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Robert J. Serafin and Thomas B. Sanford

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Thomas B. Sanford and Dus̆an S. Zrnić
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Thomas B. Sanford and Robert J. Serafin

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D. P. Winkel, M. C. Gregg, and T. B. Sanford

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The Multi-Scale Profiler (MSP), a freely falling dropsonde, has been used over the past 12 years to measure oceanic shear variance. Complete resolution of oceanic shear spectra is achieved by combining the measurements of MSP’s acoustic current meter (ACM), electromagnetic current meter (ECM), and airfoil probes. The ACM detects flow relative to MSP, so the platform motion must be known to determine the water velocity. The vechicl's tilt oscillation is inferred from accelerometer data, and its gross (point mass) horizontal motion is simulated by modeling MSP's response to the relative flow. Forcing on its tail array causes MSP to react as a point mass to fluctuations with scales as small as 2-3 m. The model of Hayes et al. for the TOPS dropsonde was modified so that it reasonably parameterized the large MSP tail force. Relevant dynamics and data processing are discussed, and the point-mass model is presented along with the analytic transfer functions that are used to select parameter values, assess sensitivities, and estimate uncertainties. Because they are unaffected by MSP's horizontal motion, the ECM measurements directly reflect the flow structure and, consequently, provide an onboard reference against which the large-scale corrections to the ACM measurements are validated. Uncorrected ACM data provide a direct check on the airfoil data, which resolve microscale shear variance to within a factor of 2, aside from some noted exceptions in warm, turbulent waters. The motion-corrected ACM profiles are shown to resolve shear variance to within 10%–15% at vertical scales from over 200 m down to 1 m (with minor anomalies at 5-m and 2-3-m scales).

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Thomas B. Sanford, Robert G. Driver, and John H. Dunlap

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A freely failing current meter called the Absolute Velocity Profiler (AVP) is described. This profiler is an expansion of a previously developed instrument, the Electro-Magnetic Velocity Profiler (EMVP), with the additional capability of acoustic Doppler (AD) measurements to determine the reference velocity for the EM profiles. The AVP measures the motional electric currents in the sea and the Doppler frequency shin of bottom-scattered echoes. The EM measurements yield a profile of the horizontal components of velocity relative to a depth-independent reference velocity; the AD measurements determine the absolute velocity of the AVP with respect to the seafloor. The EM profile is obtained from the sea surface to the bottom, and the AD measurements are obtained within about 60–300 m of the seafloor. The combination of the EM and AD measurements yields an absolute velocity profile throughout the water column. Performance analyses show the method is accurate to within 1–2 cm s−1 rms. The profiler also measures temperature and its gradient.

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R. H. Käse, H-H. Hinrichsen, and T. B. Sanford

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A method is presented for determining salinity and density from temperature data in conjunction with historical or contemporaneous (but not collocated) CTD observations. The horizontal density ratio r(z) is determined from the temperature and salinity differences at each depth (δT, δS) between pairs or ensembles of profiles. These differences are expressed as a density ratio r=αδT/βδS, where α and β are the expansion coefficients for temperature and salinity, respectively. Salinity at a site where only temperature is measured, as with an expendable bathythermograph (XBT), is computed based on the temperature and salinity at a reference station (S R,T R); that is, S=S R+(TT RST. The method is restrictive in its application because it is most accurate when all water masses in the region of a survey are linear extrapolations from the water masses at each of the reference stations. In reality, it provides useful results when the T and S fields are not simply linear functions of horizontal distance. This approach is particularly useful in regions where, the T(z)−S(z) relation is nonunique, as in the Mediterranean Water in the North Atlantic. The corresponding expression for the lateral density difference for an observed temperature difference (δT) is δρ=−αρ0δT(1−r −1). Observations from regions offshore and along the coast of Portugal are used to evaluate the method. Errors of less than 0.05 psu are exhibited in the evaluation of salinity determined from T-5 XBT drops compared with nearly simultaneous CTD casts. A comparison of water properties and cyclostrophic velocities is made using XCP temperatures and XCP velocities in a meddy.

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Je-Yuan Hsu, Ren-Chieh Lien, Eric A. D’Asaro, and Thomas B. Sanford

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Seven subsurface Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats measured the voltage induced by the motional induction of seawater under Typhoon Fanapi in 2010. Measurements were processed to estimate high-frequency oceanic velocity variance associated with surface waves. Surface wave peak frequency f p and significant wave height H s are estimated by a nonlinear least squares fitting to , assuming a broadband JONSWAP surface wave spectrum. The H s is further corrected for the effects of float rotation, Earth’s geomagnetic field inclination, and surface wave propagation direction. The f p is 0.08–0.10 Hz, with the maximum f p of 0.10 Hz in the rear-left quadrant of Fanapi, which is ~0.02 Hz higher than in the rear-right quadrant. The H s is 6–12 m, with the maximum in the rear sector of Fanapi. Comparing the estimated f p and H s with those assuming a single dominant surface wave yields differences of more than 0.02 Hz and 4 m, respectively. The surface waves under Fanapi simulated in the WAVEWATCH III (ww3) model are used to assess and compare to float estimates. Differences in the surface wave spectra of JONSWAP and ww3 yield uncertainties of <5% outside Fanapi’s eyewall and >10% within the eyewall. The estimated f p is 10% less than the simulated before the passage of Fanapi’s eye and 20% less after eye passage. Most differences between H s and simulated are <2 m except those in the rear-left quadrant of Fanapi, which are ~5 m. Surface wave estimates are important for guiding future model studies of tropical cyclone wave–ocean interactions.

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Thomas B. Sanford, James A. Carlson, John H. Dunlap, Mark D. Prater, and Ren-Chieh Lien

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An instrument has been developed that measures finescale velocity and vorticity in seawater based on the principles of motional induction. This instrument, the electromagnetic vorticity meter (EMVM), measures components of the gradient and Laplacian of the electrostatic potential field induced by the motion of seawater through an applied magnetic field. The principal innovation described here is the development of a sensor for measuring small-scale vorticity. The sensor head consists of a strong NdFeB magnet, a five-electrode array, low-noise preamplifiers, and 20-Hz digitizers. The main electronics includes attitude sensors, batteries, a microprocessor, and a hard disk. The vorticity sensors are usually carried on a heavy towed vehicle capable of vertically profiling to 200 m and at tow speeds of several knots.

The theoretical response functions of the EMVM are evaluated for velocity and vorticity. Extensive measurements were obtained in Pickering Passage, Washington, as the sensor vertically profiled in an unstratified tidal channel. During periods of strong flow, the vertical structure of all properties confirmed expectations for a fully developed turbulent bottom boundary layer. EMVM observations of velocity and vorticity are shown to be in agreement with the theoretical response function for isotropic turbulence. A principal result is that the vertical flux of spanwise vorticity (i.e., wωy) is positive (i.e., flux is away from seabed) and vertically uniform. The vertical eddy diffusivity for vorticity is about 5 × 10−2 m2 s−1, which is about the same value as for momentum.

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