Modeling the SAR Signature of Nonlinear Internal Waves

Abstract

LONG-TERM GOALSThe long-term goal of this project is to develop a forward model that predicts the Synthetic Aperture Radar (SAR) signature of Non-Linear Internal Waves (NLIWs) under a range of environmental conditions. OBJECTIVESThe objectives of this project are to understand, quantify and model the factors that influence the SAR signature of internal waves (IWs) using in situ and remote measurements. To accomplish this we will determine the factors that impact the both the surface roughness and the corresponding radar backscattering cross section. Two factors that influence surface roughness that have not been included explicitly in earlier models for predicting the SAR signature of IWs are compound modulation and breaking waves. We are developing models for both of these contributions. APPROACHThe technical approach pursued in this work is to implement an enhanced form of the model developed by Lyzenga and Bennett (1988) (L&B). The L&B model uses the action balance equation to model the spatial/temporal changes to the wave action spectral density produced by interactions arising between surface waves and currents generated by the passage of an IW. The L&B model predicts the modulated surface roughness and corresponding backscattering cross section. The enhancements we are incorporating are to include the effects of compound modulation (NLIW modulation of intermediate-scale waves which in turn modulate centimeter-scale waves) and breaking waves into the framework of the L&B model. Though the current implementation of this model uses the PiersonMoskowitz spectrum, future versions we are developing will use other ocean wave spectral models. In addition, we are taking advantage of recent developments in numerical methods to make this implementation of the model both more computationally efficient and more robust. WORK COMPLETEDIn support of model development and validation we have produced a database that includes collocated satellite data (ENVISAT, ERS, SAR imagery as well as QuikScat wind vector maps), ship-based radar measurements (from the US-based R/V Revelle and the Taiwanese ship OR3), and moored thermistor chain (S7) measurements acquired in the South China Sea during the intensive data collection conducted in the summer of 2005. Though the experiment was conducted over the course of a few