HYCOM and the need for overflowentrainment parameterizations

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HYCOM and the need for overflow/entrainment
parameterizations
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Overflow Representation in Numerical Models

• Strongly dependent on the choice of the vertical
  coordinate
• In fixed coordinate models (z and s), the
  numerically induced entrainment (i.e. mixing) is
  larger than observed gt no need for
  parameterization, the focus is on reducing the
  numerically-induced mixing to below observations
  (DYNAMO, Griffies et al. (2000), )
• In density coordinate models, the densest fluid
  will sink to the bottom gt need for an
  entrainment parameterization

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Entrainment parameterization

• The specific parameterization will depend on the
  resolution large difference between coarse
  climate models (horizontal resolution of 1º or
  lower ) and truly eddy-resolving models
  (horizontal resolution of 1/10º). Different
  viscosity gt different modeled Ri/Fr numbers
• How do we address this?

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HYCOM

• Investigation in HYCOM 2º global configuration
  for the NCAR CCSM and OMIP
• Investigation in the 1/12º North Atlantic Ocean
  prediction system (U.S. GODAE)
• What are the targets? One needs to define the
  metrics.

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U.S. GODAE Global Ocean Prediction with the
Hybrid Coordinate Ocean Model (HYCOM)
Representation of the Mediterranean Outflow and
Dennmark Straits Overflow
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Objectives and Goals

• A broad partnership of institutions that will
  collaborate in developing and demonstrating the
  performance and application of eddy-resolving,
  real-time global and basin-scale ocean prediction
  systems using HYCOM
• To be transitioned for operational use

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Opportunities

• Global high resolution (1/12º) model outputs
  available to the community at large
• Strong participation of the coastal ocean
  modeling community in using and evaluating
  boundary conditions from the global and
  basin-scale ocean modeling prediction systems

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HYCOM

• HYCOM is the result of a very effective
  collaboration between the U. of Miami,
  NRL/Stennis, and the Los Alamos National
  Laboratory.
• HYCOM has been configured globally (up to 3/4º
  60km mid-latitude resolution) and basin-scale
  (up to 1/12º 7km mid-latitude resolution North
  Atlantic and Pacific)

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The hybrid coordinate is one that is isopycnal in
the open, stratified ocean, but smoothly reverts
to a terrain-following coordinate in shallow
coastal regions, and to pressure coordinate in
the mixed layer and/or unstratified seas.
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1/25 HYCOM East Asian Seas Model
Nested inside 1/6 HYCOM Pacific Basin Model
Boundary conditions via one-way nesting and 6
hrly ECMWF 10 m atmospheric forcing
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1/25 East Asian Seas HYCOM (nested inside 1/6
Pacific HYCOM)
North-south velocity cross-section along 124.5E,
upper 400 m
redeastward flow
bluewestward flow
density front associated with sharp topographic
feature (cannot be resolved with fixed
coordinates)
Yellow Sea flow reversal with depth
Isopycnals over shelf region
Snapshot on 14 October
East China Sea
Yellow Sea
z-levels and sigma-levels over shelf and in mixed
layer
Snapshot on 12 April
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Configuration of the Prediction Systems

• Basin-scale (NRL/Miami and NOAA)

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Configuration of the Prediction Systems

• Global
• Sea Ice Options
• Energy loan
• 4-layer thermodynamic (Russel et al., 2000)
• Los Alamos CICE
• Target
• 1/12º for NAVOCEANO
• 1/4º (20 km) for FNMOC (ocean component of
  coupled ocean-atmosphere)

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Denmark Straits Overflow Along 31W
Colder fresher water forms over the shelf in the
Nordic Seas and spills over the Denmark Strait
Temperature
and entrains more saline Irminger Sea water
Salinity
Results from 1/12 Atlantic HYCOM
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Results from 1/12 Atlantic HYCOM
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Results from 1/12 Atlantic HYCOM
More on Dennmark Straits plans in Xiaobiao Xus
presentation
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Representation of the Mediterranean Outflow in
MICOM and HYCOM

• Entrainment parameterization in MICOM gt
  Papadakis et al. (2003)
• Outflow representation in MICOM and HYCOM
• Entrainment parameterization in HYCOM vs. MICOM
• Discussion

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Entrainment Parameterization in MICOM (Papadakis
et al., 2003)

• Based in the implicit diapycnal mixing scheme of
  Hallberg (2000)
• Uses a Richardson number dependent entrainment
  parameterization of Turner (1986) based on
  laboratory experiments
• Evaluated in a realistic MICOM regional model
  (1/12º grid) with imposed boundary conditions at
  the Strait of Gibraltar

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Domain Configuration
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Boundary Conditions at the Strait of Gibraltar
Inflow/outflow .8 Sv
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Sensitivity to Entrainment Parameterization
(Turner, 1986)
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Outflow Representation
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Outflow Representation
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Sensitivity to Mixing Frequency
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Implementation in 1/12º North Atlantic MICOM
Layer 10 thickness 1979-1986
Salinity Layer 10 1979-1986
Meddies dissipate quickly, outflow too shallow
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MICOM 1/12º
Transport .7 Sv
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MICOM 1/12º
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HYCOM 1/12º
Transport 1 Sv
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HYCOM 1/12º
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KPP vs. Turner (1986)

• The K-Profile parameterization (Large et al.,
  1994) is widely used in ocean models
• KPP is derived from observations while Turner
  (1986) is primarily derived from laboratory
  experiments
• There are non-oceanic aspect aspects in Turner
  (1986), i.e. lack of rotation,
• KPP is however a broad representation of the
  processes and may not be very relevant to
  overflows

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Present Strategy

• Evaluation of KPP and Turner (1986) possibly
  other parameterizations as they are developed gt
  CPTs on overflows
• Idealized configurations
• Comparison to 2D non-hydrostatic configurations
  (i.e. Ozgokmen and Chassignet, 2002)
• Document the sensitivity to numerical choices
  such as stratification, mixing frequency,..