Ocean Surface Current Observations in PWS Carter Ohlmann Institute for Computational Earth System...
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Transcript of Ocean Surface Current Observations in PWS Carter Ohlmann Institute for Computational Earth System...
Ocean Surface Current Observations in PWS
Carter Ohlmann
Institute for Computational Earth System Science, University of California, Santa Barbara, CA 93106
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
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
Lagrangian PDF vs Drifter Lagrangian PDF vs Drifter DistributionDistribution
Drifter locationsDrifter locations
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)
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.
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
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
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
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
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
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
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
PWS HF radar surface current map – spatial extent of coverage is highly variable.
PWS HF radar locations
starting positions
ending positions
USCG SLDMBsMicrostar drifters
Preliminary analysis of data:
Q: What can be learned of SLDMB water-following capabilities?
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
Preliminary analysis of data:
Ocean turbulence, u’(x,y,t), complicates comparative analyses.
starting positions
ending positions
USCG SLDMBsMicrostar drifters
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
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.
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.
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
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
Preliminary analysis of data:
Looking at a single radial cell comparison.
> 25 cm/s difference between drifter and HF radar derived surface velocities
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
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
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
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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