Capes and form drag: the role of stratification Marcello G. Magaldi 1 , Tamay M. Özgökmen 1 , Annalisa Griffa 1 , Eric P. Chassignet 2 , Hartmut Peters 1 and Mohamed Iskandarani 1 1 RSMAS, University of Miami 2 COAPS, Florida State University 10/02/2007
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Capes and form drag: the role of stratification Marcello G. Magaldi 1, Tamay M. Özgökmen 1, Annalisa Griffa 1, Eric P. Chassignet 2, Hartmut Peters 1 and.
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Capes and form drag: the role of stratification
Marcello G. Magaldi1, Tamay M. Özgökmen1, Annalisa Griffa1, Eric P. Chassignet2, Hartmut Peters1 and Mohamed Iskandarani1
1 RSMAS, University of Miami2 COAPS, Florida State University
10/02/2007
Outline
• Motivation and open questions
• Numerical setup
• Results and form drag
• Conclusions and future work
10/02/2007
Why studying capes?
10/02/2007
One of many capes in Puget Sound, (WA)
A case of current separation?
1. Current separation and eddy formation (Signell and Geyer, 1991)
2. Lee wave generation (MacCready and Pawlak, 2001)
3. Vortex tilting and stretching (Farmer et al., 2002)
4. Secondary circulations (Geyer, 1993)
5. Upwelling (Alee et al., 2004)
6. High values of dissipation and mixing (Pawlak et al., 2003)
• Movie for a no sloping case
Eddy generation 10/02/2007
CyclonesPositive vorticity
AnticyclonesNegative vorticity
Red conesFlow visualization
No slope
Bu = 0.10
τ = 25.92
Eddy generation 10/02/2007
No slope
Bu = 0.10
Dissipation dilemma 10/02/2007
(Lavelle et al, 1988)
More than the 70% of the whole energy
loss
Bottom drag coefficients are
210 times larger than usual in order to match observations
(Lavelle et al, 1988; Foreman et al, 1995)
Eddy generation 10/02/2007
Re = 28.4
Van Dyke, 1982
Re = 0.16
Re = 140
We know the role of lateral friction...
Dong et al., 2007
Re*
Eddy generation 10/02/2007
We know the role of bottom friction...
Doglioli, A. M., Griffa, A., Magaldi, M. G., 2004. JGR.
Increasing Ref
Main controlling parameter
Ref = H / CD D
Equivalent Reynolds Number
Advection / bottom friction
Open questions 10/02/2007
...but what is the role of stratification in the generation of eddies behind capes?
Only three papers:
• Boyer and Tao, 1987; big aspect ratio (1:1), not realistic (ocean 1:10 is steep)
• Davies et al., 1990;
• MacCready and Pawlak, 2001No rotation
Aims of the study
• Identify how stratification affects the eddy regime
• Understand how this translates in terms of dissipation (and mixing)
10/02/2007
Numerical setup 10/02/2007
Boyer and Tao, 1987
Initial condition:
fluid in motion, linearly stratified
(U= - 0.078 m/s, V= 0.0 m/s, η = geos.)
U
Open BCs:
M2 Flather, Chapman, M3 and Tracer radiation relaxation on tracers to the initial value on 6 pts
1. We are able to reproduce 2 of the 3 regimes by Boyer and Tao (1987). We are not able to reproduce the eddy attached regime (non-hydrostatic implications, top lid?)
2. The eddy shedding regime is enhanced by larger Burger numbers more horizontal flow, more tendency of separation stronger and more coherent vortices
3. Flow regimes do not significantly change for gentler and more realistic slopes (less inertial for bottom friction!)
4. The form drag coefficients change with Bu and increase significantly for gentler slopes (equal increase keeping a constant slope)
Future work 10/02/2007
• Quantification of mixing (energetic approach,
open boundary conditions)
• Application to the Gargano Cape
The Gargano Cape 10/02/2007
MODIS-Aqua Chlorophyll ConcentrationDaily average
NCOM surface velocity
Courtesy of Dr. Angelique Haza
Dynamics of the Adriatic in Real-Time (DART project)