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Form drag atThree Tree Point: What does the work?
Sally WarnerParker MacCreadySchool of Oceanography, University of Washington
Jim MoumCOAS, Oregon State University
February 23, 2010 Ocean Sciences Conference, Portland, OR
Form drag and mixing
U0
Form dragpressure
Tidalenergyconversion
Form drag causes:
- internal wave generation
- eddy generation
- local mixing and dissipation
Motivation for this study
Seattle
Tacoma
Edwards et al., 2004
Puget Sound, WA
PointThree Tree
Previous work
McCabe et al., 2006
> Measured the internal form drag
> Measured the external form drag
Our goal> Measure the total form drag with bottom pressure sensors
Oscillatory dynamics
tidal energy convertedto internal waves, eddies and mixing
no tidally averaged work done on system slack tide: background tilt
flood & ebb tides: waves and eddies
Warner and MacCready, 2009
0
-108
108
N
0
-0.4
0.4
m/s
0
-2x107
2x107
W
-4x107
0
-107
W
lunar hours
Velocity
Form drag
Power
Cumulative power
floodslack
0 3 6 9 12 15 18 21 24
– drag due to tilt
= residual drag
total form drag
Cruise at TTP in Dec. 2009
Seattle
Tacoma
Puget Sound, WA
PointThree Tree
Pressure sensors (PPODs) at Three Tree Point (TTP)
PPODs
PPOD pressure signals
12/03 12/05 12/07 12/09−6
−4
−2
0
2
4
psi
PPOD signals with mean from a common time period removed
TTP01TTPANTTP02TTP03TTP04TTP05TTP06TTP07TTP08TTPASTTP09TTPHS
12/03 12/05 12/07 12/09−0.14
−0.12
−0.1
−0.08
−0.06
−0.04
−0.02
0
psi
highpass filtered PPOD data
TTP01TTPANTTP02TTP03TTP04TTP05TTP06TTP07TTP08TTPASTTP09TTPHS
PPOD locations Raw PPOD data PPOD data with means removed
−4.4
−4.38
−4.36
−4.34
−4.32
−4.3
−4.28
−4.26
−4.24
−4.22
psi
PPOD signals with mean from a common time period removed
TTP01TTPANTTP02TTP03TTP04TTP05TTP06TTP07TTP08TTPASTTP09TTPHS
12/03 12/04 12/05 12/06 12/07 12/08 12/09 12/10−0.14
−0.12
−0.1
−0.08
−0.06
−0.04
−0.02
0
psi
highpass filtered PPOD data
TTP01TTPANTTP02TTP03TTP04TTP05TTP06TTP07TTP08TTPASTTP09TTPHS
12/03 12/04 12/05 12/06 12/07 12/08 12/09 12/1050
100
150
200
250
300
350
GMT 2009
psi
TTP01TTPANTTP02TTP03TTP04TTP05TTP06TTP07TTP08TTPASTTP09TTPHS
−0.2
−0.1
0
0.1
0.2
ps_t
ides
cur
rent
flood
m
/s
e
bb
−101234
ps_t
ides
hei
ght
m
12/03 12/04 12/05 12/06 12/07 12/08 12/09 12/10−4
−2
0
2
4
GMT 2009
back
grou
nd s
urfa
ce s
lope
mm
hei
ght /
km
dis
tanc
eBackground sea surface slope
flood
ebb
tidalcurrent[m/s]
seasurfaceslope[mm/km]
Sea surface slope from PPODs
Estimated background sea surface slope based on a tidal prediction model of Puget Sound (in quadrature with velocity)
PPOD pressure difference between North and South gives background slopetidal
height[m]
12/03 12/04 12/05 12/06 12/07 12/08 12/09 12/10−0.05
−0.04
−0.03
−0.02
−0.01
0
0.01
0.02
0.03
0.04
0.05
GMT 2009
m
Calculating the residual pressure
North
South
12/03 12/05 12/07 12/09−6
−4
−2
0
2
4
psi
PPOD signals with mean from a common time period removed
TTP01TTPANTTP02TTP03TTP04TTP05TTP06TTP07TTP08TTPASTTP09TTPHS
12/03 12/05 12/07 12/09−0.14
−0.12
−0.1
−0.08
−0.06
−0.04
−0.02
0
psi
highpass filtered PPOD data
TTP01TTPANTTP02TTP03TTP04TTP05TTP06TTP07TTP08TTPASTTP09TTPHS
PPOD data with means removed
minus average tidal signal
Residual pressure signals1
2
3
4
minus background tilt
−0.2
−0.1
0
0.1
0.2
ps_t
ides
cur
rent
flood
m
/s
e
bb
−101234
ps_t
ides
hei
ght
m
12/03 12/04 12/05 12/06 12/07 12/08 12/09 12/10−4
−2
0
2
4
GMT 2009
back
grou
nd s
urfa
ce s
lope
mm
hei
ght /
km
dis
tanc
eseasurfaceslope mm km[ ]
−2
−1
0
1
2
N/m
12/05 12/06 12/07 12/08 12/09 12/10−500
0500
100015002000
GMT 2009
W/m
Residual dragResidual pressures
Residual form drag and wave drag
}
W/m
N/m
x 104
}
}1 cmof SSH
N
S
Tidal conversion rate
−250
−200
−150
−100
−50
m
−3000 −2000 −1000 0 1000 2000 3000−0.4
−0.2
0
0.2
0.4
m/m
meters from tip of TTP
topographicheight
topographicslope
N
N
S
S
}}
}
Running mean: ~300 W/m
Bottom pressure sensors can be used to measure form drag on topography in the ocean.
Along our transect line, the residual drag estimated by the PPODs compares well to a wave drag parameterization.
Conclusions>
>
Warner, S. J. and P. MacCready. (2009) “Dissecting the pressure field in tidal flow past a headland: When is form drag “real”?” J. Phys. Oceanog. 39: 2971-2984.
Thank you
First directmeasurements of
topographic form drag. U. Stöber, J. N. Moum,
J. D. Nash, P. MacCready(PO35L)
Wed
nesday poster session