George A. Fowler

Mooring Vibration as a Source of Current Meter Error and Its Correction

Abstract Data from instrumented moorings are used to show that rate underreading of Aanderaa paddle-wheel rotor current meters is a result of mooring vibration induced by vortex shedding from spherical in-line buoyancy. Direct evidence of this mooring motion is provided by accelerometer records that reveal high-frequency (0.2–0.4 Hz) displacements normal to the absolute flow direction that are as much as 0.5 m in amplitude. Current meter measurements are up to 40% lower than concurrent rates measured by a ship-mounted acoustic Doppler current profiler (ADCP). The result supports the hypothesis of Loder and Hamilton that rate underreading occurs when cross-flow motion of the current meters causes a rotor shielding effect arising from meter misalignment with the fluctuating relative water velocity. Measurements from an instrumented mooring in which spherical in-line buoyancy is replaced with streamlined flotation are also presented. Here the variance of the measured cross-flow acceleration is typically redu...

Pack ice convergence measurements by GPS–ARGOS ice beacons

Satellite-tracked ice beacons containing Global Position System (GPS) location sensors were field tested for their reliability and their position accuracy (both relative and absolute) before being deployed on the mobile pack ice to monitor pack ice motion. On flat lake ice, a single beacon on average provided hourly data 87% of the time with a position accuracy of 20 m. In ice rubble, data availability was reduced to 83% and position accuracy decreased to 35 m. Between pairs of beacons, relative distance accuracy depended on whether the positions of the beacons were derived using the same satellite constellation. For all available position data, the data availability for relative distances between beacons was 78% for the short 1-day data set of the lake ice site and 69% for the longer 25-day data set of the ice rubble site. Relative distance accuracies were respectively 15 and 20 m using all position data. When positions were derived using the same satellite constellation for beacon pairs, the data availability reduced to 60% for the lake site and 52% for the ice rubble site while the relative accuracy increased respectively to 1.5 m (lake) and 10 m (ice rubble). The beacons proved their durability by monitoring the ice motion for an additional 60 days in an offshore experiment in which three floes forming a triangle were tracked until the floes the beacons were on melted.

SeaCycler: A Moored Open-Ocean Profiling System for the Upper Ocean in Extended Self-Contained Deployments

The upper ocean, including the biologically productive euphotic zone and the mixed layer, has great relevance for studies of physical, biogeochemical, and ecosystem processes and their interaction. Observing this layer with a continuous presence, sampling many of the relevant variables, and with sufficient vertical resolution, has remained a challenge. Here a system is presented that can be deployed on the top of deep-ocean moorings, with a drive mechanism at depths of 150–200 m, which mechanically winches a large sensor float and smaller communications float tethered above it to the surface and back down again, typically twice per day for periods up to 1 year. The sensor float can carry several sizeable sensors, and it has enough buoyancy to reach the near surface and for the communications float to pierce the surface even in the presence of strong currents. The system can survive mooring blowover to 1000-m depth. The battery-powered design is made possible by using a balanced energy-conserving principle. Reliability is enhanced with a drive assembly that employs a single rotating part that has no slip rings or rotating seals. The profiling bodies can break the surface to sample the near-surface layer and to establish satellite communication for data relay or reception of new commands. An inductive pass-through mode allows communication with other mooring components throughout the water column beneath the system. A number of successful demonstration deployments have been completed.

Pack ice stress and convergence measurements by satellite-tracked ice beacons

Relationships between pack ice stress, ice convergence and atmospheric conditions are investigated with data collected by satellite-tracked ice beacons deployed on the pack ice off the Canadian east coast in March 1996. Ice beacons containing Global Positioning System (GPS) sensors monitored pack ice convergence and divergence. Stationary tests showed that on average the beacons provided hourly data 93% of the time with an absolute positional accuracy of 37m. The mean relative distance accuracy was 17m with a 87% data return when positions from all satellite constellations were used. When only positions from similar satellite constellation were used the relative distance accuracy was 1.7m with a 55% data return. Ice stress data were collected by a 3-directional ice stress sensor deployed on a floe that was the centre of a pack ice triangle monitoring ice convergence/divergence with GPS ice beacons. The pressure sensor facing the thick offshore pack ice recorded pressures between 20 and 40 kPa when ice temperatures at the sensor were between -3.5/spl deg/C and -2.5/spl deg/C. Multi-variate regression analysis showed that the response of the major principal stress was 0.36 kPa for each 1 m/s of wind forcing and that the wind and the ice temperature effects explained 85% of the variance in the major principal ice stress. Since at these warm ice temperatures both the ice volume and ice strength decrease when ice warms, the thermal response of the major principal stress was a decrease of 24.9 kPa for each 1/spl deg/C of ice warming.

The Design and Development of a Particulate In-Situ Sampler (PARISS)

The accurate determination of concentrations of particulate matter in the deep ocean is complicated by the fact that such concentrations are extremely low. Consequently, considerable quantities of water must be filtered to obtain significant results. This paper describes a device which is designed to operate on a standard CTD cable to obtain insitu samples of particulate matter by filtering large measured quantities of sea water at moderate flow rates. The unit includes six banks of filters which can be sequentially engaged on command from the surface. This allows multiple samples to be collected from a single depth, or sampling at multiple depths from a single cast. The sampler is remotely powered and controlled from the surface, with information being transferred to and from the instrumentation package and the ship via a single wire. Test results indicate correct electro-mechanical operation to 2000 m however, some discrepancy with concentrations measured by conventional systems is noted.

The Development of Thermister Chain Buoys for Use in Ice Infested Waters

The use of temperature profiles gathered in near real time can greatly increase the accuracy and timeliness of forecasts such as the prediction of freeze-up. This paper describes the development of ARGOS based thermistor chain buoys to operate in the Gulf of St. Lawrence up to an beyond the appearance of ice. The buoys employ a slack, buoyant mooring system which allows them to remain in place until the arrival of ice cuts them free to drift with the ice pack while providing uninterrupted data transmission. Data recovered from field operations are presented along with some analysis describing the significance and usefulness of observed cooling trends.