GPS-Cellular Drifter Technology for Coastal Ocean Observing Systems

Carter OhlmannUniversity of California, Santa Barbara

Andy SybrandyPacific Gyre Inc.

• drifter• descriptive summary of data• importance in coastal ocean observing systems

Argos:• 100 m spatial resolution

• Position ~5 times each day• communications costs: ~$300 drifter-month

Typical Open Ocean Drifter

High-Resolution, Recoverable “Microstar” (mfg. by Pacific Gyre Inc.)

• GPS position accurate to ~5 m

• position updates as often as every minute (variable)

• data transmitted via Mobitex™ digital, data-only, cellular network

• near real-time data and thus recoverable

• communications costs: O($10) drifter-month

• range limitations (~50 km from coast)

• drag-area-ratio = 41.3• known slip (< ~2 cm s-1)

Matlab based near real-time tracking software to facilitate drifter management and retrieval.

site of large natural hydrocarbon seep

~20 days~10 drifters/day~300 tracks

• inconsistent with wind forcing• onshore nearshore

Mean Wind: 2.4 cm/s

• Drifter data show a variety of small-scale processes in the coastal ocean

• Validation of HF radar data with current meter observations are inaccurate due to consideration of disparate scales

• How should HF radar data be interpreted?

Comparison of drifter and HF radar velocities

Drifter vs. HF radar velocities over the SBC inner-shelf

HF radar data: - hourly - 1 or 2 km grid - 10 cm/s accuracy

Much of the quoted discrepancy in HF radar is due to differences in resolution

Total separation:

• 0.5 km after 2 hrs (7 cm/s)

• 1.5 km after 4 hrs (10 cm/s)

HF radar - drifter trajectories

drifter-HF radar

drifter-drifter

Summary:

• new drifter technology for coastal regions provides economical high-resolution (meters, minutes) data

• instrument error and sub-grid-scale information in HF radar data

• separation rate between drifters and HF radar trajectories is near 10 cm/s (relative dispersion is 4 cm/s) and directional biases exist

• Routine drifter releases for interpretation of HF radar data should be part of coastal ocean observing systems

• www.drifterdata.com