1. 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

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

3. 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)

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

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

7. site of large natural hydrocarbon seep

8. • 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

10. • 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?

12. Comparison of drifter and HF radar velocities

13. 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

14. 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

15. drifter-HF radar
drifter-drifter

16. 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 •