A Feasibility Study for a Wide-Swath, Airborne, Hurricane Imaging Microwave Radiometer for Operational Hurricane Measurements W. Linwood Jones, Jun-Dung Park and Josko Zec University of Central Florida; Orlando, FL Christopher S. Ruf University of Michigan; Ann Arbor, MI Marion C. Bailey Research Triangle Institute; Hampton, VA James W. Johnson NASA Langley Research Center; Hampton, VA ABSTRACT This paper presents a conceptual design of an airborne Hurricane Imaging (microwave) Radiometer (HIRad) instrument for use in operational hurricane surveillance. The basis of the HIRad design is the Stepped Frequency Microwave Radiometer (SFMR) that has successfully measured surface wind speed and rain rate in hurricanes from the NOAA Hurricane Research Division’s P-3 aircraft. Unlike the SFMR that views only at nadir, the HIRad provides wide- swath measurements between ± 45 degrees in incidence angle with a spot-beam spatial resolution of approximately 1-3 km. The system operates at four equally spaced frequency channels that cover a range between 4 GHz and 7 GHz. 1. INTRODUCTION Contemporary hurricane numerical models such as MM- 5 (Penn State/NCAR Mesoscale Model) have the ability to predict precipitation as well as to forecast storm evolution (intensity, size and track). To aid forecasting, the most important contribution that could be made from remote sensing platforms would be daily mapping of the surface wind field from the center of the storm to a distance just outside the ring of maximum winds located in or near the eyewall cloud. In the Atlantic basin such measurements are operationally available at limited times from sensors mounted on research aircraft, but none of the world's other hurricane basins have aircraft reconnaissance capabilities. Even in the Atlantic hurricanes are out of range of the aircraft for most of their lifetimes. Timely measurements of the surface wind fields in tropical cyclones, with wide swath (10's of km) and high resolution (1 km), would dramatically improve model initialization and resulting forecasts. II HURRICANE IMAGING RADIOMETER A. Instrument Heritage Retrievals of hurricane ocean surface wind speed and rain rate have been performed operationally by the Stepped Frequency Microwave Radiometer (SFMR) from aircraft by NOAA Hurricane Research Division (HRD) for more than a decade. SFMR was originally developed by the NASA Langley Research Center in the 1970s [Jones et al., 1981] and it has continued to be an integral part of NOAA operations since. Wind speed and rain rate are retrieved simultaneously from measurements of microwave brightness temperature (T B ) made by the nadir-viewing SFMR on board a NOAA P-3 flying at ~25,000 ft (7.6 km). Winds in excess of 180 mph (150 m/s) and rain rates of greater than 100 mm/h have been successfully estimated by the SFMR and validated against weather radars, dropsondes released from aircraft, and extrapolations of flight-level winds. Even at these extreme levels, the T B responses to both wind speed and rain rate have not reached saturation. The SFMR scans between 5 and 8 GHz with a variable number of channels. Retrievals have been demonstrated with as few as two and as many as eight channels. A minimum of two T B channels is required to uniquely separate the contrasting spectral signatures of surface emission and rain. Additional channels serve to over- constrain the system of equations that relate the T B measurements to the state parameters of wind speed and rain rate. This effectively reduces the sensitivity of the retrieval algorithm to instrument noise and common-mode calibration biases. The current operational NOAA sensor uses eight channels. B. HIRad Instrument Description The Hurricane Imaging (microwave) Radiometer (HIRad) is a candidate airborne sensor for future operational surface wind speed and rain rate measurements in hurricanes and typhoons. This sensor is an interferometric microwave radiometer that uses a one-dimensional thinned synthetic aperture array antenna to synthesize multiple simultaneous beams in a push-broom configuration. When used on an operational hurricane surveillance aircraft such the NOAA HRD's Gulfstream-IV (Fig. 1), the hurricane may be imaged with high resolution as shown in Fig. 2 & 3. Unlike the SFMR, that views only at nadir, the HIRad provides wide-swath measurements with simultaneous multiple "pushbroom" fan-beams between ± 45° in incidence angle. When flying at an altitude of 35,000 ft 0-7803-7537-8/02/$17.00 (C) 2002 IEEE