Abstract. This paper presents a new technique for estimating the wind friction velocity at the ocean surface from C-band radar scatterometer measurements. This technique uses physical models of ocean surface waves and electromagnetic backscattering from a rough surface at intermediate angles of incidence to generate predictions of the normalized radar cross section (NRCS, or rr ø) of the ocean surface for a given wind friction velocity and observational geometry. The ocean spectral model used in this technique has been developed specifically for this application. It combines in situ wave measurements at low wave numbers with the Phillips [1985] equilibrium spectral model. This choice of ocean wave model is supported by a set of open ocean wave measurements summarized in this paper. A suite of models, derived from both in situ and remote measurements of the sea surface, is used to characterize the directional spreading of ocean waves relative to the wind direction. The resulting two-dimensional ocean wave spectra are used with a composite surface model to predict radar backscattering from the ocean surface at Cband. These radar cross-section predictions are combined with ERS-1 scatterometer measurements in a cost function minimization scheme to yield estimates of the friction velocity vector at the ocean surface. We present examples of this technique and compare friction velocity retrievals obtained via this scheme with buoy-based measurements under a variety of wind and wave conditions. On the basis of the analysis of a limited number of cases, this technique yielded friction velocity estimates for which the magnitude was within 22% and the direction was within _+25 ø . Given that scientific applications require magnitude estimates within 10-15% and directional estimates within _+20 ø of in situ measurements, these preliminary results suggest that this is a promising approach to wind retrieval. Introduction
The long-range objective of this work is to investigate non-Bragg sea surface scattering at intermediate angles of incidence. We seek to define the characteristics and statistical properties of these scatterers and to establish a link between radar observations and the underlying physical processes. OBJECTIVES Our objective is to investigate scattering from the sea surface that seemingly cannot be explained using composite surface models based on Bragg scattering theory. Models based on Bragg scattering adequately predict most backscatter observations, but several phenomena are not well predicted using this type of approach, suggesting the models exclude some relevant physical processes. This research examines scattering events that are apparently inconsistent with composite surface theory, and seeks to identify prospective mechanisms and to develop predictive models for these non-Bragg scatterers. APPROACH Our approach is to analyze data from the SAXON-FPN experiment conducted in 1990/1991 (described in detail by Plant and Alpers, 1994) to define the physical and statistical characteristics of non-Bragg sea surface scattering. This data set includes simultaneously acquired horizontally (HH) and vertically (VV) polarized radar cross-section observations at several frequencies, and detailed environmental measurements. The ratio of HH-to-VV cross sections is commonly used to separate Bragg and non-Bragg scattering. Since Bragg scattering predicts that on average VV cross sections should be larger than HH at moderate incidence angles, events for which the HH cross sections exceed the VV are often designated as non-Bragg scattering. We have investigated the ratio of HH-to-VV, previously used as a criterion for designating non-Bragg scattering events, by comparing the data with simulations based on composite surface theory, and through statistical analysis of the observations. We have found that the polarization ratio does not serve as a useful metric for identifying non-Bragg scattering. To explain events where HH cross-section exceeds VV, we have investigated physical mechanisms including bound waves and sea spray. In addition, we have analyzed the characteristics of extreme
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
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.