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- Author or Editor: R. F. Marsden x
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Abstract
Spatial homogeneity assumptions inherent in the conversion of directly measured acoustic Doppler current profiler (ADCP) beam to Cartesian coordinates for the Janus configuration are investigated. These assumptions may be adequate for large-scale flows, such as tidal currents and wind-forced upwelling. However, for high-frequency features, such as internal solitons and turbulence, the velocity fields may vary over scales comparable to the divergence of the acoustic beams. Equations are derived for beam spreading, and it is shown that a first-order correction can be applied to improve velocity measurement accuracy. Two cases are examined. First, the effects of the spatial and temporal convolution inherent in beam spreading from the Janus configuration ADCP are applied to a model internal solitary wave. It is shown that the corrected vertical velocities have deviations of less than 2 mm s−1 for distances up to 30 m from the transducer face and are approximately 3 times more accurate than the uncorrected velocities for distance up to 20 m from the transducer face. Next, under a “frozen turbulence” hypothesis, the method is applied to processing turbulence data. It is demonstrated that the horizontal longitudinal velocity can be markedly improved.
Abstract
Spatial homogeneity assumptions inherent in the conversion of directly measured acoustic Doppler current profiler (ADCP) beam to Cartesian coordinates for the Janus configuration are investigated. These assumptions may be adequate for large-scale flows, such as tidal currents and wind-forced upwelling. However, for high-frequency features, such as internal solitons and turbulence, the velocity fields may vary over scales comparable to the divergence of the acoustic beams. Equations are derived for beam spreading, and it is shown that a first-order correction can be applied to improve velocity measurement accuracy. Two cases are examined. First, the effects of the spatial and temporal convolution inherent in beam spreading from the Janus configuration ADCP are applied to a model internal solitary wave. It is shown that the corrected vertical velocities have deviations of less than 2 mm s−1 for distances up to 30 m from the transducer face and are approximately 3 times more accurate than the uncorrected velocities for distance up to 20 m from the transducer face. Next, under a “frozen turbulence” hypothesis, the method is applied to processing turbulence data. It is demonstrated that the horizontal longitudinal velocity can be markedly improved.
Abstract
Minor alignment errors (∼1°) can contaminate acoustic Doppler current profiler measurements of vertical velocities taken from a ship traveling at speed. The signature is consistently large positive or negative vertical velocities, recorded at all depths. A technique is proposed to postcorrect contaminated data in cases of tidally dominated flow over a flat bottom. The eigenmodes of the cross-correlation matrix of the depth-averaged velocities are found, and the data are rotated to align along the eigenvector corresponding to the smallest eigenvalue that is equivalent to the physical vertical direction. An example of corrupted data is presented, and corrections for the pitch and roll directions are found. The corrected flow field is shown to be physically plausible over both flat and sloping bottom sections of the cruise track.
Abstract
Minor alignment errors (∼1°) can contaminate acoustic Doppler current profiler measurements of vertical velocities taken from a ship traveling at speed. The signature is consistently large positive or negative vertical velocities, recorded at all depths. A technique is proposed to postcorrect contaminated data in cases of tidally dominated flow over a flat bottom. The eigenmodes of the cross-correlation matrix of the depth-averaged velocities are found, and the data are rotated to align along the eigenvector corresponding to the smallest eigenvalue that is equivalent to the physical vertical direction. An example of corrupted data is presented, and corrections for the pitch and roll directions are found. The corrected flow field is shown to be physically plausible over both flat and sloping bottom sections of the cruise track.
Abstract
Using data from the Coastal Ocean Dynamics Experiment (CODE) and the Island Coastal Current Experiment, the internal tide off the west coast of the Vancouver Island was studied. Although the semidiurnal barotropic velocity field in the area was weak, the baroclinic field was found to be relatively strong with speeds up to five times greater. The intermittent nature of the baroclinic field was resolved by estimating the M2 tidal harmonic coefficients every 24 hours in 15-day overlaping intervals. These time-evolving coefficients were then analysed using spectral analysis methods. For most of the area under investigation, the internal tide was generated mainly at the shelf break and was found to propagate cross-shore. Off Estevan point, where the bottom contour is regular, the beam structure was identified and traced to distances 40 km from the generation region on the slope. The downward-propagating beam was found to follow the seasonal variation of the characteristic propagation paths. An empirical orthogonal function analysis revealed a highly coherent baroclinic field (78% of the M2 variance). The predictions of an internal tide model were found to he in good agreement with the experimental results on the slope. The interaction of a cyclonic eddy, present in the area during August 1980, with the internal tide was also addressed.
Abstract
Using data from the Coastal Ocean Dynamics Experiment (CODE) and the Island Coastal Current Experiment, the internal tide off the west coast of the Vancouver Island was studied. Although the semidiurnal barotropic velocity field in the area was weak, the baroclinic field was found to be relatively strong with speeds up to five times greater. The intermittent nature of the baroclinic field was resolved by estimating the M2 tidal harmonic coefficients every 24 hours in 15-day overlaping intervals. These time-evolving coefficients were then analysed using spectral analysis methods. For most of the area under investigation, the internal tide was generated mainly at the shelf break and was found to propagate cross-shore. Off Estevan point, where the bottom contour is regular, the beam structure was identified and traced to distances 40 km from the generation region on the slope. The downward-propagating beam was found to follow the seasonal variation of the characteristic propagation paths. An empirical orthogonal function analysis revealed a highly coherent baroclinic field (78% of the M2 variance). The predictions of an internal tide model were found to he in good agreement with the experimental results on the slope. The interaction of a cyclonic eddy, present in the area during August 1980, with the internal tide was also addressed.
Abstract
Previous studies of Knight Inlet, British Columbia, revealed the presence of a strong internal M2 tide. Most of the energy was found to be in either the first or second dynamic modes. Due to difficulties in sampling of the surface layer, objective estimates of the distribution of tidal energy and reflection coefficients, until now, have been impossible to obtain. During June and July 1989, an intense acoustic Doppler current profiler (ADCP) sampling of the inlet was conducted. These results are used to show the existence of a strong semidiurnal flow in the baroclinic field. The authors propose that the horizontal phase information inherent in the spatial sampling by the ADCP can be used to resolve the distribution of energy between dynamic modes. Through a least squares fit of the data to a simple free wave propagation model of the inlet, the authors arrive at objective estimates of the distribution of M2 internal tide energy. The fitting procedure is found to be sensitive to fluctuations in the basin width. When an accurate estimate of width is incorporated into the fit, the authors arrive at net energy fluxes of 0.44 × 106 W toward the mouth of the inlet at Protection Point and 1.17 × 106 W toward the head of the inlet at Tomakstum Island. It is shown that these results do not display the degeneracy inherent in other estimates and that they are in agreement with a recent numerical model of the inlet by Stacey and Pond.
Abstract
Previous studies of Knight Inlet, British Columbia, revealed the presence of a strong internal M2 tide. Most of the energy was found to be in either the first or second dynamic modes. Due to difficulties in sampling of the surface layer, objective estimates of the distribution of tidal energy and reflection coefficients, until now, have been impossible to obtain. During June and July 1989, an intense acoustic Doppler current profiler (ADCP) sampling of the inlet was conducted. These results are used to show the existence of a strong semidiurnal flow in the baroclinic field. The authors propose that the horizontal phase information inherent in the spatial sampling by the ADCP can be used to resolve the distribution of energy between dynamic modes. Through a least squares fit of the data to a simple free wave propagation model of the inlet, the authors arrive at objective estimates of the distribution of M2 internal tide energy. The fitting procedure is found to be sensitive to fluctuations in the basin width. When an accurate estimate of width is incorporated into the fit, the authors arrive at net energy fluxes of 0.44 × 106 W toward the mouth of the inlet at Protection Point and 1.17 × 106 W toward the head of the inlet at Tomakstum Island. It is shown that these results do not display the degeneracy inherent in other estimates and that they are in agreement with a recent numerical model of the inlet by Stacey and Pond.
Abstract
An eigenvector (EV) method for the determination of directional spectra from heave, pitch and roll buoy data is presented. Both a direct and an iterative form (based on an algorithm by Pawka) of this data-adaptive procedure are developed. The direct form outperforms the Longuet-Higgins et al. method, the cosine spread model and the maximum likelihood (ML) method for both simulated and real data. In the iterative form, the iterative EV method is superior to the other methods tested, including the iterative ML method for unimodal peaks when the noise-to-signal ratio is greater than 0.2 and for bimodal peaks at all noise levels. With real data, the direct EV method produced errors within an 80% confidence zone of the data cross-spectral matrix within the stopping criteria of Lawson and Long. Errors of the iterative EV and iterative ML methods were lower than for both the direct EV and ML results and were about the same magnitude when compared to each other. The iterative EV method, however, produced narrower, more sharply defined unimodal and bimodal spectra.
Abstract
An eigenvector (EV) method for the determination of directional spectra from heave, pitch and roll buoy data is presented. Both a direct and an iterative form (based on an algorithm by Pawka) of this data-adaptive procedure are developed. The direct form outperforms the Longuet-Higgins et al. method, the cosine spread model and the maximum likelihood (ML) method for both simulated and real data. In the iterative form, the iterative EV method is superior to the other methods tested, including the iterative ML method for unimodal peaks when the noise-to-signal ratio is greater than 0.2 and for bimodal peaks at all noise levels. With real data, the direct EV method produced errors within an 80% confidence zone of the data cross-spectral matrix within the stopping criteria of Lawson and Long. Errors of the iterative EV and iterative ML methods were lower than for both the direct EV and ML results and were about the same magnitude when compared to each other. The iterative EV method, however, produced narrower, more sharply defined unimodal and bimodal spectra.
Abstract
A simple, approximate formula for mean wind stress is given in terms of the mean and variance of the wind fluctuations over the averaging period. The formula is nonlinear with respect to the mean wind speed.
The formula is tested using 3 h wind observations from eight North Atlantic Ocean Weather Ships. Mean wind stress is calculated 1) by vector averaging the 3 h wind stresses and 2) by applying the approximate formula. For an averaging period of 4 months the two methods agree to within ±0.025 Pa, 95% of the time. For an averaging period of 1 month the approximate formula slightly overestimates the stress. This is due to skewness in the probability density function of the observed 3 h wind fluctuations. An expression for the modification of the mean stress due to skewness is given.
A straightforward method is described for the estimation of vector mean wind and variance fields, and thus mean stress fields, over the open ocean. To cheek the method, the long-term stress field of the North Atlantic, and the seasonal Sverdrup transport across 31°N, are computed and compared with the values given by Willebrand, and Bunker and Leetma. Good agreement is obtained. The zonally integrated Sverdrup transport across 45°N is also calculated and shown to exhibit significant interannual fluctuations.
Abstract
A simple, approximate formula for mean wind stress is given in terms of the mean and variance of the wind fluctuations over the averaging period. The formula is nonlinear with respect to the mean wind speed.
The formula is tested using 3 h wind observations from eight North Atlantic Ocean Weather Ships. Mean wind stress is calculated 1) by vector averaging the 3 h wind stresses and 2) by applying the approximate formula. For an averaging period of 4 months the two methods agree to within ±0.025 Pa, 95% of the time. For an averaging period of 1 month the approximate formula slightly overestimates the stress. This is due to skewness in the probability density function of the observed 3 h wind fluctuations. An expression for the modification of the mean stress due to skewness is given.
A straightforward method is described for the estimation of vector mean wind and variance fields, and thus mean stress fields, over the open ocean. To cheek the method, the long-term stress field of the North Atlantic, and the seasonal Sverdrup transport across 31°N, are computed and compared with the values given by Willebrand, and Bunker and Leetma. Good agreement is obtained. The zonally integrated Sverdrup transport across 45°N is also calculated and shown to exhibit significant interannual fluctuations.
Abstract
An open ocean, deep water air–sea interaction experiment was conducted in the Gulf of Alaska. Wave amplitude and slope data were measured using a WAVEC heave, pitch, and roll buoy that was let drift in the Alaska gyre. Wind stress estimates were obtained from a fast-sample anemometer using the dissipation technique and from synoptic measurements through a boundary-layer model. The fundamental correlation and predictive relationships between wind friction velocity and wave spectral properties were established. A comparison of the slope spectrum to simultaneous wind stresses allowed us to estimate the Phillips proposed universal constant β. Reintroducing this constant β into the Phillips slope spectrum and using measured slope spectral characteristics, an inferred wind stress was calculated that was shown to agree well with both the dissipation and model stresses thereby validating both Phillips theory and the boundary-layer model. Any discrepancies with the model stresses were attributed to second-order wave age effects. The roughness length z 0, nondimensionalized by the sea rms wave height, was shown to decrease with wave age in a manner consistent with Kitaigorodskii's functional form. A general expression for Cd as a function of wind speed or friction velocity and wave age was proposed and verified with independent data.
Abstract
An open ocean, deep water air–sea interaction experiment was conducted in the Gulf of Alaska. Wave amplitude and slope data were measured using a WAVEC heave, pitch, and roll buoy that was let drift in the Alaska gyre. Wind stress estimates were obtained from a fast-sample anemometer using the dissipation technique and from synoptic measurements through a boundary-layer model. The fundamental correlation and predictive relationships between wind friction velocity and wave spectral properties were established. A comparison of the slope spectrum to simultaneous wind stresses allowed us to estimate the Phillips proposed universal constant β. Reintroducing this constant β into the Phillips slope spectrum and using measured slope spectral characteristics, an inferred wind stress was calculated that was shown to agree well with both the dissipation and model stresses thereby validating both Phillips theory and the boundary-layer model. Any discrepancies with the model stresses were attributed to second-order wave age effects. The roughness length z 0, nondimensionalized by the sea rms wave height, was shown to decrease with wave age in a manner consistent with Kitaigorodskii's functional form. A general expression for Cd as a function of wind speed or friction velocity and wave age was proposed and verified with independent data.
