Retrieving capillary-gravity wave spectrum from polarimetric microwave radiation of ocean surface

Abstract

A new simple two-scale model on the polarimetric microwave emission of ocean surface is derived at first, which can be expressed as an integral of weighting functions (M 0 and M 2 ) and ocean surface curvature spectrum coefficients (C 0 and C 2 ). This provides a simple way to investigate the effect of curvature spectrum on ocean emission. It is found that ocean waves with wavelengths both comparable to and much greater than the electromagnetic wavelength can contribute to the harmonics of ocean surface microwave emission, depending on the magnitude of the ocean surface spectrum in these length scales. Brightness temperature predictions differ significantly due to present diverse spectrum models, and thus a study on wave spectrum obtained inversely from brightness temperature measurements is necessary. From the ocean surface radiation data measured by polarimetric microwave radiometer, we derived an ocean wave spectrum with a wider wave number range, using the proposed two-scale model and constrained linear least-squares method. The derived ocean wave spectrum is useful for comparing with present diverse models. microwave radiation of ocean surface, two-scale model, ocean wave, curvature spectrum Citation: Yin X B, Wang Z Z, Liu Y G. Retrieving capillary-gravity wave spectrum from polarimetric microwave radiation of ocean surface. Sci China Earth Sci, 2010, 53: 901-910,Communication theory is first applied to ocean wave study in 1964 [1], and the concept of the spectrum is used to describe ocean waves. The frequency spectrum gives the energy distribution among different frequencies of the sea surface wind waves. The directional spectrum can also give the wind wave energy distribution among different directions on the sea surface. The frequency spectrum, significant wave height, dominant wave period, mean frequency, peak frequency, spectrum width, wavelength, direction, and so on can be obtained from the directional spectrum. There are two main methods to measure ocean wind wave spectrum, i.e., in situ measurements and remote sensing measurements. Buoy and its arrays can measure the gravity wave spectrum, but cannot measure capillarygravity wave spectrum due to too short a wavelength. The present models on capillary-gravity wave spectrum are obtained based on optical image and remote sensing measurements of SAR (Synthetic Aperture Radar) and active radar scatterometer, but none based on microwave radiometer measurements until now.There are many spectrum models of ocean wind waves. Note that there are obvious differences among present models, some emphasize more energy in long gravity waves regions, and others predict more energy in short gravity regions. Therefore, it is significant to obtain an ocean wind wave spectrum with a better accuracy in a wider frequency range.