting capabilities for global communities who must contend with the serious impacts of tropical cyclones without adequate observational assets.