Abstract
The rain-free normalized radar cross-section (NRCS) measurements from the Ku-band precipitation radars (PRs) aboard the tropical rainfall measuring mission (TRMM) and the global precipitation measurement (GPM) mission, along with simultaneous sea surface wind truth from buoy observations, stepped-frequency microwave radiometer (SFMR) measurements, and H*Wind analyses, are used to investigate the abilities of the quasi-specular scattering models, i.e., the physical optics model (PO) and the classical and improved geometrical optics models (GO and GO4), to reproduce the Ku-band NRCS at low incidence angles of 0–18° over the wind speed range of 0–45 m/s. On this basis, the limitations of the quasi-specular scattering theory and the effects of wave breaking are discussed. The results show that the return caused by quasi-specular reflection is affected significantly by the presence of background swell waves at low winds. At moderate wind speeds of 5–15 m/s, the NRCS is still dominated by the quasi-specular reflection, and the wave breaking starts to work but its contribution is very small, thus, the models are found in excellent agreement with the measurements. With wind speed increasing, the impact of wave breaking increases, whereas the role of standard quasi-specular reflection decreases. The wave breaking impact on NRCS is first visible at incidence angles near 18° as wind speed exceeds about 20 m/s, then it becomes dominant when wind speed exceeds about 37 m/s where the NRCS is insensitive to wind speed and depends linearly on incidence angle, which cannot be explained by the standard quasi-specular scattering theory.
Highlights
Microwave radar sensing of the sea surface at low incidence angles is presently at the heart of many applications, such as wave spectrum estimation from wave spectrometer observations, wind speed retrieval from radar altimeter returns, and wave spectrum and wind speed estimations from wide swath interferometric imaging radar altimeter measurements (e.g., [1,2,3])
The buoy observations of sea surface continuous winds are from 58 moored buoys in the waters around the US operated by the National Data Buoy Center (NDBC)
The stepped-frequency microwave radiometer (SFMR) is an airborne remote sensing instrument built by ProSensing, Inc. of Amherst, MA, USA and used by the National Oceanic and Atmospheric Administration (NOAA) Hurricane Research Division (HRD) for operational surface wind estimation in hurricanes
Summary
Microwave radar sensing of the sea surface at low incidence angles is presently at the heart of many applications, such as wave spectrum estimation from wave spectrometer observations, wind speed retrieval from radar altimeter returns, and wave spectrum and wind speed estimations from wide swath interferometric imaging radar altimeter measurements (e.g., [1,2,3]). The GO model is only valid in the optical limit and exhibits significant deviations in the microwave regime, that is, it should be improved at finite wavelength [7,10] In this context, it is often resorted to a “radar-filtered” mss, which is contributed by ocean waves whose lengths are greater than the radar wavelength (e.g., [11]). In this paper, the abilities of the PO, GO, and GO4 models to reproduce the low-incidence NRCS are systematically compared and analyzed, and on this basis, the limitations of the quasi-specular scattering theory and the effects of wave breaking are discussed.
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