Ocean Science www.ocean-sci.net 1812-0784 1812-0792 2 2 2006 10.5194/os-2-213-2006 http://www.ocean-sci.net/2/213/2006/ http://www.ocean-sci.net/2/213/2006/os-2-213-2006.html http://www.ocean-sci.net/2/213/2006/os-2-213-2006.pdf 213 222 2006-10-31 Study of the combined effects of data assimilation and grid nesting in ocean models – application to the Gulf of Lions L. Vandenbulcke luc.vandenbulcke@ulg.ac.be A. Barth M. Rixen A. Alvera-Azcarate Z. Ben Bouallegue J. M. Beckers GeoHydrodynamics and Environment Research, Université de Liège, Belgium Ocean Circulation Group, University of South Florida, College of Marine Science. Florida 33701, USA NATO/SACLANT Undersea Research Centre, La Spezia, Italy Modern operational ocean forecasting systems routinely use data assimilation techniques in order to take observations into account in the hydrodynamic model. Moreover, as end users require higher and higher resolution predictions, especially in coastal zones, it is now common to run nested models, where the coastal model gets its open-sea boundary conditions from a low-resolution global model. This configuration is used in the "Mediterranean Forecasting System: Towards environmental predictions" (MFSTEP) project. A global model covering the whole Mediterranean Sea is run weekly, performing 1 week of hindcast and a 10-day forecast. Regional models, using different codes and covering different areas, then use this forecast to implement boundary conditions. Local models in turn use the regional model forecasts for their own boundary conditions. This nested system has proven to be a viable and efficient system to achieve high-resolution weekly forecasts. However, when observations are available in some coastal zone, it remains unclear whether it is better to assimilate them in the global or local model. We perform twin experiments and assimilate observations in the global or in the local model, or in both of them together. We show that, when interested in the local models forecast and provided the global model fields are approximately correct, the best results are obtained when assimilating observations in the local model. Auclair, F., Casitas, S., and Marsaleix, P.: Application of an inverse method to coastal modeling, J. Atmos Oceanic Technol., 17, 1368–1391, 2000. Auclair, F., Marsaleix, P., and Estournel, C.: The penetration of the Northern Current over the Gulf of Lions (Mediterranean) as a downscaling problem, Oceanologica Acta, 24, 529–544, 2001. Auclair, F., Marsaleix, P., and De Mey, P.: Space-time structure and dynamics of the forecast error in a coastal circulation model of the Gulf of Lions, Dynamics of Atmospheres and Oceans, 36, 309–346, 2003. Barth, A., Alvera-Azcárate, A., Beckers, J.-M., Rixen, M., and Vandenbulcke, L.: Multigrid state vector for data assimilation in a two-way nested model of the Ligurian Sea, J. Marine Syst., in press, 2006. Barth, A., Alvera-Azcárate, A., Rixen, M., and Beckers, J.-M.: Two-way nested model of mesoscale circulation features in the Ligurian Sea, Progress In Oceanography, 66, 171–189, 2005. Beckers, J.-M.: Application of a 3D model to the Western Mediterranean, J. Mar. Syst., 1, 315–332, 1991. Blayo, E. and Debreu, L.: Adaptive mesh refinement for finite difference ocean model: some first experiments, J. Phys. Ocean., 29, 1239–1250, 1999. Brasseur, P., Ballabrera, J., and Verron, J.: Assimilation of altimetric data in the mid-latitude oceans using the Singular Evolutive Extended Kalman filter with an eddy-resolving, primitive equation model, J. Mar. Syst., 22, 269–294, 1999. Crépon, M., Wald, L., and Monget, J.: Low-frequency waves in the Ligurian Sea during December., J. Phys. Oceanogr., 87, 595–600, 1982. Echevin, V., Crépon, M., and Mortier, L.: Simulation and analysis of the mesoscale circulation in the northwestern Mediterranean Sea, Ann. Geophys., 21, 281–297, 2003. Estournel, C., Durrieu de Madron, X., Marseleix, P., Auclair, F., Julliand, C., and Vehil, R.: Observation and modeling of the winter coastal oceanic circulation in the Gulf of Lion under wind conditions influenced by the continental orography (FETCH experiment), J. Geophys. Res., 108, 8059–8076, 2003. Fox, A. D. and Maskell, S. J.: Two-Way Interactive Nesting of Primitive Equation Ocean Models with Topography, J. Phys. Oceanogr., 25, 2977–2996, 1995. Fox, A. D. and Maskell, S. J.: A nested primitive equation model of the Iceland-Faeroe front, J. Geophys. Res., 101, 18 259–18 278, 1996. Ginis, I., Richardson, R. A., and Rothstein, L. M.: Design of a Multiply Nested Primitive Equation Ocean Model, Mon. Wea. Rev., 126, 1054–1079, 1998. Grilli, F. and Pinardi, N.: The computation of Rossby radii of deformation for the Mediterranean sea, MTP News No. 6, March, 1998. Korres, G. and Lascaratos, A.: A one-way nested eddy resolving model of the Aegean and Levantine basins: implementation and climatological runs, Ann. Geophys., 21, 205–220, 2003. Lacombe, H. and Tchernia, P.: Les zones de formation d'eau profonde océanique: Caractères–processus–problèmes, in: Processus de formation des eaux océaniques profondes, Colloq. Int. Centre Natl. Rech. Sci., 215, 249–262, 1974. Millot, C.: The Gulf of Lions' hydrodynamics, Cont. Shelf Res., 10, 885–894, 1990. Millot, C.: Circulation in the Western Mediterranean Sea, J. Mar. Syst., 20, 423–442, 1999. Nihoul, J. C. J., Deleersnijder, E., and Djenidi, S.: Modelling the general circulation of shelf seas by 3D $k-\epsilon$ Models., Earth Sci. Rev., 26, 163–189, 1989. Oey, L.-Y. and Chen, P.: A Nested-Grid Ocean Model: With Application to the Simulation of Meanders and Eddies in the Norwegian Coastal Current, J. Geophys. Res., 97, 20 063–20 086, 1992. Petrenko, A.: Circulation features in the Gulf of Lions, NW Mediterranean Sea; importance of inertial currents., Oceanologica Acta, 26, 323–338, 2003. Pham, D. T., Verron, J., and Roubaud, M. C.: A singular evolutive extended Kalman filter for data assimilation in oceanography., J. Mar. Syst., 16, 323–340, 1998. Pinardi, N., Allen, I., Demirov, E., De Mey, P., Lascaratos, A., Le Traon, P.-Y., Maillard, C., Manzella, G., and Tziavos, C.: The Mediterranean ocean forecasting system: first phase of implementation (1998–2001), Ann. Geophys., 21, 3–20, 2003. Smith, W. H. F. and Sandwell, D. T.: Global Sea Floor Topography from Satellite Altimetry and Ship Depth Soundings, Science, 277, 1956–1962, 1997. Spall, M. A. and Robinson, A. R.: A new open-ocean hybrid coordinate primitive equation model, Math. Comput., 31, 241–269, 1989. Spall, M. A. and Holland, W. R.: A nested primitive equation model of oceanic application, J. Phys. Oceanogr., 21, 205–220, 1991. Sparnocchia, S., Picco, P., Manzella, G. M. R., Ribotti, A., Copello, S., and Brasey, P.: Intermediate water formation in the Ligurian Sea, Oceanologica Acta, 18, 151–162, 1995. Tusseau, M. and Mouchel, J.: Nitrogen inputs to the Gulf of Lions via the Rhône river, in: Water Pollution Research Reports, Proceedings of the EROS 2000 Workshop, Hamburg, edited by: Martin, J. and Barth, H., commission of the European Communities, 1994. Verlaan, M. and Heemink, A. W.: Tidal flow forecasting using reduced rank square filters, Stochastic Hydrology and Hydraulics, 11, 349–368, 1997. Zavatarelli, M. and Pinardi, N.: The Adriatic Sea modelling system: a nested approach, Ann. Geophys., 21, 345–364, 2003.