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D. A. Huntley

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D. A. Huntley

Abstract

The inertial dissipation method for estimating seabed friction velocities from near-bed turbulence spectra requires few measurements; it is relatively insensitive to errors in sensor orientation and measurement of mean flows. However, the method is only valid if turbulence spectra are measured at a height above the seabed that is small enough to be within the constant stress layer but large enough to produce an inertial subrange. It is shown that such a height exists only if the friction velocity exceeds a critical value (typically 0.8 cm s−1 for a midlatitude ocean). Recent measurements from combined wave and mean flow conditions on the continental shelf do not satisfy this requirement. However, an empirical modification to the inertial dissipation method is suggested to allow estimation of the friction velocity even when a true inertial subrange does not exist. The modified method is applied to the combined wave and mean flow field data; it virtually removes an increase in estimated friction velocity with height, and results in values which are in good agreement with theoretical expectation. It generally applicable, the modified method will significantly extend the range of conditions in which the inertial dissipation method can be used.

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D. A. Huntley and D. G. Hazen

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A tripod holding electromagnetic flowmeters at two heights within 1 m above the seabed has been deployed at two shallow sites (25 and 45 m depths) on the continental shelf off Nova Scotia, Canada. Wave flows are comparable with the small mean flows at both sites. Friction velocities have been estimated from the observed spectra of vertical turbulent velocities, using a modification of the dissipation method appropriate to low Reynolds number conditions. The results from each site show no significant change of friction velocity with height, as expected for measurements from within the constant stress layer. However, in each case the observed friction velocities are considerably larger than would be predicted on the basis of the observed bottom roughness and the mean flows alone, indicating that the wave flows were important in enhancing the friction velocity. The theory of Grant and Madsen (1979) has been used to predict the friction velocities, based on the observed mean and wave velocities and on the bottom roughness estimated from stereophotography of the seabed. Good agreement is found between the predicted and observed friction velocities at both sites provided that the significant orbital velocity amplitude is used in the predictions. This is in general agreement with the results of Grant et at.

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Philip Muscarella, Matthew J. Carrier, Hans Ngodock, Scott Smith, B. L. Lipphardt Jr., A. D. Kirwan Jr., and Helga S. Huntley

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The Lagrangian predictability of general circulation models is limited by the need for high-resolution data streams to constrain small-scale dynamical features. Here velocity observations from Lagrangian drifters deployed in the Gulf of Mexico during the summer 2012 Grand Lagrangian Deployment (GLAD) experiment are assimilated into the Naval Coastal Ocean Model (NCOM) 4D variational (4DVAR) analysis system to examine their impact on Lagrangian predictability. NCOM-4DVAR is a weak-constraint assimilation system using the indirect representer method. Velocities derived from drifter trajectories, as well as satellite and in situ observations, are assimilated. Lagrangian forecast skill is assessed using separation distance and angular differences between simulated and observed trajectory positions. Results show that assimilating drifter velocities substantially improves the model forecast shape and position of a Loop Current ring. These gains in mesoscale Eulerian forecast skill also improve Lagrangian forecasts, reducing the growth rate of separation distances between observed and simulated drifters by approximately 7.3 km day−1 on average, when compared with forecasts that assimilate only temperature and salinity observations. Trajectory angular differences are also reduced.

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A. C. Haza, E. D’Asaro, H. Chang, S. Chen, M. Curcic, C. Guigand, H. S. Huntley, G. Jacobs, G. Novelli, T. M. Özgökmen, A. C. Poje, E. Ryan, and A. Shcherbina

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The Lagrangian Submesoscale Experiment (LASER) was designed to study surface flows during winter conditions in the northern Gulf of Mexico. More than 1000 mostly biodegradable drifters were launched. The drifters consisted of a surface floater extending 5 cm below the surface, containing the satellite tracking system, and a drogue extending 60 cm below the surface, hanging beneath the floater on a flexible tether. On some floats, the drogue separated from the floater during storms. This paper describes methods to detect drogue loss based on two properties that distinguish drogued from undrogued drifters. First, undrogued drifters often flip over, pointing their satellite antenna downward and thus intermittently reducing the frequency of GPS fixes. Second, undrogued drifters respond to wind forcing more than drogued drifters. A multistage analysis is used: first, two properties are used to create a preliminary drifter classification; then, the motion of each unclassified drifter is compared to that of its classified neighbors in an iterative process for nearly all of the drifters. The algorithm classified drifters with a known drogue status with an accuracy of virtually 100%. Drogue loss times were estimated with a precision of less than 0.5 and 3 h for 60% and 85% of the drifters, respectively. An estimated 40% of the drifters lost their drogues in the first 7 weeks, with drogue loss coinciding with storm events, particularly those with steep waves. Once the drogued and undrogued drifters are classified, they can be used to quantify the differences in material dispersion at different depths.

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Henry Chang, Helga S. Huntley, A. D. Kirwan Jr., Daniel F. Carlson, Jean A. Mensa, Sanchit Mehta, Guillaume Novelli, Tamay M. Özgökmen, Baylor Fox-Kemper, Brodie Pearson, Jenna Pearson, Ramsey R. Harcourt, and Andrew C. Poje

Abstract

We present an analysis of ocean surface dispersion characteristics, on 1–100-m scales, obtained by optically tracking a release of O(600) bamboo plates for 2 h in the northern Gulf of Mexico. Under sustained 5–6 m s−1 winds, energetic Langmuir cells are clearly delineated in the spatially dense plate observations. Within 10 min of release, the plates collect in windrows with 15-m spacing aligned with the wind. Windrow spacing grows, through windrow merger, to 40 m after 20 min and then expands at a slower rate to 50 m. The presence of Langmuir cells produces strong horizontal anisotropy and scale dependence in all surface dispersion statistics computed from the plate observations. Relative dispersion in the crosswind direction initially dominates but eventually saturates, while downwind dispersion exhibits continual growth consistent with contributions from both turbulent fluctuations and organized mean shear. Longitudinal velocity differences in the crosswind direction indicate mean convergence at scales below the Langmuir cell diameter and mean divergence at larger scales. Although the second-order structure function measured by contemporaneous GPS-tracked surface drifters drogued at ~0.5 m shows persistent r 2/3 power law scaling down to 100–200-m separation scales, the second-order structure function for the very near surface plates observations has considerably higher energy and significantly shallower slope at scales below 100 m. This is consistent with contemporaneous data from undrogued surface drifters and previously published model results indicating shallowing spectra in the presence of direct wind-wave forcing mechanisms.

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