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Regional models

Par ECOLA Dernière modification 20/08/2018 16:48

Development of regional oceanic and atmospheric modeling

nested ROMS TIW

Patrick Marchesiello, Gildas Cambon, Laurent Debreu, Pierrick Penven, Jerome Lefevre, Florian Lemarie, Yves Soufflet, Swen Jullien ...

In 2008, the developers of oceanic models from various French institutes (INRIA, IRD, CNRS, Ifremer, SHOM) participated in a first national meeting (Autrans, October 2008). One objective of this meeting was to unite our efforts and aim for a joint project: COMODO. The French branch of the Regional Oceanic Modeling System, developed at IRD (in collaboration with INRIA) and based at LEGOS, played an essential role in this joint venture. The first appeal of the model is the sophistication of its dynamic core based on high-order numerical methods that minimize the effects of dispersion and diffusion errors, with special care to diapycnal diffusion (Soufflet et al., 2016). The IRD version of ROMS used AGRIF, an efficient library, applied for 2-way nesting at the fast mode level (see Debreu et al., 2012 and figure above from Marchesiello et al., 2011). In addition, great efforts were devoted to the model's accessibility. This strategy has lead us to a community of hundreds of users around the world, many in southern countries. This model was chosen as a base for CROCO, a joint national project for a common high-standard numerical model. 

Numerical methods

 The development of numerical methods with finite differences is guided by a tradeoff between accuracy and cost. In a paper by Sanderson (1998): "order and resolution for computational ocean dynamics", it is shown that the optimal discretization order of accuracy for three-dimensional oceanic models is the fifth order. ROMS and WRF regional models are some of the best examples, for the ocean and the atmosphere respectively. These theoretical considerations have put a limit on the development of finite difference methods, although this limit may not be fully understood (Soufflet et al., 2016; Menesguen et al., 2018). In the future, refinements may be less expected from formal order of accuracy than actual accuracy of space-time numerical schemes, and those of nonlinear monotonic schemes. Progress will also be related to the development of physical parameterizations and couplings between various environments, including the wave field and atmosphere.

Effective resolution

One aspect of numerical analysis that is of particular interest to us is the assessment of diffusion mechanisms in the context of scheme order, turbulent cascade, submesoscale and microscale modeling. The goal is to advance fundamental understanding of the processes of numerical dispersion/diffusion, define effective resolution of models based on the statistical patterns and contribute to design models for future generation (Marchesiello et al., 2011; Soufflet et al, 2016; Menesguen et al, 2018). See more in the next topic.

Topography

An aspect that remains problematic in regional models is related to the treatment of topography. Terrain-following (sigma) coordinates propose proper bottom boundary conditions that are not subject to any approximation due to discretization, as opposed to the case of geopotential coordinates. This coordinate allows flow sensitivity to the structure of the seabed, such as slope-current interactions, internal tides generation, or benthic boundary layer dynamics. We have shown the fundamental role of topography in different regions and at different scales (Marchesiello et al, 1998; Marchesiello and Middleton, 2000; Marchesiello et al., 2003,. Couvelard et al, 2008), which confirmed the value of sigma coordinates.

But sigma coordinates are not without problems. After advances were made to the pressure gradient computation (Shchepetkin and McWilliams, 2003), our work has revealed a new challenge for ocean modeling (Marchesiello et al. 2009a). This is related to the Veronis effect, i.e., an excessive amount of spurious diapycnal diffusion. The Veronis effect is greatly amplified by the orientation of sigma levels with respect to isopycnals. The effect becomes acceptable at very high resolution (<1 km), but at intermediate resolutions, the advective and diffusive parts of the advection scheme must be separated to redirect diffusion along isopycnals and thus correct the Veronis effect. This approach was consolidated by Lemarié et al. (2012) by including an implicit treatment of vertical fluxes, thus avoiding stability constraints (providing the first isopycnal hyperdiffusion scheme). But if this technique is efficient for linear schemes, it is not so for nonlinear monotonous schemes (FCT/TVD type schemes popular with biological modelers). We are thus testing new ideas while refining the actual schemes.

Ocean-Wave-Atmosphere coupling

A major development for integrated regional modeling is air-sea coupling. We have addressed the physical problem in relation to cyclone activity in the South Pacific (Jullien et al., 2014),  after considering some mathematical issues (thèse de Florian Lemarié). For regional atmospheric models, CROCO is coupled with WRF (Weather Research Forcast model) and with Meso-NH, using OASIS. Besides our work on tropical cyclones, CROCO-WRF modeling has permitted some breakthrough in important small-scale OA processes, e.g., eddy killing, a coupled process that affects mesoscale eddies and major oceanic currents through the turbulent inverse cascade (Renault et al., 2016). 

However, usual bulk formulations of air-sea exchange were proposed for well-developed seas, away from the coast, neglecting fine-scale processes and extreme events (winds greater than 30 m/s). Recent observations show that waves have a significant but complex impact on the oceanic and atmospheric surface layers. In particular, young seas (high frequency waves) can increase the roughness of the ocean (Doyle, 2002). In extreme events, the layer formed by foam patches ("foam layer") can also affect exchanges by forming a slippery layer at the interface (Powell et al. 2003) and sea-spray seems to intensify latent and sensible heat fluxes. Under these conditions, sea roughness usually inferred from the work of Charnock in bulk formula is not suitable for fine-scale, coastal or extreme air-sea exchanges and the coupling of a wave model (WW3) is required for further refinement. WW3 is now coupled with CROCO-WRF and will allow us studying these fairly unexplored aspects of oceanography.

Wave-current interaction is another part of OAW coupling. In Marchesiello et al. (2015), we implemented the formalism of McWilliams et al. (2004) following Uchiyama et al. (2010). Besides the importance of these interactions for offshore OA dynamics, modeling momentum transfer between waves and currents is also used to study nearshore dynamics and address issues of coastal vulnerability.

References:

  • Menesguen C., S. Le Gentil, P. Marchesiello, and N. Ducousso, 2018: Destabilization of an oceanic meddy-like vortex: energy transfers and significance of numerical settings. Journal of Physical Oceanography, 48, 1151-1168.
  • Soufflet Y., P. Marchesiello, J. Jouanno, X. Capet, L. Debreu, F. Lemarie, 2016: On effective resolution in ocean models. Ocean Modelling, 98, 36-50.
  • Renault, L., Molemaker, M. J., Gula, J., Masson, S., and McWilliams, J. C., 2016: Control and Stabilization of the Gulf Stream by Oceanic Current Interaction with the Atmo- sphere. Journal of Physical Oceanography, 46(11), 3439–3453. 
  • Marchesiello P., R. Benshila, R. Almar, Y. Uchiyama, J.C. McWillams and A. Shchepetkin, 2015: On tridimensional rip current modeling. Ocean Modelling, 96, 36-48.
  • Jullien S., P. Marchesiello, C. E. Menkes, J. Lefèvre, N. C. Jourdain, G. Samson, and M. Lengaigne, 2014: Ocean feedback to tropical cyclones: climatology and processes. Climate Dynamics, 43, 2831-2854.
  • Debreu, L., P. Marchesiello, P. Penven, and G. Cambon, 2012 : Two-way nesting in split-explicit ocean models: algorithms, implementation and validation. Ocean Modelling, 49-50, 1-21.
  • Marchesiello P., X. Capet, C. Menkes, and S.C. Kennan, 2011: Submesoscale dynamics in Tropical Instability Waves. Ocean Modelling, 39, 31-46.
  • Lemarié F., 2009 : Algorithmes de Schwarz et couplage océan-atmosphère. Thèse de l’Université Joseph Fourier, Grenoble, France.
  • Marchesiello P. L. Debreu and X. Couvelard, 2009: Spurious diapycnal mixing in terrain-following coordinate models: the problem and a solution. Ocean Modelling, 26, 156-169.
  • Marchesiello, P., J.C. McWilliams, and A. Shchepetkin, 2001: Open boundary conditions for long-term integration of regional oceanic models. Ocean Modelling, 3, 1-20.
  • Penven P., L. Debreu, P. Marchesiello, and J.C. McWilliams, 2006: Evaluation and application of the ROMS 1-way embedding procedure to the central california upwelling system. Ocean Modelling, 12, 157-187.
  • Penven P., P. Marchesiello, L. Debreu, and J. Lefevre, 2008: Software tools for pre- and post-processing of oceanic regional simulations. Environ. Model. Softw., 23, 660-662.
  • Sanderson, B. G., 1998: Order and resolution for computational ocean dynamics. J. Phys. Oceanogr., 28, 1271–1286.
  • Shchepetkin, A., and J.C. McWilliams, 2005: The Regional Oceanic Modeling System: A split-explicit, free-surface, topography-following-coordinate ocean model. Ocean Modelling, 9, 347-404.
  • Shchepetkin, A.F., and J.C. McWilliams, 2003: A method for computing horizontal pressure-gradient force in an ocean model with a non-aligned vertical coordinate. J. Geophys. Res., 108, C3, 3090.
  • Skamarock, W. C., J. B. Klemp, 2007: A Time-Split Nonhydrostatic Atmospheric Model for Research and NWP Applications.  J. Comp. Phys. special issue on environmental modeling.

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