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  • Author or Editor: Anthony R. Kirincich x
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Anthony R. Kirincich and Johanna H. Rosman

Abstract

Turbulent Reynolds stresses are now routinely estimated from acoustic Doppler current profiler (ADCP) measurements in estuaries and tidal channels using the variance method, yet biases due to surface gravity waves limit its use in the coastal ocean. Recent modifications to this method, including spatially filtering velocities to isolate the turbulence from wave velocities and fitting a cospectral model to the below-wave band cospectra, have been used to remove this bias. Individually, each modification performed well for the published test datasets, but a comparative analysis over the range of conditions in the coastal ocean has not yet been performed. This work uses ADCP velocity measurements from five previously published coastal ocean and estuarine datasets, which span a range of wave and current conditions as well as instrument configurations, to directly compare methods for estimating stresses in the presence of waves. The computed stresses from each were compared to bottom stress estimates from a quadratic drag law and, where available, estimates of wind stress. These comparisons, along with an analysis of the cospectra, indicated that spectral fitting performs well when the wave climate is wide-banded and/or multidirectional as well as when instrument noise is high. In contrast, spatial filtering performs better when waves are narrow-banded, low frequency, and when wave orbital velocities are strong relative to currents. However, as spatial filtering uses vertically separated velocity bins to remove the wave bias, spectral fitting is able to resolve stresses over a larger fraction of the water column.

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Anthony R. Kirincich, Tony de Paolo, and Eric Terrill

Abstract

Estimates of surface currents over the continental shelf are now regularly made using high-frequency radar (HFR) systems along much of the U.S. coastline. The recently deployed HFR system at the Martha’s Vineyard Coastal Observatory (MVCO) is a unique addition to these systems, focusing on high spatial resolution over a relatively small coastal ocean domain with high accuracy. However, initial results from the system showed sizable errors and biased estimates of M 2 tidal currents, prompting an examination of new methods to improve the quality of radar-based velocity data. The analysis described here utilizes the radial metric output of CODAR Ocean Systems’ version 7 release of the SeaSonde Radial Site Software Suite to examine both the characteristics of the received signal and the output of the direction-finding algorithm to provide data quality controls on the estimated radial currents that are independent of the estimated velocity. Additionally, the effect of weighting spatial averages of radials falling within the same range and azimuthal bin is examined to account for differences in signal quality. Applied to two month-long datasets from the MVCO high-resolution system, these new methods are found to improve the rms difference comparisons with in situ current measurements by up to 2 cm s−1, as well as reduce or eliminate observed biases of tidal ellipses estimated using standard methods.

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Anthony R. Kirincich, Steven J. Lentz, and Gregory P. Gerbi

Abstract

Recently, the velocity observations of acoustic Doppler current profilers (ADCPs) have been successfully used to estimate turbulent Reynolds stresses in estuaries and tidal channels. However, the presence of surface gravity waves can significantly bias stress estimates, limiting application of the technique in the coastal ocean. This work describes a new approach to estimate Reynolds stresses from ADCP velocities obtained in the presence of waves. The method fits an established semiempirical model of boundary layer turbulence to the measured turbulent cospectra at frequencies below those of surface gravity waves to estimate the stress. Applied to ADCP observations made in weakly stratified waters and variable significant wave heights, estimated near-bottom and near-surface stresses using this method compared well with independent estimates of the boundary stresses in contrast to previous methods. Additionally, the vertical structure of tidal stress estimated using the new approach matched that inferred from a linear momentum balance at stress levels below the estimated stress uncertainties. Because the method makes an estimate of the horizontal turbulent length scales present as part of the model fit, these results can also enable a direct correction for the mean bias errors resulting from instrument tilt, if these scales are long relative to the beam separation.

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