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
OBJECTIVES AND IMPORTANCENonlinear Internal Waves are pervasive globally, particularly in coastal waters. The currents and displacements associated with internal waves influence acoustic propagation and underwater navigation, as well as ocean transport and mixing. Synthetic Aperture Radar (SAR) imagery can reveal the surface manifestations of internal waves (IWs) in satellite imagery and so is routinely used to locate and characterize these features. Though some of the mechanisms that link the SAR signatures, surface processes, and the underlying internal structures have been understood for decades, a complete characterization has yet to emerge, making SAR imagery useful only as a qualitative tool. The objective of this research is to develop and validate a forward model to predict the SAR signature of NLIWs that explicitly includes relevant mechanisms that impact the sea surface roughness and corresponding backscattering cross section, such as wind speed and direction, compound modulation (i.e. modulation of intermediate-scale waves by IWs, which in turn modulate smaller waves), microscale breaking and breaking waves. SIGNIFICANT RESULTS AND LESSONS LEARNED BackgroundFor the first part of this effort we developed a computationally efficient, web-based implementation of the Lyzenga and Bennett (1988) (LB) model, which uses the action balance equation to model the interactions between surface waves and currents, such as those generated by the passage of an 1W. The action balance equation can be expressed, in general as:where N(k) is the wave action spectrum (see Phillips, 1980) at a given wavenumber, k; Q(k) is the action source term that includes contributions from wind input, wave interactions and dissipative processes; x indexes spatial location along the current profile attributable to an IW, o is the radian frequency, cgi is the group velocity and ui is the current due to the internal wave in the direction indexed by x.
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
Grant Number: N00014-01-1-0153 http://www.apl.washington.edu LONG-TERM GOALSThe long-range objective of this work is to develop a detailed characterization of non-Bragg scattering from a wind-roughened water surface, or a breaking wave region, and establish a link between radar observations and physical processes, leading ultimately to a more complete, physically-based model for predicting radar scattering from the sea surface. OBJECTIVESOur objective is to characterize non-Bragg scattering from the air-sea interface as a function of environmental conditions and radar parameters to improve our understanding of the mechanisms responsible for non-Bragg sea surface scattering and to develop models appropriate for this type of scattering. Most predictive models of radar backscattering from the sea surface rely on Bragg-based characterization of sea surface scattering. Evidence suggests that although Bragg scattering from the sea surface accounts well for many observations, some phenomena exist that it is unable to explain, indicating that it does not adequately represent all of the physical processes. A physically-based model of sea surface scattering that takes into account both Bragg and non-Bragg scattering will ameliorate this situation. APPROACHOur approach is to study non-Bragg scattering from the air-sea interface using existing data sets that include both environmental and radar measurements. Our investigations focus on the following issues:Non-Bragg Criterion. The ratio of normalized radar backscatter from the roughened water surface
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 have been studying the variables that impact the both the surface roughness and the corresponding radar backscattering cross section. Previous efforts focused on modeling the the modulation of surface waves by IWs using the action balance equation with source terms of energy input from the wind and energy loss from viscous dissipation. Our work seeks to include additional action source terms such as wave-wave interaction, generation of parasitic capillary waves and breaking waves. APPROACHTwo different methods have been pursued for this project. The first is based on the work of Lyzenga and Bennett (1988), who used the action balance equation (Equation 1) to model the spatial and temporal changes to the wave action spectral density arising from interactions with currents.The L&B model assumes sources and sinks (Q(k)) of action are input from the wind and energy loss from viscous dissipation. Using the action balance equation they predict the surface roughness modulation produced by the passage of an IW and the corresponding changes to backscattering cross section. We first implemented the L&B technique using the spectral model described in that paper; we subsequently modified the technique to use the Donelan and Pierson (1987) spectral model, which uses the same action source terms as the L&B model, but has more physically realistic directional characteristics.We implemented a second technique (Kudryavtsev et al. (2003(Kudryavtsev et al. ( , 2005) to predict the modulation of surface waves by IW-produced currents. This approach includes an innovative spectral model (Kudryavtsev, 1999) and additional action source terms associated with wave-wave interactions and 1
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