Ocean Surface Current Observations in PWS Carter Ohlmann

1. Ocean Surface Current Observations in PWS Carter Ohlmann Institute for Computational Earth System Science, University of California, Santa Barbara, CA 93106

2. ROMS-based dispersal simulation Lagrangian PDFs are calculated for 1 – 14 day advection times Deployment sites have 5 km radius and are adjacent to the coast From each site, around 100 particles are released every 12 hours from Jan. 1996 – Dec. 2002 PDFs = probability density functions

3. Drifter data(CODE 1 meter; MMS SBC-SMB study) SCB drifter data on the regional scale Drifter dispersal from a single site Drifters deployed ~ quarterly from 1993 – 1999. 568 drifters sampling for an average of ~24 days give ~13,500 drifter days of data. Red circle: “release” site Blue dots: drifter locations for a give advection time

4. Lagrangian PDF vs Drifter Distribution Drifter locations

5. Project Goal: • Provide improved real-time ocean current and wind forecasts with error estimates for inclusion in USGC DSTs. • Pathway to Project Goal: • Benchmark DSTs (year 1) • Develop and evaluate improved data assimilating models (year 2)

6. Motivation for this research component: Benchmarking, evaluating, and assimilating data into DSTs (focused on transport pathways) requires a thorough understanding of surface current observations. Data from drifting buoys are key as drifters provide direct observations of both advection and diffusion, the two processes responsible for defining a search area. 1000 m 100 m 10 m 24 hrs

7. Outline: • Instrumentation for measuring ocean surface currents • - HF radar derived surface currents • - Drifting buoys • - SLDMBs • Ocean surface current data collected during year 1 field program • - 54 drifter tracks w/ 12 drifters • Preliminary analysis of year 1 surface current data • - SLDMB performance • HF radar “ground truth” • Work plan for year 2

8. Microstar Drifters: • tri-star drogue centered at 1 m depth • 10 minute position sampling w/ GPS • data transmission through Iridium • 1 cm/s slip in 10 m/s wind • 7 day life expectancy • real time data on web • recoverable www.drifterdata.com Ohlmann et al. 2005, and Ohlmann et al. 2007

9. Microstar drifter data during PWS FE: • 12 drifters used; 12 drifters worked; 1 drifter lost • 54 drifter trajectories sampled • mostly ~2 days in length • positions every 10 minutes

10. USCG SLDMB • marker buoy used by USCG • based on 1970’s design • altered dimensions • water-following characteristics not found in scientific literature • 30 minute position data • data transmission: Argos • difficult to recover

11. USCG SLDMB data during PWS FE: • 9 drifters used; 8 drifters worked; 9 drifters lost • 8 drifter trajectories sampled • mostly numerous days in length • positions every 30 minutes

12. HF radar surface currents – Bragg scattering off surface gravity waves with known wavelength, extract wave speed, get surface current. Typically 15 – 30 minute averages reported hourly for a 1 – 10 km grid. Velocity “errors” of 10 cm/s typically quoted

13. PWS HF radar locations HF radar surface currents – time-space (1 hr - 1 km) average surface current maps such as this were produced throughout the PWS FE (~14 days).

14. PWS HF radar locations PWS HF radar surface current map – spatial extent of coverage is highly variable.

15. Preliminary analysis of data: Q: What can be learned of SLDMB water-following capabilities? ending positions USCG SLDMBs Microstar drifters starting positions

16. Preliminary analysis of data: A: SLDMBs move ~1.0 cm/s slower. similar diffusion characteristics for first 19 hours diffusion difference advection difference ~400 m separation after ~18 hours

17. Preliminary analysis of data: Ocean turbulence, u’(x,y,t), complicates comparative analyses. USCG SLDMBs Microstar drifters ending positions starting positions

18. Preliminary analysis of data: A: SLDMBs move ~3 – 4 cm/s “differently”. Need to understand why? similar diffusion characteristics diffusion difference advection difference ~8000 m separation after ~55 hours

19. Preliminary analysis of data: Q: How well do drifter and HF radar observations agree? 7 HF radar radial cells 20 drifter tracks Need to compute time-space averages from drifter clusters for HF radar ground truth.

20. Preliminary analysis of data: Q: How well do drifter and HF radar observations agree? 14 HF radar radial cells 20 drifter tracks Need to compute time-space averages from drifter clusters for HF radar ground truth.

21. Preliminary analysis of data: Q: How well do drifter and HF radar observations agree? HF radar velocities show large variance on few km space scales > 70 cm/s range

22. Preliminary analysis of data: Q: How well do drifter and HF radar observations agree? HF radar velocities show large variance on few km space scales > 40 cm/s range

23. Preliminary analysis of data: Looking at a single radial cell comparison. > 25 cm/s difference between drifter and HF radar derived surface velocities

24. Preliminary analysis of data: Looking at a single radial cell comparison. > 40 cm/s difference between drifter and HF radar derived surface velocities drifter and HF radar velocities agree to within a few cm/s

25. Summary: Year 1 accomplishments • Successful field experiment. 12 drifters were used to sample 54 drifter tracks, only 1 drifter lost • First set of coincident SLDMB and drifter observations • Observations for evaluating HF radar surface currents Year 2 workplan • SLDMB performance analysis with wind data • HF radar ground truth analysis • Benchmark for ROMS simulations • Quantify parameters for a PWS Lagrangian Stochastic Model

27. Mean Dispersion Values: 1000 m D2(t) = exp(At) ; A-1 = 60 min ; r2 = 0.91 100 m 10 m exponential growth during first 4 hours

28. Definitions: Relative Dispersion •Spread (or variance) of a set of particles relative to coordinate frame fixed to the cloud’s center of mass (“two particle” statistics) Eddy Diffusivity • Time rate of change of dispersion