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Ocean Surface Current Observations in PWS
Carter Ohlmann
Institute for Computational Earth System Science, University of California, Santa Barbara, CA 93106
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ROMS-based dispersal simulationROMS-based dispersal simulation
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
Lagrangian PDFs are calculated for 1 – 14 day advection times
PDFs = probability density functions
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Drifter dataDrifter data (CODE 1 meter; MMS SBC-SMB (CODE 1 meter; MMS SBC-SMB study)study)
SCB drifter data on the regional scale
Drifters deployed ~ quarterly from 1993 – 1999. 568 drifters sampling for an average of ~24 days give ~13,500 drifter days of data.
Drifter dispersal from a single site
Red circle: “release” site
Blue dots: drifter locations for a give advection time
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Lagrangian PDF vs Drifter Lagrangian PDF vs Drifter DistributionDistribution
Drifter locationsDrifter locations
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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)
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24 hrs
1000 m
100 m
10 m
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.
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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
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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
Ohlmann et al. 2005, and Ohlmann et al. 2007
www.drifterdata.com
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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
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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
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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
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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
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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).
PWS HF radar locations
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PWS HF radar surface current map – spatial extent of coverage is highly variable.
PWS HF radar locations
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starting positions
ending positions
USCG SLDMBsMicrostar drifters
Preliminary analysis of data:
Q: What can be learned of SLDMB water-following capabilities?
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Preliminary analysis of data:
A: SLDMBs move ~1.0 cm/s slower.
~400 m separation after ~18 hours
advection difference
diffusion differencesimilar diffusion characteristics for first 19 hours
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Preliminary analysis of data:
Ocean turbulence, u’(x,y,t), complicates comparative analyses.
starting positions
ending positions
USCG SLDMBsMicrostar drifters
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Preliminary analysis of data:
A: SLDMBs move ~3 – 4 cm/s “differently”. Need to understand why?
~8000 m separation after ~55 hours
advection difference
diffusion difference
similar diffusion characteristics
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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.
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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.
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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
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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
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Preliminary analysis of data:
Looking at a single radial cell comparison.
> 25 cm/s difference between drifter and HF radar derived surface velocities
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Preliminary analysis of data:
Looking at a single radial cell comparison.
drifter and HF radar velocities agree to within a few cm/s
> 40 cm/s difference between drifter and HF radar derived surface velocities
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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
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exponential growth during first 4 hours
Mean Dispersion Values:
D2(t) = exp(At) ; A-1 = 60 min ; r2 = 0.911000 m
100 m
10 m
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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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