Abstract. In this paper a description is given of a physically based theoretical ocean backscatter model (called the VIERS-1 model) for intermediate incidence angles, and a comparison of its performance against the CMOD4 empirical model is made. The VIERS-1 scatterometer algorithm is based on a two-scale composite surface model which includes both specular and Bragg scattering. Its short wave model is based on the energy balance equation and accounts for viscous damping, slicks, dissipation due to whitecapping, and nonlinear three-and four-wave interactions. A number of parameters in the model have been determined by means of laboratory data and analyzed European Centre for MediumRange Weather Forecasts (ECMWF) winds. Because of the two-scale approach the wave number up to which Bragg scattering applies should be determined. This is done by means of laboratory data at X band. In addition, laboratory data of the wave spectrum have been utilized to validate the VIERS-1 short wave spectrum. An inverse of the algorithm is developed to derive wind speed and direction from the observed (ERS-1) backscatter and by comparison with ECMWF analyzed winds' three parameters for the short wave spectrum, namely, the Phillips parameter, the directional width of the spectrum, and the wave number boundary between gravity waves and short waves have been obtained. Comparisons between VIERS-I, C band model, version 4 (CMOD4), and ECMWF analyses are made. VIERS-1 performs better in the high wind speed range, and this feature is of importance when scatterometer winds are assimilated into an atmospheric model. However, in terms of backscatter rather than wind speed, CMOD4 shows better results. It is suggested that this is caused by the too simple directional distribution of the VIERS-1 short wave spectrum.
With the operation of the European Remote Sensing (ERS) satellites and RADARSAT, radar images are now readily available. One of the new applications of radar images is their use for bathymetric mapping in shallow seas. The Bathymetry Assessment System (BAS), described in detail in this paper, constructs accurate depth maps from radar images and a limited number of echo soundings. The BAS consists of a forward imaging model and an inversion part. The system needs a rst guess depth map that may be derived from echo soundings or an old map of the area. The forward model calculates a simulated radar image. This is compared with the actual radar image by evaluating a penalty function. The penalty function also contains a term that compares model depths with measured depths and a term that contains a smoothness criterion, prohibiting speckle noise to be interpreted as depth variations. The inversion part of the system consists of optimization of the penalty function. This leads to an iterative procedure in which some model parameters are also estimated. When converged, the model depth is an estimate for the real depth. This model depth matches radar images and echo soundings as closely as possible. The system may be regarded as an intelligent interpolator: it interpolates depth between transects of echo soundings steered by the bathymetric information in radar images. The system has been applied many times and some examples are given in this paper. Its accuracy depends on the number of echo soundings fed into the system, the number and quality of the radar images, and the nature of the area under consideration. When a root mean squared error of 30 cm (compared to echo soundings) is acceptable, the distance between the tracks of echo soundings needed by the BAS varies between 600 m to 1 km or more. This should be compared to the usual track distance that is 200 m at most. Use of the BAS may therefore lead to a considerable improvement in eYciency. The accuracy of the system can be improved by using airborne radar images with higher resolution.
Presented here is a simple analytical model based on established physics of the magnitude of the hydrodynamic modulations caused by sand waves. The model describes the modulations of the radar backscatter when first-order Bragg scattering is assumed. The major difference between this model and existing analytical models is that the specific shape of sand waves is incorporated. An assessment is made of the key parameters (shape, oceanographic, meteorological and radar) that influence the radar backscatter modulation. It is shown that depth, steepness of the slope, and height of the sand wave are the shape parameters that determine the radar backscatter modulation. The maximum backscatter modulation that can be found for sand waves in nature is approximately 3 dB. It is shown that sand waves in the NorthSea near the Dutch coast have a linear relation between their heights and slopes. Implementation of this relation simplifies the model further. Furthermore, backscatter modulations calculated with two radar backscatter models are compared and discussed. The correlation between predictions and measurements with the airborne imaging radar of NASA's Jet Propulsion Laboratory is considered encouraging. Measurements from the images indicate a relation between sand wave height and brightness modulation similar to that predicted by the model. 1. Introduction Knowledge of sea bottom topography is of vital hnportance for shipping, fisheries and all kinds of off shore activities. Particularly near harbors and shipping lanes, updated information on translation and height changes of sandbanks and sand waves is essential for the safety of navigation by deep-draught shipping. An example of the thne-and moneyconsulning ship-based surveys necessary to obtain this valuable information is given by Burton [1977]. Morphodynamic models describing the translation and height changes of these bed forths are presently under development. However, this development and the validation and calibration of these models require the input of many traditional, costly observations of this type. Radar hnages of the sea recorded under suitable conditions from aircraft or spacecraft offer an alternative way to obtain this information with high resolution for a large area. It is well known that under favorable meteorological and hydrodynamical conditions, sand waves become visible in airborne and spaceborne radar hnages of shallow seas. This phenomenon was observed for the first thne by Copyright 1995 by the American Geophysical Union. Paper number 94JC00957. 0148-0227/95/94JC-00957505.00 de Loor and coworkers in Q-band side-looking airborne radar imagery of the North Sea [De Loor and BrunsveM van Hulten, 1978; De Loor, 1981].It is now generally accepted that the hnaging mechanism consists of three steps: (1) interaction between tidal current and sand waves causes spatial modulations in the surface current velocity;(2) modulations in the surface current velocity give rise to variations in the spectrum of wind-generated waves (wave-current interaction): an...
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