Understanding Ku-Band Ocean Radar Backscatter at Low Incidence Angles under Weak to Severe Wind Conditions by Comparison of Measurements and Models

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.

Ocean Wind Observation Based on the Mean Square Slope Using a Self-Developed Miniature Wave Buoy

“…However, even under ideal wavemaking conditions, there is always a limit to the frequency range in which the slope ratio equals one. When calculating MSS, the ocean surface low-pass-filtered mean square slope with an upper bound is frequently used (as shown in Equation (5)): where ufc is the upper cutoff frequency, which usually ranges from one-sixth to one-third of the working wavenumber of the altimeter (e.g., [ 20 , 21 , 22 , 23 ]). The first systematic LPMSS data set is reported by Cox and Munk [ 20 ].…”

“…The corresponding electromagnetic wavenumber of the L band frequency (1.41 GHz) is 29.53 rad m −1 . The upper bound of the MSS integration is taken as one-third of the electromagnetic wave number [ 22 , 23 ], and the corresponding ufc is 1.56 Hz based on the deep water–wave dispersion relationship. Two vertical solid lines are shown in Figure 3 d, with the right one marking the position of 1.56 Hz.…”