GPS-Cellular Drifter Technology for Coastal Ocean Observing Systems Carter Ohlmann University of California, Santa Barbara Andy Sybrandy Pacific Gyre Inc. • drifter • descriptive summary of data • importance in coastal ocean observing systems

Typical Open Ocean Drifter Argos: • 100 m spatial resolution • Position ~5 times each day • communications costs: ~$300 drifter-month

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

• inconsistent with wind forcing • onshore nearshore ~20 days ~10 drifters/day ~300 tracks Mean Wind: 2.4 cm/s • inconsistent with wind forcing • onshore nearshore

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

HF radar - drifter trajectories 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