on the dynamics of oceanic gravity currents · the vertical resolution is key to correctly...
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![Page 1: On the Dynamics of Oceanic Gravity Currents · The vertical resolution is key to correctly represent oceanic gravity currents. A few σ (< 6) levels in the bottom layer are sufficient](https://reader034.vdocuments.net/reader034/viewer/2022052000/60123fb9376c937a160c7272/html5/thumbnails/1.jpg)
On the Dynamics of Oceanic GravityCurrents
Achim Wirth
LEGI / CNRS
LEGOS, Dec 6, 2011
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Ocean Circulation
Gyre“weather”
Overturning“climat”
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Ocean Dynamics by Scale
-k
energy injection
(107m)−1 (105m)−1 (10m)−1 (10−2m)−1
energy dissip
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Ocean Dynamics by Scale
-k
energy injection
(107m)−1
inverse energy cascade
(105m)−1
bc/bt inst.
geostrophy QG turbulence ? ? ? ? ? strat. rot. turb.
(10m)−1
turb. 3D
(10−2m)−1
energy dissip
? ? ? ? ? ? direct energy cascade
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Ocean Dynamics by Scale
-k
energy injection
(107m)−1
inverse energy cascade
(105m)−1
bc/bt inst.
geostrophy QG turbulence ? ? ? ? ? strat. rot. turb.
(10m)−1
turb. 3D
(10−2m)−1
energy dissip
? ? ? ? ? ? direct energy cascade
geophys. fluid dyn. turbulence
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Ocean Dynamics by Scale
-k
energy injection
(107m)−1
inverse energy cascade
(105m)−1
bc/bt inst.
geostrophy QG turbulence ? ? ? ? ? strat. rot. turb.
(10m)−1
turb. 3D
(10−2m)−1
energy dissip
? ? ? ? ? ? direct energy cascade
convectiongravity currentinteract topographyfrontfilamentint. waves..........
geophys. fluid dyn. turbulence
small scale processes
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Ocean Dynamics by Scale
-k
energy injection
(107m)−1
inverse energy cascade
(105m)−1
bc/bt inst.
geostrophy QG turbulence ? ? ? ? ? strat. rot. turb.
(10m)−1
turb. 3D
(10−2m)−1
energy dissip
? ? ? ? ? ? direct energy cascade
convectiongravity currentinteract topographyfrontfilamentint. waves..........
geophys. fluid dyn. turbulence
small scale processes
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Gravity Current
••
•• •
•
••
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Geostrophy
������α
��
��
��
��
��
��
���
�� F ′
g
F ′
c
������������������������������
-x
α
⊗-
?
���9���:
Fg
Fc
F ′
gF ′
cF ′
c
u =g′
ftan α
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Forced Geostrophy
vein
friction layer
Ekman layer
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Forced Geostrophie
veine
friction layer
Ekman layer
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Forced Geostrophy
vein
friction layer
Ekman layer
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Heat Equation
∂th = −∂xUEk =δ
2∂xvgeo =
δg′
2f∂xxh = ∂x (κH∂xh)
24h 60h
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Friction... determines the dynamics of oceanic gravity currents.
The frictional processes can not be explicitely representedin today’s (and tomorrow’s) ocean models
τ + cD|u|
linear Rayleigh friction (τ ) quadratic drag law (cD)
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Initial Conditions
Temp Vg
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Reference Exp. (2D)
Z σ
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Grid σ
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Convective Adjustment and Classic (2D)
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2+4+3 Levels (2D)
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Down-slope transport
2.5D 3D
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Conclusions A
◮ The vertical resolution is key to correctly represent oceanicgravity currents.
◮ A few σ (< 6) levels in the bottom layer are sufficient.
◮ The refinement of the vertical resoltuion at the bottom ismore important than at the surface.
◮ These results are NOT restricted to gravity currents.
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How do we find REAL bottom-friction
◮ Optimist : Study non-hydorstatic PBL-dynamics.◮ Pessimist : Use data-assimilation to determine friction
parameters from obs. (that we do not have).
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Non-hydrostatic simulation :
HAROMOD
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Coherent Structures
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Coherent Structures
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Coherent Structures
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?roughness of the ocean floor ?
variability of roughness ?
multiscale roughness (bio) ?
roughness type “k” vrs. “d” ?
orientation of roughness elements ?
suspension of sediments ?
tidal currents ?
waves ?
retroaction of currents on roughness ?
And : “The matter is far from beeing understood” Jiménez, Ann.Rev. Fluid Mech. (2004).
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Friction... determines the dynamics of oceanic gravity currents.
The frictional processes can not be explicitely representedin today’s (and tomorrow’s) ocean models
τ + cD|u|
linear Rayleigh friction (τ ) quadratic drag law (cD)
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Data Assimilation :Estimation of parameters and friction laws :detetction of transition from linear to quadratic law.
0 500 1000 1500 2000 2500Re
0
5
10
15
C_D
(10
^-4)
c̃D = cD +τ
|u|
cD
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Conclusions
Convection
Gravity Current
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Conclusions & Perspectives
Convection
Gravity CurrentDWBC
?m
ixin
g?
mixed layer, air-sea interaction