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Louis Goodman and Edward R. Levine

Abstract

The vertical motion of a neutrally buoyant float is determined from the solution to the nonlinear forced harmonic oscillator equation originally set forth by Voorhis. Float response to forced vertical oscillations is characterized by the response ratio, r = ξrw, where ξr, is the vertical displacement of an isopycnal relative to the float, and ξw is the vertical displacement of an isopycnal relative to its initial equilibrium position. For isopycnal displacements with frequencies much less than the resonant frequency of the float, the goat can be considered to be in near dynamic equilibrium with the forcing, and r is a function of the relative compressibility between the float and seawater, s = γf/ γw, and the normalized buoyancy frequency N = N/Ω, where Ω is a characteristic float frequency defined by Ω2 = gξw[1 − (αfαw −1)]− 1, where αf, αw are the coefficients of thermal expansion of the float and water, respectively. For the new dynamic equilibrium case, data obtained from a float deployment in a Gulf Stream meander result in an observed r value close to the predicted value. For the case of float response to an isopycnal displacement of frequency near the resonant frequency of the float, vertical motion depends on drag, in addition to the material properties of the float and seawater. The bandwidth over which resonance can occur is parametrized by the ‘Q’ factor, the inverse of the normalized bandwidth, which for cylindrical floats is predicted to be greater than 1, indicating sharp resonance. From a float deployment in the Gulf Stream region it was estimated that Q ≈ 5. For this case, the spectrum of float temperature, which was used as an indicator of the relative response between the float and a displaced isopycnal, and the spectrum of the float pressure, used as an indicator of float displacement, did scale according to that predicted by the condition of near equilibrium response, up to of order the resonant frequency of the float.

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Edward R. Levine and Rolf G. Lueck

Abstract

Horizontal profiles of the microstructure of velocity and temperature were obtained with a large autonomous underwater vehicle (AUV) using two piezoelectric shear probes, an FP07 thermistor, and three orthogonal accelerometers mounted on a sting at the forward end of the vehicle. A winter field trial in Narragansett Bay provided a run in the midwater pycnocline at 8-m depth that contained a thermal front, an ascending profile to 3-m depth, and a run in the weakly stratified surface layer at this depth. Although shear spectra were strongly contaminated by narrowband vibrations produced by the motor and actuators, this contamination was highly coherent with the measured acceleration and was removed with standard signal processing techniques. The corrected spectra agreed well with the Nasmyth universal spectrum for wavenumbers up to 40 cpm. The estimated rate of dissipation of kinetic energy varied from 0.8 to 250 × 10−8 W kg−1 and was consistent with the rate of production of turbulence by surface wind forcing and bottom stress.

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Louis Goodman, Edward R. Levine, and Rolf G. Lueck

Abstract

The terms of the steady-state, homogeneous turbulent kinetic energy budgets are obtained from measurements of turbulence and fine structure from the small autonomous underwater vehicle (AUV) Remote Environmental Measuring Units (REMUS). The transverse component of Reynolds stress and the vertical flux of heat are obtained from the correlation of vertical and transverse horizontal velocity, and the correlation of vertical velocity and temperature fluctuations, respectively. The data were obtained using a turbulence package, with two shear probes, a fast-response thermistor, and three accelerometers. To obtain the vector horizontal Reynolds stress, a generalized eddy viscosity formulation is invoked. This allows the downstream component of the Reynolds stress to be related to the transverse component by the direction of the finescale vector vertical shear. The Reynolds stress and the vector vertical shear then allow an estimate of the rate of production of turbulent kinetic energy (TKE). Heat flux is obtained by correlating the vertical velocity with temperature fluctuations obtained from the FP-07 thermistor. The buoyancy flux term is estimated from the vertical flux of heat with the assumption of a constant temperature–salinity (T–S) relationship. Turbulent dissipation is obtained directly from the usage of shear probes.

A multivariate correction procedure is developed to remove vehicle motion and vibration contamination from the estimates of the TKE terms. A technique is also developed to estimate the statistical uncertainty of using this estimation technique for the TKE budget terms. Within the statistical uncertainty of the estimates herein, the TKE budget on average closes for measurements taken in the weakly stratified waters at the entrance to Long Island Sound. In the strongly stratified waters of Narragansett Bay, the TKE budget closes when the buoyancy Reynolds number exceeds 20, an indicator and threshold for the initiation of turbulence in stratified conditions. A discussion is made regarding the role of the turbulent kinetic energy length scale relative to the length of the AUV in obtaining these estimates, and in the TKE budget closure.

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Edward R. Levine, Donald N. Connors, Richard R. Shell, and Robert C. Hanson

Abstract

An autonomous underwater vehicle (AUV), the U.S. Navy’s Large Diameter Unmanned Underwater Vehicle (LDUUV), was used as a stable platform for rapid, repeated, near-synoptic CTD measurements of estuarine variability in Narragansett Bay. Surveys were made in lawnmower-like patterns at middepth to obtain horizontal profiles and maps, and in vertical yo-yo patterns to obtain vertical profiles. These observations were ground-truthed using standard CTDs on the fixed position of the launch cage and on ship-based surveys around the perimeter of the study area before and after the runs. For the horizontal surveys, a comparison of temperature and salinity time series from the LDUUV and the launch cage CTDs shows that differences are within the range of lateral variability around the study area observed at run depth from the ship. For the yo-yo surveys, a comparison of LDUUV CTD and standard CTD profiles shows indications of hysteresis in the vehicle-obtained data, which can be minimized with improved sampling techniques. Horizontal profiles and maps were obtained for a 2000 m × 300 m area, which was repeatedly sampled during 1994. An example of a time series from the vehicle shows three crossings of a salinity front with no significant temperature variability. Accompanying platform data show the effects of turns and speed changes on the data acquisition process. In the maps, temperature and salinity variability were observed with horizontal scales of order 100 m, finestructure advected by the tidal current. Temperature–salinity relationships in the AUV-derived data show unique differences related to seasonal changes in Narragansett Bay, including the winter–spring and the subsequent summer–fall transition of 1994. These measurements show the viability of the AUV as a unique tool for hydrographic characterization, under vehicle, environmental, and oceanographic constraints.

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Edward R. Levine, D. N. Connors, Peter C. Cornillon, and H. Thomas Rossby

Abstract

No abstract available.

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Matthew T. Morris, Jacob R. Carley, Edward Colón, Annette Gibbs, Manuel S. F. V. De Pondeca, and Steven Levine

Abstract

Missing observations at airports can cause delays in commercial and general aviation in the United States owing to Federal Aviation Administration (FAA) safety regulations. The Environmental Modeling Center (EMC) has provided interpolated temperature data from the National Oceanic and Atmospheric Administration’s Real-Time Mesoscale Analysis (RTMA) at airport locations throughout the United States since 2015, with these data substituting for missing temperature observations and mitigating impacts on air travel. A quality assessment of the RTMA is performed to determine if the RTMA could be used in a similar fashion for other weather observations, such as 10-m wind, ceiling, and visibility. Retrospective, data-denial experiments are used to perform the quality assessment by withholding observations from FAA-specified airports. Outliers seen in the RTMA ceiling and visibility analyses during events meeting or exceeding instrument flight rules suggest the RTMA should not be substituted for missing ceiling and visibility observations at this time. The RTMA is a suitable replacement for missing temperature observations for a subset of airports throughout most of the CONUS and Alaska, but not at all stations. Likewise, the RTMA is a suitable substitute for missing surface pressure observations at a subset of airports, with notable exceptions in regions of complex terrain. The RTMA may also be a suitable substitute for missing wind speed observations, provided the wind speed is ≤15 kt (1 kt ≈ 0.51 m s−1). Overall, these results suggest the potential for RTMA to substitute for additional missing observations while highlighting priority areas of future work for improving the RTMA.

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