2814 IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 41, NO. 12, DECEMBER 2003 Calibrating the Quikscat/SeaWinds Radar for Measuring Rainrate Over the Oceans David E. Weissman, Fellow, IEEE, Mark A. Bourassa, James J. O’Brien, and Jeffrey S. Tongue Abstract—This effort continues a study of the effects of rain, over the oceans, on the signal retrieved by the SeaWinds scatterom- eter. It is determined that the backscatter radar cross section can be used to estimate the volumetric rain rate, averaged horizontally, across the surface resolution cells of the scatterometer. The dual polarization of the radar has a key role in developing this capa- bility. The relative magnitudes of the radar backscatter depends on the volumetric rain rate, the rain column height and surface wind velocity, the viewing angle, as well as the polarization (due to the oblateness of raindrops at the higher rain rates). The ap- proach to calibrating the SeaWinds normalized radar cross section (NRCS) is to collect National Weather Service Next Generation Weather Radar (NEXRAD) radar-derived rain rate measurements (4-km spatial resolution and 6-min rotating cycles) colocated in space (offshore) and time with scatterometer observations. These calibration functions lead to a Z–R relationship, which is then used at mid-ocean locations to estimate the rain rate in 0.25 or larger resolution cells, which are compared with Tropical Rainfall Map- ping Mission (TRMM) Microwave Imager (TMI) rain estimates. Experimental results to date are in general agreement with sim- plified theoretical models of backscatter from rain, for this fre- quency, 14 GHz. These comparisons show very good agreement on a cell-by-cell basis with the TMI estimates for both wide areas (1000 km) and smaller area rain events. Index Terms—Next Generation Weather Radar (NEXRAD), precipitation, radar reflectivity, scatterometer normalized radar cross section (NRCS), space-based radar, Tropical Rain Mea- suring Mission (TRMM). I. INTRODUCTION R AIN INTERFERES with the intended application of the measurements of the sea surface’s radar cross section by the SeaWinds scatterometer: the estimation of the sea surface wind speed and direction [1], [2]. The presence of rain regions within the boundaries of the illuminated surface area results in additional backscatter from these atmospheric volumes and pos- sibly attenuation of the beam directed toward the surface and the backscatter from the surface [3]. In many cases, depending on the wind magnitude, the backscatter from the rain will augment or dominate the received signal and the measured normalized Manuscript received September 19, 2002; revised July 27, 2003. This work was supported in part by the Physical Oceanography Program of the National Aeronautics and Space Administration (NASA) through grants to Hofstra Uni- versity and the Center for Ocean-Atmospheric Prediction Studies, Florida State University (through support by the NASA OVWST Project, the NASA/OSU SeaWinds Project, and NOPP), and in part by the National Weather Service through a COMET Partners Project grant to Hofstra University administered by the University Corporation for Atmospheric Research, Boulder, CO. D. E. Weissman is with the Department of Engineering, Hofstra University, Hempstead, NY 11549 USA. M. A. Bourassa and J. J. O’Brien are with the Center for Ocean-Atmospheric Prediction Studies, The Florida State University, Tallahassee, FL 32306-2840 USA. J. S. Tongue is with the National Weather Service, Upton, NY 11973 USA. Digital Object Identifier 10.1109/TGRS.2003.817975 radar cross section (NRCS). This results in overestimates of the wind magnitude and meaningless wind direction values [4]. The relatively long duration transmitted pulse provides the equiva- lent of a continuous-wave radar. The backscatter from the rain volume is the path-integrated value that may include the com- plete column of rain or possibly a thinner layer when the beam attenuation is large enough to suppress most of the power den- sity before it reaches the sea surface. The pencil beam geometry of the conically scanning Sea- Winds radar is shown in Fig. 1 [2]. The antenna consists of a single 1-m parabolic reflecting dish that accommodates feeds for both vertical and horizontal polarizations. The beams are coplanar and are incident on the surface with angles of 54 and 46 , respectively, from the local zenith. These result in differing radii for the illumination circles on the surface, of about 900 and 700 km. The distance of any wind vector cell (WVC) from the nadir track of the satellite determines the time differential between the V-pol and H-pol observations. With a space- craft speed of 7 km/s, the difference in time between the suc- cessive illumination of a cell by the V-pol and H-pol beams can be limited to vary only between 29 s (forward look) and 80 s (side look). Relative to the time rates of change of rain struc- tures in the atmosphere and rain rates at the surface, this time differential is viewed as not significant with respect to causing errors in measurements and interpretations within this current study. Collocating measurements with the two different polar- izations is straightforward, since each measurement cell is fully documented. When it is desirable to collect pairs of H-pol and V-pol measurements, the distances between the centers of cells for a selected pair are mostly 3–8 km. A separation and grouping was made between those cells viewed by the beams in the “for- ward” azimuth swing and those viewed in the “aft” azimuth angles. In certain comparisons to be shown below, it was de- sirable to eliminate the effects of the time difference when a cell is viewed by the forward beam followed by the aft beam, which may be two or more minutes later. This procedure also suppresses some of what would be the azimuth variation of , when the wind-driven sea surface backscattering is also con- tributing to the measurement. Steps were taken to quantify the effect of rain on SeaWinds NRCS, using Next Generation Weather Radar (NEXRAD) Level III rain data for in situ measurements, over the ocean and sufficiently far offshore to avoid land contamination issues. The nearly simultaneous NEXRAD composite reflectivity data (“Z” values; e.g., Fig. 2), were converted to rain rate in 4-km cells; then they were combined into an average over an approximate scatterometer footprint (a 28-km ), centered at each scatterometer cell. Z is the logarithmic radar reflectivity factor measured in units of decibels relative to a reflectivity of 1 mm m (dBZ). Each of the colocated SeaWinds NRCSs, 0196-2892/03$17.00 © 2003 IEEE