Duncan B. Ross

Comparison ofin situ and remotely sensed ocean waves off Marineland, Florida

Some early results from an oceanographic experiment staged off Marineland, Florida, in December 1975 are presented, viz., intercomparisons between the X-band and L-band imagery obtained by the Environmental Research Institute of Michigans (ERIM) dual-wavelength, dualpolarization multiplexed radar. This radar allows direct comparison since the images are produced simultaneously. The wave data obtained from the radar imagery are compared with surface measurements of waves obtained with a pitch- and-roll buoy. The conclusions are only applicable to medium and low wind and wave conditions encountered during the Marineland test. The results indicate that X-band images provide superior quality wave imagery and more useful Fourier Transforms compared to L-band under equivalent signal-to-noise ratios and resolution. Optimum wave imagery is seen when waves propagate in the range direction. Comparisons betweenin situ measurements and X-band imagery of the same area indicate that the dominant wave direction can be obtained from imagery to within a few degrees. A one-dimensional spectrum obtained from X-band imagery compares favorably with an equivalent wave frequency spectrum obtained from the pitch- and-roll buoy after suitable transformation asing linear wave theory.

Laser and Microwave Observations of Sea-Surface Condition for Fetch-Limited 17- to 25-m/s Winds

The variability of sea-surface conditions has been observed from a low-flying aircraft by a laser-wave profiling system and a scanning horizontally polarized 19.35-GHz passive microwave radiometer for fetch-limited wind speeds of 17 to 25 m/s in the North Sea. Wave profiles obtained with the laser system have been analyzed and show that wave growth occurs simultaneously at all frequencies and that an equilibrium value for the higher frequency components is eventually reached, but not before substantially higher (overshoot) values are obtained. Simultaneous observations of the microwave brightness temperature at vertical incidence show an increase with wind speed (or roughness of the sea surface) of 1°K/m/s. This increase, not in accord with theory, is shown to be a function of the percentage of foam coverage of the ocean surface.

Ocean surface patterns above sea-floor bedforms as recorded by radar, Southern Bight of North Sea

Abstract Airborne radar observations of the Southern Bight of the North Sea have revealed sea-surface patterns that reflect a constructional sea-floor topography of tide-built sand waves and sand ridges. The patterns result from local horizontal differences in the amplitude of water surface waves with wavelengths less than 1 cm. Such modulations result from surface convergences and divergences as the water flows across structures on the ocean floor. Three sand ridges were observed which form a topological series defined by side slope, asymmetry, and sand-wave pattern. Depth appears to be the controlling variable, with shallow water leading to the steepest slopes, most intense asymmetry and the most marked separation of sand waves into ebb and flood dominated domains.

On the relationship of radar backscatter to wind speed and fetch

The physics of the interaction of electromagnetic waves with the ocean surface has been an active area of research for a number of years. We present here the results of satellite and aircraft experiments to investigate the ability of active microwave radars to infer surface wind speeds remotely. Data obtained from the recent National Aeronautics and Space Administration (NASA) Skylab experiment are compared with surface wind speeds measured by low-flying aircraft and ships-of-opportunity and found to give useful estimates of the oceanic wind field. We also investigate the influence of varying wave height on radar measurements of wind speed by measuring the backscattering cross-section for constant wind speed but variable wave conditions. We conclude that this effect is of little importance.

A Comparison of Parametric and Spectral Hurricane Wave Prediction Products

Statistics of extreme wave heights created by tropical cyclones are urgently needed for the design of harbor breakwaters, ships, and offshore structures to prevent accidents from underde-sign and to avoid the untenable costs of overdesign. Wave data alone cannot provide the needed statistics due to the long periods required to collect statistically reliable data sets and the inability to deploy enough systems to obtain measurements of maximum conditions. Needed statistics, therefore, are usually extracted from models which attempt to specify the wave environment associated with particular storm systems. These models are either largely untested against measurements, or are validated over a limited range of storm conditions. One such model is that of Cardone et al. (1976a) which was developed and calibrated using a data set obtained in hurricane Camille, a severe Gulf of Mexico storm which struck the coast of the U. S. in August 1969. Another is an empirically-based parametric model proposed by Ross (1976), based upon forty measurements of one-dimensional frequency spectra obtained in Camille, Pacific hurricane Ava (Ross, 1975), and Gulf of Mexico hurricane Eloise (Withee and Johnson, 1975).

Southern Ocean Waves and Winds Derived from SEASAT Altimeter Measurements

SEASAT altimeter measurements of significant wave height in the Southern Ocean during August and September 1978 have been examined. Significant wave heights of greater than 10 m were observed. Background levels of 2–4 m indicate generally rougher seas than occur in the North Atlantic during a comparable season. The altimeter data set is examined statistically and the spectrum of the worst case situations is determined parametrically based upon a JONSWAP form of the spectrum. Results of a statistical analysis of the altimeter data are presented together with an evaluation of the probable spectral characteristics based upon a parametric derivation of the spectrum.

State-of-the-Art Wave Prediction Methods and Data Requirements

In the past decade, there has been a general shift in wave prediction methods from simple empirical techniques developed in the 1940’s to the application of numerical models based upon the spectral energy balance equation. Such models simulate the physical processes governing the growth of surface waves which have been identified as a result of extensive theoretical and experimental study of wave generation. The importance of nonlinear wave-wave interactions in wave generation has prompted the recent development of an alternate model context for wave prediction, which is based upon a parametric representation of the wave spectrum. This paper reviews the structure of state-of-the-art spectral and parametric methods and validation, and wave data requirements for further improvements in such methods.

On the Use of Aircraft in the Observation of One- and Two-Dimensional Ocean Wave Spectra

Aircraft have been utilized for a number of years to observe various characteristics of wind-generated ocean waves. The techniques involved are both active and passive and incorporate sensors which operate in the microwave and optical regions of the electromagnetic spectrum. This paper reviews a number of such sensors and presents data in support of their utility. It is found that aircraft can play an important role in a wave monitoring program.

Comparisons of Hurricane Fico Winds and Waves from Numerical Models with Observations from SEASAT-A

Several types of satellite data collected during SEASAT-A overpasses of hurricane Fico are used to adjust input parameters in a hurricane wind prediction model. Derived winds are then used in a complex discrete spectral wave prediction model to calculate wave heights and directional spectrum distributions. Three different areas of the storm with winds ranging from 12 m/s to 23 m/s and hindcast wave heights varying from 4 m to 8 m were used to compare model normalized directional wave spectra with those from SEASAT synthetic aperture radar (SAR) imagery. The two-dimensional normalized spectral energy results from the SAR and wave model showed an average difference in peak directions of 0.9° with a standard deviation of 6.8°. Wind and wave model results are also compared with altimeter data along a subsatellite track. This study shows the value of satellite observations in model validation and, conversely, the value of the model results in verification of the efficacy of the satellite data sets. Especially noteworthy is the snapshot view of the characteristics of the storm, a demonstration of the potential power of satellite remote sensing.