HOME: A Community Ocean Circulation Modeling Environment with Generalized Vertical Coordinates

1. The Hybrid Ocean Modeling Environment (HOME)A vision for Community Ocean Circulation Models:Generalized Vertical Coordinates Presented on behalf of the HOME Team: R. Hallberg, R. Bleck, E. Chassignet, R. deSzoeke, S. Griffies, P. Schopf, S. Springer and A. Wallcraft

2. What is a “Modeling Environment”? A “Model”: • A specific collection of algorithms – e.g. MICOM v2.8 - or - • A specific configuration, including parameter settings, geometry, forcing fields, etc. – e.g. The 1/12° North Atlantic MICOM model A “Modeling Environment”: • Uniform code comprising a diverse collection of interchangeable algorithms and supporting software from which a model can be selected.

3. -Rotating and stratified fluids=>dominance of • lateral over vertical transport. • -Hence, it is traditional in ocean modeling to orient • the two horizontal coordinates orthogonal to the • local vertical direction as determined by gravity. • -The choice of the vertical coordinate system is the • single most important aspect of an ocean model's • design (DYNAMO, DAMÉE-NAB). • -The practical issues of representation and • parameterization are often directly linked to the • vertical coordinate choice(Griffies et al., 2000).

4. Currently, there arethree main vertical coordinates in use, none of which provides universal utility. Hence, many developers have been motivated to pursue research intohybrid approaches.

5. Vertical Coordinates

6. σ-z z σ Hybrid

7. HYCOM The hybrid coordinate in HYCOM is isopycnalin the open stratified ocean, but smoothly reverts to a terrain-following coordinate in shallow coastal regions, and to a pressure coordinate in the mixed layer and/or unstratified seas.

8. Examples of HOME predecessor model applications: 1/25° East Asian Seas HYCOM (nested inside 1/6° Pacific) North-south velocity cross-section along 124.5°E, upper 400 m blue=westward flow red=eastward flow density front associated with sharp topographic feature (cannot be resolved with fixed coordinates) Isopycnals over shelf region Yellow Sea flow reversal with depth Snapshot on14 October z-levels and sigma-levels over shelf and in mixed layer East China Sea Snapshot on 12 April Yellow Sea

9. There is broad agreement among ocean modelers that generalized vertical coordinates are desirable.A large fraction of the U.S. ocean model development community will therefore participate in HOME development. • HOME Predecessor Models: • HIM (NOAA/GFDL) • HYCOM (U. Miami, Navy NRL, & DOE LANL) • HYPOP (DOE LANL) • Poseidon (NASA/GMAO & George Mason U.) • POSUM (Oregon State U.) • Contributing Models: • MITgcm (MIT) – A. Adcroft • MOM4 (NOAA/GFDL) – S. Griffies • ROMS (Rutgers U. & UCLA) – D. Haidvogel, J. McWilliams, A. Shchepetkin

10. Examples of HOME predecessor model applications:Studies of the role of resolution and eddies in climate variability

11. Examples of HOME predecessor model applications:Tropical Instability Waves in an ENSO forecast

12. Examples of HOME predecessor model applications:1/12° Pacific HYCOM with regional nesting Forced with high frequency ECMWF winds and thermal forcing SSH Snapshot – 21 March

13. Examples of HOME predecessor model applications:Overflows? (Papadakis et al or other?)

14. Examples of HOME predecessor model applications:Global climate simulations at NASA/GISS

15. The HOME white paper advocates the development of a versatile, open-source, community Ocean Modeling Environment using a generalized hybrid vertical coordinate. In addition to collecting the common code base, HOME would identify best practices for modeling key oceanic phenomena.

16. HOME would be a joint effort of several Federal Agencies (NOAA, Navy/NRL, NASA, DOE) and university groups, with a structure that invites contributions from the broader domestic and international community. HOME could be construed to partially address Rec. 28-2 of the U.S. Commission on Ocean Policy Report: “NOAA and the U.S. Navy should establish a joint ocean and coastal information management and communications program… [that] should create a research and development component…to generate new models and forecasts in collaboration with Ocean.IT, taking full advantage of the expertise found in academia and the private sector.”

17. Advantages of HOME: Community Cohesion • An organic trust-base already exists within the Lagrangian-vertical-coordinate ocean modeling community. • All major developers recognize the practical benefits from trading “code ownership” for “community modeling” • Reduced redundancy of efforts • Increased collaborations and intellectual cross-fertilization • Increased capabilities available to all researchers and applications • More rapid solution of common difficulties

18. Advantages of HOME: Ingenuity • More rapid model improvements • Combine existing capabilities of predecessor models to identify optimal configurations • Provide a target for new developments to be rapidly evaluated and transitioned to realistic ocean model applications. • Enable direct comparison of new techniques with existing in idealized and actual applications Biological metaphor: “Breed” better ocean models from a bigger “gene pool” of algorithms.

19. Advantages of HOME: Technology Components Layer: Gridded Components Coupler Components ESMF or PRISM Superstructure • Built upon ESMF & PRISM standards • Single code-base readily deployable to a wide variety of computer architectures • Facilitates long-term support & stability • ESMF adoption provides a window of opportunity when the transition to HOME will be less disruptive. Selected HOME Code Model Layer ESMF Infrastructure Fields and Grids Layer Low Level Utilities External Libraries BLAS, MPI, NetCDF, …

20. Advantages of HOME: Flexibility • Enable a single modeling system to be tailored for the demands of a particular application. • Lagrangian vertical coordinate solution techniques have proven adept at emulating all of the existing vertical coordinates in both atmospheric and oceanic applications (OR: Atmospheric and oceanic applications have shown the versatility of generalized coordinates.)

21. Advantages of HOME: Education • Students would learn about using ocean models with the same code-base as is widely used for real applications. • The HOME code will allow for easy-to-use, pedagogically interesting examples. • Code will be adaptable for a wide variety of student research topics.

22. Coastal applications of HOME • HOME predecessor models already work in coastal applications • HOME offers the prospect of moving from the blue-water into the coastal zones seamlessly. • Nesting, open boundary conditions, and data assimilation capabilities have been used successfully. • Key coastal model developers (J. McWilliams, D. Haidvogel, A. Shchepetkin) have agreed to collaborate in making world-class HOME-based coastal models.

23. Physical-Biogeochemical Model: Fei Chai Air-Sea Exchange Small Phytoplankton [P1] Micro- Zooplankton [Z1] Biological Uptake Total CO2 [TCO2] Grazing NO3 Uptake NH4 Uptake Predation Nitrate [NO3] Excretion Meso- Zooplankton [Z2] Ammonium [NH4] Fecal Pellet Advection & Mixing N-Uptake Fecal Pellet Grazing Lost Silicate [Si(OH)4] Detritus-N [DN] Detritus-Si [DSi] Si Uptake Diatoms [P2] Physical Model Sinking Sinking Sinking

24. HOME and Biogeochemical modeling Biogeochemical modeling places unique demands on ocean models: • Use lots of tracers • Typically evolve with slower timescales than internal gravity waves • The HOME techniques allow very clean separation of tracer time stepping from dynamics. • GFDL/HIM is ~10 times faster than GFDL/MOM4 in 1° global models with 20 tracers and same number of layers! • Depend strongly on material conservation of properties • Isopycnal coordinate models ability to control diapycnal diffusion is invaluable.

25. HOME and Climate modeling • HOME-predecessor models are under active consideration for IPCC use at NOAA/GFDL & NASA/GISS. • Most coupled climate models have traditionally used Z-coordinates, leading to common biases. • Dense overflows mediate many climatically important slow processes (e.g. North Atlantic thermohaline circulation). Controling the level and location of diapycnal diffusion in overflows is of particulary importance for long climate simulations. • Eddy-permitting models using Z-coordinates exhibit large advective diapycnal watermass modification (Griffies et al., MWR, 2000). Isopycnal coordinate may prove uniquely useful for eddy-permitting climate simulations.

26. HOME and Ocean Data Assimilation • Some HOME predecessors have demonstrated value from multiple data assimilation approaches. • Optimal Interpolation ; EnVOI ; EnKF ; SEEK ; ROIF ; Adjoint • HOME will be a natural host for this collection of methodologies. • Some data assimilation techniques require a tangent linear model and adjoint of the forward model, which can be naturally incorporated in the framework.

27. The 10-year vision • Distinct HOME predecessor codes disappear within 3-5 years. • HOME group will have extensive consultation with European and other international counterparts (e.g. NEMO) • Strong coordination with existing Z- and s- modeling groups over 3-5 years sets the stage for next step. • Broad unification of all ocean model development activities within a generalized ocean modeling environment over 10 years. • Algorithmic diversity will persist, with the selection dictated by the needs of specific applications.

28. Voluntary participation of existing isopycnal & hybrid ocean model community in developing a common ocean modeling environment. Collaboration from the broader ocean model development community, especially in the extension of HOME to applications that have not traditionally used isopycnal coordinates. Contributions of new capabilities from beyond the circle of developers of existing models. HOME Measures of Success (1)

29. A code base that is easy to configure and use for a variety of applications, with clear, consistent, and explicit documentation Widespread adoption of the HOME code-base for ocean applications. Useful and extensive best-practice guidance for HOME users. Pedagogically useful examples derived from the HOME code base. HOME Measures of Success (2)