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The Gravity Current Entrainment Climate Process Team. Sonya Legg Princeton University, NOAA-GFDL. Accomplishments.

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the gravity current entrainment climate process team

The Gravity Current Entrainment Climate Process Team

Sonya Legg

Princeton University, NOAA-GFDL

  • New and improved shear mixing parameterizations have been developed and implemented in HYCOM and HIM. Jackson and Hallberg parameterization (in HIM) is also suitable for z-coordinate models, and appears to parameterize shear-driven mixing in equatorial undercurrent as well as in overflows.
  • Parameterization of frictional bottom boundary mixing in overflows: implementation in Hallberg Isopycnal Model has made a 1st order difference to its credibility as a climate ocean model at 1 degree.
  • The Marginal Sea Boundary Condition has been implemented for Med Sea outflow in climate models, NCAR POP and HYCOM: Med Sea Outflow can now be represented credibly at coarse resolution. Implementation in NCAR POP for Nordic overflows is underway and improvements are being explored.
  • Partially open barrier method: identified as a promising method for representing narrow straits.
  • Reduction in spurious mixing in z-coordinate models: use of new advection schemes is being explored.

All of these developments have involved input from observations, idealized and regional numerical simulations, in addition to GCMs.

  • 1.Table of observations
  • Compiled by team observationalists, lead by Arnold Gordon.
  • Quick reference for:
  • Parameters needed in GCM overflow representations
  • Intercomparison of observed overflow characteristics
  • Comparison with regional and climate simulations.

2. New parameterizations and code implemented in climate models

3. Benchmark calculations

Idealized and high-res regional calculations against which to compare coarse res models. Outside groups have already been comparing their simulations with these calculations.

  • 4. Publications
  • Numerous individual publications
  • 2 group publications in USCLIVAR Variations
  • 1 synthesis article in preparation for BAMS
  • 1 review article in AGU monograph

5. Community

Collaboration between the CPT observationalists, process modelers and GCM developers will likely continue long after the CPT ends.

challenges and lessons learned
Challenges and lessons learned
  • CPTs place considerable demand on modeling centers: Transforming understanding into parameterizations requires an inside knowledge of the model code, and can’t always be done by a postdoc. CPTs will only succeed if one or more people at the modeling center(s) are committed and excited by the project. Modeling centers also have other obligations – e.g. IPCC deadlines to meet.
  • A successful CPT has parameterization ideas ready to go from the start (new ones may also arise along the way).
  • It is beneficial to simultaneously explore several different approaches to parameterizing the same problem, not putting all hopes on one solution.
  • Sometimes the effort required to parameterize a process in an existing code might be less than the effort to build a new code with more appropriate formulation for that process.
  • One committed person has to take responsibility for the coordination, synthesis, motivation.
representing overflows in climate models

Hydraulic control


Shear instability, entrainment



Geostrophic eddies

Bottom friction

Downslope descent


Physical processes in overflows

Representing overflows in climate models

Dense water masses formed in marginal seas enter open ocean through overflows, e.g. Denmark Straits, Faroe Bank Channel, Antarctic slope overflows.

Mixing and transport in overflows determines properties of ocean bottom waters, e.g. NADW, AABW.

  • Ocean models need to represent overflow processes correctly to get dense water properties right.
  • Z-coordinate models have difficulties getting water downslope without excessive mixing at coarse climate resolution.
  • Isopycnal models need to parameterize diapycnal mixing.
  • Narrow straits are an issue for all coarse resolution models.
how do we improve ocean model overflow representation

Laboratory and numerical process studies



Improved parameterizations



Simulations in idealized configurations

Regional simulations

Global climate simulations

How do we improve ocean model overflow representation?

The Gravity Current Entrainment Climate Process Team: a multi-institutional collaboration between those studying processes in detail and those building and running climate models.

A US CLIVAR project funded by NSF and NOAA, 2003-2008.

cpt gce participants models numerical and lab process studies observations
CPT-GCE participants: Models, Numerical and Lab process studies, Observations

Core PIs

Stephen Griffies (GFDL), Robert Hallberg (GFDL),William Large (NCAR), Gokhan Danabasoglu (NCAR), Peter Gent (NCAR), Jim Price (WHOI), Jiayan Yang (WHOI), Sonya Legg (WHOI/Princeton), Hartmut Peters (Miami), Eric Chassignet (Miami), Tamay Ozgokmen (Miami), Tal Ezer (Princeton), Arnold Gordon (Columbia), Paul Schopf (GMU)


Ulrike Riemenschneider (WHOI), Laura Jackson (GFDL), Yeon Chang (Miami), Wanli Wu (NCAR)

For more details see

focus on frictional bottom boundary mixing

Legg et al., 2005

Focus on Frictional bottom boundary mixing

With thick plumes both interfacial shear mixing and drag-induced near bottom mixing are needed. (Legg, Hallberg and Girton,2006).

Interior Ri# Mixing Only


10 km

Actively mixing

Interfacial Layer

Shear Ri# Param.

appropriate here.

Interior Ri# + Drag Mixing


10 km

Well-mixed Bottom Boundary Layer

Mixing driven by bottom stresses

Resolved mixing (LES)


500m x 30m

Observed profiles from Red Sea plume from RedSOX

(Hartmut Peters)


Impact of frictional bottom boundary mixing on GCM results

Mediterranean outflow salinity: comparison between 1 degree Hallberg Isopycnal Model and observed climatology.

Spurious bottom plume

New parameterization eliminates spurious bottom plume


Focus on the Marginal Sea Boundary Condition

Parameterizes both narrow straits and entrainment, suitable for both isopycnal and z-coord models. Developed by Yang and Price, implemented in NCAR POP for Med Sea and Faroe Bank Channel, and in HYCOM for Med Sea.





(Ent. density based on 12 grid points)


(Source density based on 12 grid points)



g’ = g (ρS-ρO)/ρref




QS = g’ (hS)2 / 2f

NCAR MSBC implementation for FBC

QP = QSFr2/3

QP = QS + QE


Heat and salt conservation




team interactions involved in msbc implementation
Team interactions involved in MSBC implementation
  • Parameterization originally developed by Yang and Price, who continue to consult on implementation issues.
  • Numerous parameters must be specified: values are taken from Table of observations, and/or from regional simulations (Ulrike Riemenschneider, FBC).
  • Whole team provides input on scenarios for examining impact in GCMs.
  • Yang and Price continue to improve/update MSBC, with new developments to be ported to GCMs when ready.
results impact of msbc in gcm simulations
Results: Impact of MSBC in GCM simulations

Salinity at 1100m depth: comparison between climatology and NCAR 3 degree simulations for Med outflow. MSBC leads to credible Med salt tongue.

cpt plans for years 4 and 5 include
CPT plans for years 4 and 5 include:
  • Complete implementation of MSBC in NCAR POP and HYCOM, for all climatologically important overflows.
  • Complete new shear-driven mixing parameterizations, implement in HIM, MOM4 and other models.
  • Complete implementation of partially open barriers in global models and compare with other methods of representing narrow straits.
  • Focus on impact of parameterizations on Nordic overflows and Antarctic overflows.
  • Examine impact of parameterizations on global climate simulations, including climate change scenarios, Greenland ice-melt scenarios.
results table of observations
Results:Table of observations
  • Purpose: Quick reference for:
  • Parameters needed in GCM overflow representations
  • Intercomparison of observed overflow characteristics
  • Comparison with regional and climate simulations.
  • Effort led by Arnold Gordon.

Abbreviated version: for full version see

results improved parameterizations of shear driven mixing
Results: Improved parameterizations of shear-driven mixing.

Calibration of entrainment parameterization by comparison with nonhydrostatic benchmark calculations (Miami CPT members)

Calibrated parameterization is validated by comparison between regional simulations and observations. Example: Mediterranean outflow simulated in 0.08 degree HYCOM regional implementation (Xu et al, 2006).

Downstream evolution of entrainment in HYCOM simulations with different entrainment parameterizations compared to nonhydrostatic (Nek) benchmark calculations.

with E0= 0.2 and Ric=0.25 (Xu et al, 2006)

results improved parameterizations of shear driven mixing ii
Results: Improved parameterizations of shear-driven mixing II

Jackson and Hallberg (GFDL) are developing a new parameterization of shear-driven mixing for both isopycnal and z-coord models, with a parameterized diffusivity of the form:

where S is the vertical shear of the resolved horizontal velocity

is the buoyancy length scale (the scale of the overturns), N is the buoyancy frequency, and Q is the turbulent kinetic energy, found from an energy budget.

F(Ri) is a function of shear Richardson number Ri such as

Initial comparison with DNS of shear layers and jets looks promising.

Validation with LES is continuing.

DNS data

ET parameterisation

New parameterisation

F0 = 0.14, cN = 0.41, cS = 0.10,  = 0.6

F0 = 0.11, cN = 0.20, cS = 0.10,  = 0.7

F0 = 0.12, cN = 1.87, cS = 0.10,  = 0.9

results impact of shear driven mixing parameterization on climate
Results: Impact of shear-driven mixing parameterization on climate

In coupled simulations using Hallberg Isopycnal model, with entrainment in Nordic overflows SSTs are warmer near entrainment site, and cooler to south, due to change in location of Gulf Stream induced by DWBC transport changes.

results treatment of narrow straits
Results: Treatment of narrow straits

Regional simulations of Red Sea (Chang et al, 2006), Mediterranean (Xu et al, 2006) and Faroe Bank Channel (Riemenschneider and Legg, 2006) show results are more sensitive to resolution of topography than to mixing parameterization.

Representation of channels below grid scale by thin walls and partially open barriers is under development by Adcroft and Hallberg (GFDL).

This technique improves Mediterranean outflow simulations in Hallberg Isopycnal model at 1 degree (110km) resolution by reducing Gibraltar width to 12km.

Salinity in HIM global simulations and observations

results reducing spurious mixing in z coordinate models
Results: reducing spurious mixing in z-coordinate models
  • Idealized simulations (Legg et al, 2006) and regional simulations (Riemenschneider and Legg, 2006) show that z-coordinate models produce too much numerical mixing, principally due to advection schemes.
  • Efforts to reduce spurious mixing in MOM4 (Griffies, GFDL) include:
  • New, less diffusive advection schemes
  • Improved sigma-diffusion schemes (Beckmann and Doescher, 1997).

Difference between CM2.1 simulations with and without sigma-diffusion at 100m in Med outflow region.

Promising recent development includes non-local horizontal communication in sigma-diffusion.