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Surface Drifter Deployments

Principal Investigators:
-- mark halverson
prince william sound science center and

-- carter ohlmanN
UNIversity of California, Santa barbara

Oil spill response and the search and rescue of vessels and people in Prince William Sound (PWS) and on the high seas are highly reliant upon accurate and timely surface current information.  The computer simulations under development by the Alaska Ocean Observing System (AOOS) will greatly improve the response time to marine accidents.  But the forecasts provided by computer models need to be tested against observations and measurements in the ocean.  A field experiment in 2004 used drifting buoys to measure ocean currents in PWS.  The results showed a strong horizontal circulation of water moving counterclockwise around the central basin.  Mathematical models predict a similar circulation around the perimeter of the sound.  This summer a second field experiment will again use drifting buoys to test the accuracy and precision of the computer forecasts.

Four types of drifting buoys will be used: Argosphere drifters made by Metocean Data Systems, Surface Velocity Program (SVP) drifters made by Pacific Gyre, Microstar drifters also made by Pacific Gyre, and the U.S. Coast Guard (USCG) Self Locating Data Marker Buoy (SLDMB) made by Metocean.

The Argospheres (Fig. 1) are 28-cm diameter spherical buoys designed to track oil floating on the water.  Position determination is through the Argos satellite system and a GPS receiver.  For the GPS positions, an accuracy of +/- 10 m is normal.  Position updates will be obtained at 0.5-hour intervals.  A hand-held tracking unit will be used to provide location information in the field.

The SVP drifters (Fig. 2) are 38-cm diameter spherical buoys to which a drogue is attached, and they are expected to drift with the water at the depth of the center of the drogue.  The drogue is a holeysock 2.5 m in height and positioned to be centered at 10 m.  Location determination is through the Argos satellite system and GPS receiver.  

The Microstar drifters (Fig. 3) are designed to track the mean current at a depth of about 1 meter.  The key elements of the drifter include the drogue, the surface float and the connecting tether.  The drogue acts as a sea anchor locking the drifter to a parcel of water.  The surface float contains the telemetry system, antenna, batteries and sensors.  Drifter positions are calculated by an onboard GPS receiver.  Positions are transmitted to the user providing the information necessary to calculate mean current velocities and dircetion.

The Metocean SLDMB (Fig. 4) used by the USCG is designed specifically for deployment from a vessel or aircraft and for unattended operation during a 30-day lifetime.  The SLDMB is accompanied by an onboard electronics package which includes GPS positioning.  Service Argo, Inc, receives the data and forwards it to the National Oceanic and Atmospheric Administration polar-orbiting n-series satellites every 30 minutes.  The data is transmitted to a secure USCG website for use by trained Search and Rescue Personnel.  The USCG uses these drifters to construct current vectors from sequential SLDMB positions.  In conjunction with the Search and Rescue Optimal Search Planning System (SAROPS) the USCG is able to establish the ocean surface motion and use this to better predict the motion of vessels and people during search and rescue operations.

These drifters will be deployed in groups that contain at least three of each style of drifter.  During the main study periods the drifters will be released at three sites 60 35N 146 56W, 60 35N 146 47W, and 60 31N 146 56W.  The first location listed is where drifters were deployed in the 2004 experiment.  By using three locations we provide a spread to check the model results and are situated to maximize time in the cyclonic circulation that was observed in 2004.  In 2004 the primary drifter trajectory was a cyclonic circulation with the drifters leaving the circulation pattern from the northern and western edge.  The drifters will be picked up if they leave a circle 10 nm in radius with 60 35N 146 56W as the center of the circle.  Once picked up they will be redeployed at their original starting location.

During the period between the two main observation times the drifters will be used to test circulation in other parts of the system, such as starting at a more northwestern location where the freshwater flow is expected to be greater.  They will also be deployed within the HF-radar viewing area with a smaller watch circle used to pick up and redeploy the drifters.  This will allow better quantification of the dispersion of the drifters from a grid cell of the model or HF radar footprint.  Drifter position will be monitored through Argos and Iridium telemetry as well as hand-held receivers.  A file with at least hourly drifter positions will be provided to AOOS each evening before 10 p.m. ADST.

The Alaska Ocean Observing System is building a network of observation platforms and forecast models. The goal of this network is to provide information products and tools to improve our understanding of Alaska’s ocean ecosystem and allow us to make better decisions about our use of the marine environment.

AOOS is affiliated with the Integrated Ocean Observing System and the
University of Alaska Fairbanks School of Fisheries and Ocean Sciences.
UAF is an affirmative action/equal opportunity employer and educational institution.

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