Measurements of ocean surface backscattering using an airborne 94-GHz cloud radar - Implication for calibration of airborne and spaceborne W-band radars

Backscattering properties of the ocean surface have been widely used as a calibration reference for airborne and spaceborne microwave sensors. However, at millimeter-wave frequencies, the ocean surface backscattering mechanism is still not well understood, in part, due to the lack of experimental measurements. During the Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment (CRYSTAL-FACE), measurements of ocean surface backscattering were made using a 94-GHz (W band) cloud radar on board a NASA ER-2 high-altitude aircraft. This unprecedented dataset enhances our knowledge about the ocean surface scattering mechanism at 94 GHz. The measurement set includes the normalized ocean surface cross section over a range of the incidence angles under a variety of wind conditions. It was confirmed that even at 94 GHz, the normalized ocean surface radar cross section, sigma(o) is insensitive to surface wind conditions near a 10 degrees incidence angle, a finding similar to what has been found in the literature for lower frequencies. Analysis of the radar measurements also shows good agreement with a quasi-specular scattering model at low incidence angles. The results of this work support the proposition of using the ocean surface as a calibration reference for airborne millimeter-wave cloud radars and for the ongoing NASA CloudSat mission, which will use a 94-GHz spaceborne cloud radar for global cloud measurements.