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Ocean Surface Current Observations in PWS Carter Ohlmann

Ocean Surface Current Observations in PWS Carter Ohlmann Institute for Computational Earth System Science, University of California, Santa Barbara, CA 93106. ROMS-based dispersal simulation. Lagrangian PDFs are calculated for 1 – 14 day advection times.

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Ocean Surface Current Observations in PWS Carter Ohlmann

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  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

  26. 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

  27. 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

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