Fredrik T. Thwaites

Ekman Veering, Internal Waves, and Turbulence Observed under Arctic Sea Ice

The ice‐ocean system is investigated on inertial to monthly time scales using winter 2009‐10 observations from the first ice-tethered profiler (ITP) equipped with a velocity sensor (ITP-V). Fluctuations in surface winds, ice velocity, and ocean velocity at 7-m depth were correlated. Observed ocean velocity was primarily directed to the right of the ice velocity and spiraled clockwise while decaying with depth through the mixed layer. Inertial and tidal motions of the ice and in the underlying ocean were observed throughout the record. Just below the ice‐ocean interface, direct estimates of the turbulent vertical heat, salt, and momentum fluxes and the turbulent dissipation rate were obtained. Periods of elevated internal wave activity were associated with changes to the turbulent heat and salt fluxes as well as stratification primarily within the mixed layer. Turbulent heat and salt fluxes were correlated particularly when the mixed layer was closest to the freezing temperature. Momentum flux is adequately related to velocity shear using a constant ice‐ocean drag coefficient, mixing length based on the planetary and geometric scales, or Rossby similarity theory. Ekman viscosity described velocity shear over the mixed layer. The ice‐ocean drag coefficient was elevated for certain directions of the ice‐ocean shear, implying an ice topography that was characterized by linear ridges. Mixinglengthwasbestestimatedusingthewavenumber ofthebeginningoftheinertialsubrangeoravariable drag coefficient. Analyses of this and future ITP-V datasets will advance understanding of ice‐ocean interactions and their parameterizations in numerical models.

Development of a modular acoustic velocity sensor

The authors are developing a Modular Acoustic Velocity Sensor (MAVS), a three-axis current meter that measures the differential acoustic-travel time in a small measurement volume. The requirements of this sensor are: low cost, small size, high accuracy, good cosine response to current direction, lack of bias in a wave environment, ability to measure turbulence and the Reynolds stress, resistance to fouling, ability to measure near a boundary, and accuracy at low current speeds. The sensor is a derivative of the Benthic Acoustic Stress Sensor (BASS) which has been very successful at measuring currents, shear, and Reynolds stress. A first MAVS sensor prototype has been built and tested. The sensor was designed to reduce flow disturbance error when flows are steeper than thirty degrees from the horizontal. This paper describes performance measurements of the first prototype sensor. The sensor was towed in a tow tank to measure its accuracy and to characterize the vortex shedding noise inherent in any flow measurement made local to an ocean sensor. The first prototype was found to have better cosine response but more flow noise than BASS. A second prototype is now being built to have better cosine response than BASS and lower noise than the first prototype.

Motion tracking in an acoustic point-measurement current meter

Measurements of velocity structure in the water column under Arctic ice from an Ice-Tethered Profiler (ITP) employed an acoustic point-measurement current meter, MAVS (Modular Acoustic Velocity Sensor) [1]. With the velocity sensor it becomes the Ice-Tethered Profiler with Velocity (ITPV). The profiler, containing a Seabird CTD, MAVS, batteries, an inductive modem, and a wire crawling engine, integrated by McLane Labs, was constrained to be deployed through a 24″ diameter hole drilled in the ice. The anchor to the ice via a buoy with a satellite transmitter fixed the top of the mooring to a drifting but GPS tracked location while the profiler descending to a depth of 800m measured velocity relative to the moving mooring and the climbing profiler. A large current-orienting alignment fin was not possible on the ITP due to the limit of the ice hole diameter yet it was known that vortex shedding by the profiler body in the current would cause the instrument to swing and the current sensor to measure horizontal velocities due to the rotation of the profiler around the center of gyration of the package. To remove this platform motion from the current measurement, an inertial sensor, Analog Devices ADIS 16355 [2], was added to the MAVS current meter and three axes of angular velocity and three axes of linear acceleration were added to each data record of time, velocity, temperature, three-axis magnetic vector components and two axes of tilt. From the rate gyro value of angular velocity around the vertical axis, the platform rotation is determined and using the distance that the velocity sensor is displaced from the axis of rotation the horizontal current sensor velocity can be subtracted from the horizontal velocity measured by the sensor. This ITP was deployed in October, 2009 and data from the first profile indicates expected and unexpected performance.

Earth coordinate 3-D currents from a modular acoustic velocity sensor

MAVS, modular acoustic velocity sensor, measures 3D flow components along four acoustic paths using differential travel-time techniques, removes the offset from each acoustic axis due to individual characteristics of the cabling and any electronic offsets, and rotates the corrected components into Earth coordinates with a gimbaled two-axis magnetometer. In the present version of MAVS, the sensor is assumed to be aligned vertically to within 15/spl deg/. Three requirements had to be met to allow the conversion of the three-axis current measurement into Earth coordinates in situ without introducing non-removable bias. First, flow distortion by the sensor had to be reduced to an acceptable level so that a cosine response in elevation as well as azimuth could be applied. Second, the determination of offsets for zero flow had to be easily measured and compensated, and these offsets had to be stable. Third, the measurement of sensor-frame heading had to be accurate and tilt tolerant. Fairing of the transducer support rings achieved the first requirement. Enabling the sensor to be auto-zeroed and rigidly capturing the cables inside the transducer supports achieved the second. Rotating the precisely measured sensor-frame components of velocity into Earth coordinates with the relatively imprecise magnetometer components and vector averaging the resultants achieved the third. That these can be done in a small instrument with low power permits a modular approach to current sensing. Direct reading, stand-alone logging, and modular component applications of a modular acoustic velocity sensor, MAVS, have been implemented.

Use of tow tanks to study sensitive current meters

The importance of accurate calibration of current sensors has increased as current sensors have become more sensitive, and as scientists study turbulence and mixing in boundary layers. Current sensors are often calibrated in tow tanks. This paper discusses and presents some measurements of some of the complexities of calibration in tow tanks. Residual convective currents cause a noise floor in calibration. Sensor wakes cause gain errors and increase the ambient turbulence if not dissipated before a successive measurement is made in the same water. Proximity to surfaces can affect the velocity gain of current sensors. Density stratification in water and the internal waves it allows add complexity to measuring slow flows. Strumming of current meter mounts greatly increases the velocity noise of current meters. Measurements of velocities in current sensor wakes are compared with far-field turbulent wake models to characterize how long one must wait between tank tows in order for wakes to dissipate. Wake velocities caused by sensors that looked like they would leave two-dimensional wakes, in fact, decayed as velocities in three-dimensional wakes. Three-dimensional wake velocities dissipate more rapidly than two-dimensional wakes velocities, reducing the time required to wait between measurements in tow tanks.

Noise in Ice-Tethered Profiler and McLane Moored Profiler velocity measurements

In order to measure current profiles, and most recently, turbulent fluxes, moored profiling instrument have been equipped with acoustic travel-time current sensors. Noise in the measured currents has exceeded expectations. A customized Falmouth Scientific acoustic current sensor on a McLane Moored Profiler (MMP) has a standard deviation of measured velocity that is 4.4% of the profiler velocity in still water and a modified Modular Acoustic Velocity Sensor (MAVS) on an MMP and an Ice-Tethered Profiler (ITP) has a standard deviation of 4.6% of profiler velocity. Both of these sensors measure velocity along four acoustic paths and the water velocities were computed neglecting their downstream paths.

A winch and cable for the Autonomous Vertically Profiling Plankton Observatory

High frequency (of order hours) sampling of plankton over extended periods (of order months) is essential in studying the response of plankton to their environment. The Autonomous Vertically Profiling Plankton Observatory (AVPPO) has been designed and built to meet this need in coastal areas. For this configuration of the video plankton recorder, an autonomous winch, resting on the sea floor, and custom cable have been developed. The system requirements were: low power, tolerance of slack in the table permitting the buoyant instrument platform to profile to and from the surface in 100 m of water, ability to telemeter video signals down to the system base which houses video recorders, and autonomous operation over extended periods of time. A custom, high-helix angle, electromechanical cable, with a central Kevlar strength member, was made to be light, flexible in bending, stiff in stretch, and tolerant of bending while containing two coaxial cables. The winch has two independent, oil-filled, pressure-compensated, brushless DC motors. The drum motor controls speed and position of the cable, while the powered sheave keeps cable tension on the drum even when the cable from the fish is slack, as when the fish surfaces. The system has been built and twice deployed autonomously in 80 m of water on Georges Bank off the coast of Massachusetts.

Oversampling MAVS for reduction of vortex-shedding velocity-sensing noise

Reduction of vortex-induced velocity-sampling noise was demonstrated in an acoustic travel-time current meter, MAVS, by oversampling and averaging bursts of 20Hz measurements every 0.5s sampling interval during tows in the Woods Hole Oceanographic Institution flume. A characteristic vortex size of 7.37cm (from frequency and tow speed) translated into narrow band spectral peaks of velocity fluctuation with frequency proportional to tow speed. A selection of burst-sampling intervals during which 20Hz measurements were averaged was tuned to the profiling speed of the moored profiler upon which the MAVS was to be mounted. Burst-sampling improved the signal to noise ratio by almost a factor of 3 with a 4-point burst every 0.5s at 30cm/s tow (profiling) speed. This closely matched the dominant 4.3Hz vortex at 30cm/s tow speed.

A leveling system for an ocean-bottom seismometer

As part of a new broad-band, ocean-bottom seismometer (OBS) system that has been developed at the Woods Hole Oceanographic Institution, a gimbaled leveling system was designed and built. The goal of the broad-band system is to measure ocean-bottom vibrations from a period of 120 seconds up to 20 Hertz. During system deployment, a sphere containing the seismic sensor is dropped into sediment on the ocean bottom. Seismic sensors need to be leveled before use, and it is not practical to accurately control the attitude of the sphere as it settles on the ocean bottom. The sphere holds the seismic sensor in gimbals whose axes have brakes. The gimbal axes have brakes to prevent tilting in response to slow horizontal accelerations that would complicate long-period seismometer response. To level the seismic sensor, the brakes are released, the righting moment of the seismic sensor in the gimbals levels the seismometer attitude, and the brakes are reasserted. The brake systems were designed to have zero play and had to be modified to raise the lowest system natural frequency above 20 Hertz. This paper describes the mechanical aspects of the system and the modifications needed to push up the mechanical resonances. Twenty-five of the OBS systems have been built and deployed off Hawaii as part of the Plume-Lithosphere Undersea Melt Experiment (PLUME)

Acoustic vorticity meter for benthic boundary layer flow measurements

An acoustic travel-time flow-meter that had been configured to measure vorticity on scales of 15 and 45 centimeters has now been configured to measure benthic boundary layer vorticity over a scale of 1.5 meters. Closed square paths in each sensor define three orthogonal axes of vorticity. Originally constructed to measure shear in the presence of waves, the 15 cm and 45 cm sensors have an expected accuracy for shear of 10/sup -2/ s/sup -1/ and 3/spl times/10/sup -3/ s/sup -1/ respectively, The 1.5 m sensor appears to have a noise floor of 2/spl times/10/sup -5/ s/sup -1/. Strong boundary layer shear generates turbulence and turbulent vorticity. Measurements of vorticity are being made with these sensors at several scales in strong tidal flows from 1.2 m to 3.2 m above the bottom. Data are being stored internally and in a pop-up data logger for recovery at the end of the deployment and part way through the deployment, Absolute shear can be determined with the 45 cm sensor through careful zero flow calibration before the deployment. The 1.5 m sensor is part of the deployment tripod and too large to be calibrated in controlled conditions. Consequently, fluctuations in vorticity can be determined with the 1.5 m sensor.<<ETX>>