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