We report laboratory measurements of nonlinear parasitic capillary waves generated by longer waves in a channel. The experiments are conducted for three frequencies of longer waves ͑4, 5, and 6 Hz͒, corresponding to wavelengths of approximately 11, 7, and 5 cm. For these wavelengths we apply a model developed recently by Fedorov and Melville ͓J. Fluid Mech. 354, 1 ͑1998͔͒ to predict the wave profile. Based on a viscous boundary layer approximation near the surface, the model enables us to efficiently calculate gravity-capillary waves. We present direct comparisons that show good agreement between the measurements and numerical predictions over a range of parameters. Finally, we give some simple estimates for a sharp cutoff in the wave number spectra observed in both the numerical solutions and the laboratory measurements of short gravity-capillary waves.

Numerical simulation results of wind wave in the Caspian Sea by using wind forcing data are presented. The numerical modeling which is applied in this study is based on numerical spectral wave model which is based on Navier-Stokes equations. It solves these equations through each of mesh elements. Moreover, in this model high-resolution unstructured grid for the Caspian Sea has been used in order to reach finer accuracy. The wind forcing data are given from European Centre for Medium-Range Weather Forecasts (ECMWF). The measurement data, which are gained from Ports and Marine Organisation (PMO) of Iran, are used in order to estimate the accuracy of the model. The results have shown better accuracy in comparison with PMO simulation. Mean of the coefficient of determination (R-squared) for significant wave height in this simulation is 0.8059, though, in PMO simulation this coefficient is reported 0.7056. Better accuracy requires more measurement data and also finer resolution of bathymetry data.

Laboratory measurements of Ku-band scattering at grazing incidence are presented. This study was motivated by the need to understand the processes which significantly contribute to scattering at grazing incidence. A dual polarized (VV, HH) coherent pulsed Ku-band scatterometer with good temporal resolution (3 ns) was used to obtain Doppler spectra and the absolute cross-section of scattered signals for pazing angles from 6-12', and winds in the range 2-12 d s. Wire wave gauges were used to measure the wind-wave field. Measurements of the first few moments of the Doppler spectra (cross-section, central frequency and bandwidth) showed that the data separated into two groups. The first grouping corresponded to HH scattering in the upwind look direction, and was clearly associated with scattering from the dominant gravity wind-waves. The second grouping corresponded to HH scattering in the downwind look direction, and all W scattering, and was consistent with Bragg scattering from free higher frequency waves. This classification of the electromagnetic scattering was consistent with comparisons of direct and Doppler measurements of the kinematics of the surface wave field. The electromagnetic classification was also consistent with asymmetries in the wave field which increased with increasing wind speed.

Laboratory measurements of the fine space-time structure of short gravity-capillary waves, as well as Ku-band scattering at grazing and moderate incidence from wind waves in the large Delft Hydraulics Laboratory wind-wave channel are presented. This study was stimulated by the need to verify the processes that significantly contribute to scattering at grazing and moderate incidence. A scanning laser slope gauge was used for measuring capillary waves from 2-mm to 2-cm wavelengths and frequencies ranging up to 100 Hz. A dual-polarized (vertical, VV, and horizontal, HH), coherent, pulsed Ku-band scatterometer with good temporal resolution (3 ns) was used to obtain simultaneous Doppler spectra and the absolute cross section of scattered signals for grazing angles 6 and 25 and for winds in the range 2.5-12.5 m/s. Two-dimensional (2-D) filtering and bispectral analyses were used to separate and study the influence of free and bound surface waves. The results of this study demonstrate that the frequency-wavenumber spectra of capillary waves consist of two parts. The first corresponds to free capillary waves, which satisfy the dispersion relationship. The second corresponds to bound parasitic capillary waves, which are located near the crests of steep wind waves. The phase velocity of these capillary waves is approximately equal to the phase velocity of the steep waves.Measurements of the Doppler frequency of the scattered signals show that the Doppler spectra also have a bimodal structure. While the first low-frequency part of the spectrum corresponds to the Bragg scattering from the free capillary waves, the highfrequency part is associated with Bragg scattering from the bound capillary waves on the crests of the steep waves. This type of scattering is predominant for the upwind direction of illumination (especially for HH-polarization).Index Terms-Bragg scattering, electromagnetic scattering from rough surfaces, remote sensing.