Fred W. Dobson

On the dependence of sea surface roughness on wave development

Abstract The aerodynamic roughness of the sea surface, z0, is investigated using data from Lake Ontario, from the North Sea near the Dutch coast, and from an exposed site in the Atlantic Ocean off the coast of Nova Scotia. Scaling z0 by rms wave height gives consistent results for all three datasets, except where wave heights in the Atlantic Ocean are dominated by swell. The normalized roughness depends strongly on wave age: younger waves (traveling slower than the wind) are rougher than mature waves. Alternatively, the roughness may be normalized using the friction velocity, u*, of the wind stress. Again, young waves are rougher than mature waves. This contradicts some recent deductions in the literature, but the contradiction arises from attempts to describe z0 in laboratory tanks and in the field with a single simple parameterization. Here, it is demonstrated that laboratory waves are inappropriate for direct comparison with field data, being much smoother than their field equivalents. In the open ocea...

The heat budget at ocean weather station Bravo

Abstract The monthly surface heat budget and wind stress are calculated from three‐hourly meteorological data obtained at OWS B from 1946 to 1974 using formulae based on the best available measurements, and are then compared with earlier estimates. The surface heating is compared with the monthly heat storage in the water column calculated from hydrographic casts from 1964 to 1973. In spite of the scatter in the individual monthly heat storage values, the mean seasonal cycle agrees well with that of the surface flux. The residual, attributed to advection, is highly variable and does not have a strong seasonal cycle at this location. The long‐term heat loss from the ocean to the atmosphere at OWS B is found to be 28 W m−2, much smaller than the 98 W m−2 loss computed from the same data using Bunkers widely accepted formulae. The difference is accounted for by Bunkers 28% underestimate of the incoming short‐wave radiation and his 74 and 43% overestimates of the outgoing sensible and latent heat fluxes, re...

On the deep‐water fetch laws for wind‐generated surface gravity waves

Abstract With the object of providing an accurate set of open‐sea wave spectra in a variety of conditions, we deployed, in conjunction with CASP, an array of 9 wave buoys (3 directional, 6 non‐directional) along a 30‐km line offshore from Martinique Beach, N.S. A large set of high‐quality wave spectra was collected in conjunction with extensive meteorological information. The data set is unique in the sense that a large onshore swell component was normally present. Offshore‐wind cases for three windows: ±5°, ±15° and ±30° with respect to the shore normal, have been considered. Wind speed was found to be a strong function of fetch, and attempts were made to allow for this in the analysis. Power‐law regressions have been produced of dimensionless sea energy, peak frequency and high‐frequency spectral level (the Kitaigorodskii “alpha” parameter) vs dimensionless fetch and wind speed (inverse wave age). The regressions are compared with earlier work: the Joint North Sea Waves Project (Jonswap) and the Canada ...

Measuring the relationship between wind stress and sea state in the open ocean in the presence of swell

Abstract Wind stress and directional wave spectra were measured during the Grand Banks ERS‐1 SAR wave spectra validation experiment. Swell dominated the wave spectral energy, so that existing relations between wind stress and sea state at sheltered sites were not directly applicable. To test the idea that simple deletion of the swell from the wave spectra would produce a sea state/wind stress relation suitable for general use, a spectral method for separating sea from swell was developed. The resulting open‐ocean wind stress/sea‐state relation does not differ from earlier results without swell, but, as for the swell‐dominated part of the HEXOS (Humidity Exchange Over the Sea) dataset, the statistical significance is low.

Ocean wave extraction from RADARSAT synthetic aperture radar inter-look image cross-spectra

This study is concerned with the extraction of directional ocean wave spectra from synthetic aperture radar (SAR) image spectra. The statistical estimation problem underlying the wave-SAR inverse problem is examined in detail in order to properly quantify the wave information content of SAR. As a concrete focus, a data set is considered comprising six RADARSAT SAR images co-located with a directional wave buoy off the east coast of Canada. These SAR data are transformed into inter-look image cross-spectra based on two looks at the same ocean scene separated by 0.4 s. The general problem of wave extraction from SAR is cast in terms of a statistical estimation problem that includes the observed SAR spectra, the wave-SAR transform, and prior spectral wave information. The central role of the weighting functions (inverse of the error covariances) is demonstrated, as well as the consequence of approximate (based on the quasilinear wave-SAR transform) versus exact linearizations on the convergence properties of the algorithm. Error estimates are derived and discussed. This statistical framework is applied to the extraction of spectral wave information from observed RADARSAT SAR image cross-spectra. A modified wave-SAR transform is used to account for case-specific geophysical and imaging effects. Analysis of the residual error of simulated and observed SAR spectra motivates a canonical form for the SAR observation error covariance. Wave estimates are then extracted from the SAR spectra, including wavenumber dependent error estimates and explicit identification of spectral null spaces where the SAR contains no wave information. Band-limited SAR wave information is also combined with prior (buoy) spectral wave estimates through parameterization of the wave spectral shape and use of regularization.

Estimates of Dissipation in the Ocean Mixed Layer Using a Quasi-Horizontal Microstructure Profiler

Abstract Some recent measurements of the mixed layer in oceans and lakes have indicated that the rate of the dissipation of turbulent kinetic energy, e, is much higher than expected from a purely shear-driven wall layer. This enhancement has usually been attributed to wave breaking. In this study, measurements of dissipation in the open-ocean mixed layer on the continental shelf off Nova Scotia are integrated with air–sea flux estimates and directional wave spectra to further study this issue. A microstructure profiler gliding quasi-horizontally provides estimates of e starting within 2 m of the ocean surface as it slowly descends through the mixed layer. Dissipation rates were found to be enhanced relative to the wind stress production and indicated that ∼6% of the wind energy at 10 m is dissipated in the ocean mixed layer. In addition, results from this experiment demonstrate that the WAVES scaling for e, based on wind and wave parameters, is valid for the case of a simple windsea in which the swell can...

Retrieval of ocean wave spectra and RAR MTF's from dual-polarization SAR data

A method for the retrieval of the real aperture radar (RAR) modulation transfer function (MTF) and ocean wave spectra from dual-polarization (i.e., simultaneously acquired HH and VV polarizations) synthetic aperture radar (SAR) image data is described. The RAR MTF is estimated by applying empirical MTF estimation methodologies to inter-look cross spectra between various combinations of individual looks and available polarizations for a given radar frequency. The concept behind the nonlinear inversion is that any combination of like- and cross-polarization image spectra should return the same wave spectrum, in agreement with in situ and model wave spectra. This permits estimation of the RAR MTF on a case-by-case basis. The results are compared with theoretical treatments of the RAR MTF, which are shown to be inadequate for the range of conditions encountered in their data set. However, the theory and measurements fit well in describing the polarization dependence of the RAR MTF. The data set consists of SIR-C/X-SAR L-band and CCRS CV-580 C-band SAR data, in situ buoy measurements, and model data from field programs in Canadian waters in October and December 1994.

The grand banks ERS‐1 SAR wave spectra validation experiment: Program overview and data summary

As part of the ERS-1 validation program, the ERS-1 Synthetic Aperture Radar (SAR) wave spectra validation experiment was carried out over the Grand Banks of Newfoundland (Canada) in Nov. 1991. The principal objective of the experiment was to obtain complete sets of wind and wave data from a variety of calibrated instruments to validate SAR measurements of ocean wave spectra. The field program activities are described and the rather complex wind and wave conditions which were observed are summarized. Spectral comparisons with ERS-1 SAR image spectra are provided. The ERS-1 SAR is shown to have measured swell and range traveling wind seas, but did not measure azimuth traveling wind seas at any time during the experiment. Results of velocity bunching forward mapping and new measurements of the relationship between wind stress and sea state are also shown.

The Grand Banks ERS‐1 SAR wave experiment

The European Space Agencys Remote Sensing Satellite (ERS-1) was launched in July 1991 [Attema and Francis, 1991]. As part of a global program to validate the ocean surface sensors on board ERS-1, we carried out a joint experiment in November 1991 on the Grand Banks of Newfoundland involving ships, moorings, aircraft, and a land-based radar. The principal objective was to provide a field validation of ERS-1 synthetic aperture radar (SAR) [Attema, 1991] measurements of ocean surface structure. Other objectives included the testing of numerical wave prediction models, the validation of both shipborne X-band and shorebased HF marine wave radars, and an investigation of the relation between sea state and wind stress. The experiment included a large number of independent wave, sea surface, and atmospheric sensors (Table 1).

Estimation of Solar Radiation at Sea

The widely-used short-wave radiation formula of Budyko (1974) underestimates the measured solar radiation at Ocean Weather Stations (OWS) A, I, J, K and P by as much as 30%, with the largest errors occurring at times of high cloud amount.