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Feature Article

Taking the Temperature Of the Arctic With UMVs
Arctic Wave Gliders Gather 900,000 Measurements During a Two-Month Mission in the Beaufort Sea

Feature Author
Christian Meinig
Director of Engineering
NOAA Pacific Marine Environmental Laboratory
Seattle, Washington

Feature Author
Dr. Michael Steele
Senior Principal Oceanographer
Applied Physics Laboratory
University of Washington
Seattle, Washington

Feature Author
Dr. Kevin Wood
Research Scientist
Joint Institute for the Study of the Atmosphere and Ocean
University of Washington
Seattle, Washington

Arctic Wave Glider system components.
Over the past five years, the loss of Arctic sea ice has been dramatic, especially in the Pacific sector. Satellite data indicate that the summertime ice-free area in the Beaufort Sea has increased by roughly 80 percent since 2007, in comparison to climatology (1981 to 2010). This has caused a tremendous increase in the air-sea exchange of heat in the upper ocean, a classic example of ice-albedo feedback, in which the dark ocean surface absorbs far more radiation than highly reflective sea ice. The result is an unprecedented warming of the upper ocean during the summer and early autumn.

Quantitative impacts of this heat transfer on ocean and atmosphere are not well known. Previous scientific studies have relied on numerical model output, satellite remote sensing or sparse in-situ (surface) observations. The major limitation has been the ability to economically carry out sustained observation in the surface layer of the seasonal ice zone, a technically and operationally challenging domain. Ship time is expensive in the Arctic and ranges between $25,000 to $80,000 per day, depending on capability.

To overcome these issues, NOAA Pacific Marine Environmental Laboratory (PMEL) and Liquid Robotics Inc. (Sunnyvale, California) deployed two Wave Glider unmanned maritime vehicles (UMVs), energy-self-sufficient surface vehicles that can be configured for extended research missions in challenging environments, such as the Beaufort Sea.

Mission Objectives
Beginning in early summer, sea ice melts and ocean heat content increases from intensifying solar radiation. Buoyant freshwater from the melting sea ice and from significant amounts of river runoff result in an increasingly stratified ocean surface layer. This effectively traps more heat at the surface, accelerating further melting and heating. Some of this heat is stored in the ocean until autumn, when it can delay the onset of freeze-up, and may, as it is released back to the atmosphere, warm and destabilize the marine atmospheric boundary layer and possibly the regional atmospheric circulation. Heat absorbed in summer also descends into the deepening winter mixed layer, which can continue to melt sea ice throughout the winter.

The mission objectives were to collect critical temperature data in the upper 6 meters, following a prescribed track with one vehicle leading the other by 12 hours to study diurnal heating effects and to take surface images from an aft-mounted camera at prescribed times for situational awareness near the ice edge to aid navigation and route planning. Another major goal was to evaluate the Wave Gliders' power generation and management at high latitudes.

Arctic Wave Glider Equipment and Testing
The Liquid Robotics Wave Glider UMV is able to harvest wave energy for platform propulsion, while solar panels provide electrical power for command, control and payload electronics. The Wave Glider is a two-part vehicle, consisting of a surface float connected to a submerged glider via a flexible umbilical. Just as an airplane's forward motion through the air allows its wings to create an upward lifting force, the submerged glider's vertical motion through the comparatively still waters at the glider's depth allows its wings to convert a portion of this upward motion into forward propulsion. The Arctic Wave Glider (AWG) proof-of-concept mission involved several modifications on the stock Wave Glider.

Six RST Instruments Ltd. (Maple Ridge, Canada) Therm'Array thermistors were molded into the tether at depths of 0.5, 1, 1.5, 3, 4.5 and 6 meters, in addition to a standard satellite-based command and control, and data-handling package. Communication was through an RS-485 transceiver to the command and control electronics. Data were recorded on board the AWG and transmitted in near real time at user-selectable intervals. A one-minute interval was chosen for this mission, which was oversampled but allowed researchers to evaluate sending large files through the Wave Glider platform.

Uncertain solar power availability during late summer motivated the use of a supplemental lithium-ion battery pack of 1,200-watt-hour capacity. This module has the capacity to provide backup power to the AWG system for 15 days of operation.

Additionally, an aft-mounted camera connected to the Iridium Communications Inc. (McLean, Virginia) link was installed to send back real-time images for situational awareness because of the unknowns of working in the seasonal ice zone.

Tests were conducted on the bench and in the field to determine the fatigue endurance of the AWG umbilical, increased drag due to the faired polyurethane overmolding, and the accuracy and response time of the thermistors. Early on in the development, a highly accelerated life test of one of the thermistor umbilicals was conducted by cycling a 720-pound load every 2.4 seconds for 30,000 cycles. Analysis showed that several thermistor wires failed under strain. While the test was ultimately judged to be too harsh, a subsequent modified design with unique strain relief loops was developed, deployed and tested at the Liquid Robotics marine engineering and test facility in Hawaii for three weeks and passed without incident.

The AWG was raced against a baseline vehicle with an unmodified umbilical and was less than 0.1 knot slower on average, with no change in station-keeping ability. All thermistors were pre- and post-mission calibrated, and accuracies for all thermistors were less than 0.01° C after calibration factors were applied. Only one thermistor showed an appreciable drift of approximately 0.15° C at lower temperatures between calibrations; all others were less than 0.02° C. Thermal response tests were also conducted by plunging a complete umbilical from room temperature (20° C) to the controlled bath at '1° C. The thermal response time, which is defined as the sensor reading 63.2 percent of a steady-state temperature, was determined to be about nine minutes for all thermistors. To continue this article please click here.

Christian Meinig is the director of engineering at NOAA Pacific Marine Environmental Laboratory in Seattle, Washington, where he develops and deploys ocean and atmospheric instruments and observing platforms. The engineering group's focus is to provide high-quality and cost-effective ocean observing systems for research and operations.

Dr. Michael Steele is a senior principal oceanographer at the Polar Science Center of the Applied Physics Lab at the University of Washington. His main interest is the physical oceanography of the Arctic and Antarctic seas, with a focus on the how the changing sea-ice cover influences the upper ocean.

Dr. Kevin Wood is a research scientist at the NOAA and University of Washington Joint Institute for the Study of Atmosphere and Oceans (JISAO), specializing in high-latitude climatology. He has more than 20 years of experience working in the Arctic and Antarctic regions.

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