Abstract

Many authors assume that the frequency peak and the wavenumber peak of an ocean wave height variance spectrum are related by the ocean wave dispersion relationship. This note shows that this is not true and that the true relationship depends on the shape of the spectrum, thereby introducing an element of randomness into the relationship.

Abstract

Steady-state energy balances of short gravity wave systems generated in a wave tank with and without airflow have been measured and compared with the predictions of perturbation theory. Wind-wave spectra were found to fit a JONSWAP form to a good approximation if a wind-dependent equilibrium range coefficient was used. Mechanically generated waves were produced which had frequency spectra similar to wind-generated wave spectra and which exhibited nonlinear effects through a decrease in the spectral peak frequency with fetch. In the wind-wave case, perturbation theory well predicted the difference between the net source function and energy input from the wind for a wide range of fetch and wind speed conditions provided that surface tension was properly taken into account. In the case of waves generated without airflow, perturbation theory predicted energy transfer rates much smaller than the measured values.

Abstract

We describe here a method for recovering directional ocean surface wave spectra obtained from height and slope measurements made over a small area, the iterative deconvolution method (IDM). We show that IDM is a more reliable method for estimating directional wave spectra than more common spectral estimation techniques by comparing it with the widely used maximum entropy method (MEM). IDM is based on the observation that pitch–roll buoys produce directional spectra that are the true spectra convolved with an angular windowing function and are therefore much broader than the true spectra. We test IDM against simulated data and find that it does a better job of retrieving the known input spectra than does MEM, which often produces false double peaks or incorrect angular widths. We compare IDM recoveries to spectra obtained using a nonstandard processing technique, the wavelet directional method (WDM) on data from a compact array of wave staffs on Lake Ontario. We find that IDM produces directional wave spectra very nearly identical to those obtained using WDM, verifying both techniques. Finally, we processed standard NDBC buoy directional spectra and showed that IDM recovers ocean wave spectra that narrow in the Strait of Juan de Fuca and that follow a changing wind in the expected manner. Neither of these phenomena are reliably obtained using MEM due to its tendency to produce false bimodal peaks and peaks that are too narrow.

Abstract

A coherent, X-band airborne radar has been developed to measure wind speed and direction simultaneously with directional wave spectra on the ocean. The coherent real aperture radar (CORAR) measures received power, mean Doppler shifts, and mean Doppler bandwidths from small-resolution cells on the ocean surface and converts them into measurements of winds and waves. The system operates with two sets of antennas, one rotating and one looking to the side of the airplane. The rotating antennas yield neutral wind vectors at a height of 10 m above the ocean surface using a scatterometer model function to relate measured cross sections to wind speed and direction. The side-looking antennas produce maps of normalized radar cross section and line-of-sight velocity from which directional ocean wave spectra may be obtained. Capabilities of CORAR for wind and wave measurement are illustrated using data taken during the Shoaling Waves Experiment (SHOWEX) sponsored by the Office of Naval Research. Wind vectors measured by CORAR agree well with those measured by nearby buoys. Directional wave spectra obtained by CORAR also agree with buoy measurements and illustrate that offshore winds can produce dominant waves at an angle to the wind vector that are in good agreement with the measurements. The best agreement is produced using the Joint North Sea Wave Project (JONSWAP) parameterizations of the development of wave height and period with fetch.

The rationale is given for a unique experiment in which microwave scatterometer and surface flux measurements are to be made from a blimp to develop an improved scatterometer model function. A principal goal of the effort is to obtain a more accurate understanding of the relationship between the surface fluxes and the microwave power backscattered from the surface of the ocean. The limitations of previous overwater surface flux and scatterometer measurements are reviewed. The accuracy of various flux measurement techniques are compared. Evidence shows that if direct surface flux measurements are to be accurate to better than 20%, the measurements should be made at an altitude of about 5 m to 10 m from a platform that is free of flow distortion. The improved surface flux measurements are required to test proposed scatterometer theories and to determine whether the radar backscatter is principally a function of surface stress or wind speed. It is concluded that scatterometer measurements accompanied by eddy-correlation technique flux measurements must be made from a platform that is highly mobile and which enables the measurements to be made over a variety of oceanic conditions. To meet these requirements, the Naval Research Laboratory is undertaking a series of air-sea interaction experiments in which a sonic anemometer and other flux measurement instrumentation are suspended 60 m beneath a blimp flying at an altitude of 70 m while multiple scatterometer measurements are made from the blimp's gondola. Experiments are planned for a wide range of oceanic environments beginning off the central east coast of the United States in 1990.

Abstract

In 1992 and 1993, the authors made measurements of the marine boundary layer off the coast of Oregon from an airship. In 1992, these measurements consisted of coherent microwave backscatter measurements at K_u band taken from the gondola of the airship and micrometeorological and wave height measurements made from an airborne platform suspended by a cable 65 m below the gondola so that it was between 5 and 20 m above the sea surface. In 1993, an infrared imaging system was added to the suite of instruments operated in the gondola and two narrowbeam infrared thermometers were mounted in the suspended platform. In both years, a sonic anemometer and a fast humidity sensor were carried on the suspended platform and used to measure surface layer fluxes in the atmosphere above the ocean. A laser altimeter gave both the altitude of the suspended platform and a point measurement of wave height. By operating all these instruments together from the slow-moving airship, the authors were able to measure atmospheric fluxes, microwave cross sections and Doppler characteristics, air and sea surface temperatures, and wave heights simultaneously and coincidentally at much higher spatial resolutions than had been possible before. Here the authors document the methods and present observations of the neutral drag coefficient between wind speeds of 2 and 10 m s⁻¹, the relationship between the wind vector and the microwave cross section, and the effect of a sharp sea surface temperature front on both the wind vector and the microwave cross section. The drag coefficients first decrease with increasing wind speed, then reach a minimum and begin to increase with further increases in the wind speed. The values of the drag coefficient at very low wind speeds are higher than those given by Smith, however, and the minimum drag coefficient seems to occur somewhat above the wind speed he indicates. The authors show that their measured azimuthally averaged cross sections fall somewhat below the SASS II model function of Wentz et al. at low wind speeds but are rather close to that model at higher wind speeds. Coefficients describing the dependence of the cross section on azimuth angle are generally close to those of SASS II. The azimuthally averaged cross sections generally fall within the 90% confidence interval of the model function based on friction velocity recently proposed by Weissman et al. but are often near the upper limit of this interval. Somewhat surprisingly, a residual dependence on atmospheric stratification is found in the neutral drag coefficients and in the microwave cross sections when plotted against a neutral wind speed obtained using the Businger–Dyer stability corrections. This indicates that these corrections are not adequate over the ocean for stable conditions and the authors suggest that wave-induced shear near the surface may be the reason. Finally, it is shown that winds around a sea surface temperature front can rapidly change direction and that the microwave cross section follows this change except very near the front where it becomes more isotropic than usual.

Abstract

Coincident measurements with a 37-GHz polarimetric radiometer and a 10-GHz scatterometer during the Coastal Ocean Probing Experiment (COPE) in September and October of 1995 offered a unique opportunity to compare their relative sensitivity and performance in observing sea surface winds. The scatterometer cross section σ _o and the radiometer's second and third Stokes parameters, Q and U, were measured. The dependence of the angular signature of the radiometer on friction velocity was investigated by combining the COPE data with data collected in the Labrador Sea in February and March of 1997 at higher wind speeds than were encountered during COPE. The results of these experiments showed that the first harmonics of the radiometer azimuthal response were relatively insensitive to the friction velocity but that their second harmonics increased rapidly in amplitude with increasing friction velocity, with approximately the same sensitivity as a scatterometer cross section. The sensitivity of the radar and radiometer to the wind direction were then compared by computing the ratio of the angular signal to the inherent variability of the measurements for both instruments, their variability ratio. These ratios were comparable for the radar and radiometer and both increased with wind speed. However, the variability ratio of the radiometer decreased with increasing incidence angle while that of the radar increased. The similar magnitudes of the variability ratios of the two instruments are interpreted by the authors to indicate that their azimuthal signatures are caused by the same geophysical process: the angular dependence of short waves on the ocean surface and their tilting by longer waves. Their different dependence on incidence angle θ _i is explained as a result of the fact that ∂σ _o /∂θ _i decreases with increasing incidence angle, while ∂Q/∂θ _i and ∂U/∂θ _i increase with incidence angle.