Estimating Energy Dissipation Rate from Breaking Waves Using Polarimetric SAR Images.

Rafael D Viana,João A Lorenzzetti,Ferdinando Nunziata,Jonas T Carvalho

doi:10.3390/s20226540

Abstract

The total energy dissipation rate on the ocean surface, (W m), provides a first-order estimation of the kinetic energy input rate at the ocean–atmosphere interface. Studies on the spatial and temporal distribution of the energy dissipation rate are important for the improvement of climate and wave models. Traditional oceanographic research normally uses remote measurements (airborne and platforms sensors) and in situ data acquisition to estimate ; however, those methods cover small areas over time and are difficult to reproduce especially in the open oceans. Satellite remote sensing has proven the potential to estimate some parameters related to breaking waves on a synoptic scale, including the energy dissipation rate. In this paper, we use polarimetric Synthetic Aperture Radar (SAR) data to estimate under different wind and sea conditions. The used methodology consisted of decomposing the backscatter SAR return in terms of two contributions: a polarized contribution, associated with the fast response of the local wind (Bragg backscattering), and a non-polarized (NP) contribution, associated with wave breaking (Non-Bragg backscattering). Wind and wave parameters were estimated from the NP contribution and used to calculate from a parametric model dependent of these parameters. The results were analyzed using wave model outputs (WAVEWATCH III) and previous measurements documented in the literature. For the prevailing wind seas conditions, the estimated from pol-SAR data showed good agreement with dissipation associated with breaking waves when compared to numerical simulations. Under prevailing swell conditions, the total energy dissipation rate was higher than expected. The methodology adopted proved to be satisfactory to estimate the total energy dissipation rate for light to moderate wind conditions (winds below 10 m s), an environmental condition for which the current SAR polarimetric methods do not estimate properly.

Highlights

Wave breaking plays a significant role in momentum, energy and gas exchanges at the ocean–atmosphere interface
This study aims to analyze the potential use of NP contribution presented in the co-polarized backscattering Synthetic Aperture Radar (SAR) return to estimate the total energy dissipation rate, extending the interval of estimation for the low-to-moderate winds
Using the simplified two-scale models (TSM) model (Figure 3), the relative contribution for both polarizations varied between 10% and 50%, with the lowest values occurring mostly in the high incidence angles

Summary

Introduction

Wave breaking plays a significant role in momentum, energy and gas exchanges at the ocean–atmosphere interface. Wave breaking controls the maximum height of surface waves, and can affect the skill of operational wave models [1]. The flux of greenhouse gases have been shown to depend on the water-side turbulence [2,3], which depends on the wave breaking [4,5,6]. The breaking of surface waves produces whitecaps, promotes the mixing and enhances the Sensors 2020, 20, 6540; doi:10.3390/s20226540 www.mdpi.com/journal/sensors. Sensors 2020, 20, 6540 insertion of bubbles in the surface layers in the ocean [1]. This influences the estimation of the inherent optical properties of the ocean, and the interpretation of the ocean color data [7,8]. Whitecap cover serves as a first step in the modeling of sea spray droplet as a sea spray source [9]

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Discussion

Conclusion