Effect of non-Gaussian properties of the sea surface on the low-incidence radar backscatter and its inversion in terms of wave spectra by an ocean wave spectrometer

Abstract

The principle of ocean wave spectrometers was fi rst presented several decades ago to detect the directional wave spectrum with real-aperture radar (Jackson, 1981). To invert wave spectra using an ocean wave spectrometer, for simplicity, the hydrodynamic forcing and wave-wave interaction effect are neglected and a Gaussian slope probability density function (pdf) is used to calculate the normalized backscattering cross-section ( σ 0 ) of the ocean surface. However, the real sea surface is non-Gaussian. It is not known whether the non-Gaussian property of the sea surface will affect the performance of the inversion of the wave spectrum if following existing inversion steps and methods. In this paper, the pdf of the sea surface slope is expressed as a Gram-Charlier fourth-order expansion, which is quasi-Gaussian. The modulation transfer function (MTF) is derived for a non-Gaussian slope pdf. The effects of non-Gaussian properties of the sea surface slope on the inversion process and result are then studied in a simulation of the SWIM (Surface Waves Investigation and Monitoring) instrument confi guration to be used on the CFOSAT (China-France Oceanography Satellite) mission. The simulation results show that the mean trend of σ 0 depends on the sea slope pdf, and the peakedness and skewness coeffi cients of the slope pdf affect the shape of the mean trend of σ 0 versus incidence and azimuth; owing to high resolution of σ 0 in the range direction, MTF obtained using the mean trend of σ 0 is almost as accurate as that set in the direct simulation; in the inversion, if ignoring the non-Gaussian assumption, the inversion performances for the wave spectrum decrease, as seen for an increase in the energy error of the inverted wave slope spectrum. However, the peak wavelength and wave direction are the same for inversions that consider and ignore the non-Gaussian property.Keyword : slope probability density of the sea surface; ocean wave spectrometer; non-Gaussian; directional wave spectrum; energy error 1 INTRODUCTION

Quasi-Gaussian probability density function of sea wave slopes from near nadir Ku-band radar observations

“…Apart from the accuracy of GO4, the sensitivity of the backscattering to the quasi-Gaussian slope PDF parameters also need to be considered for the choice of the incidence angle range. Direct modeling of σ • as proposed in Ping Chen and Huang (2015) for the QS case, shows that a variation in peakedness will more significantly affect medium incidence angles than incidence angles very close to nadir. Thus, in order to efficiently invert the peakedness coefficients from the σ • profile, the largest possible range of incidence angle should be chosen.…”

Describtion of sea surface slopes in applications related to radio wave reflection.

“…Чтобы оценить вклад волн разных масштабов в дисперсию уклонов,  от 0  , полученными на основе измерений [13]. Параметр Оценки 1-3 параметра E  получены на основе данных радиометрических измерений на трех длинах волн [17]; оценки 4 -по данным радиолокационных измерений [20]; оценки 5 и 6 -по данным измерений лазерным уклономером [21,22] соответственно; оценки 7 -по данным измерений струнными волнографами [23]; оценки 8 -по данным измерений волнографическим буем [24].…”