Assimilation of Sea Surface Temperature in a doubly, two-way nested primitive equation model of the Ligurian Sea

Conference (2003, April)

The GHER 3D primitive equation model is implemented with three different resolutions: a low resolution model (1/4^o) covering the whole Mediterranean Sea, an intermediate resolution model (1/20^o) of the ... [more ▼]

The GHER 3D primitive equation model is implemented with three different resolutions: a low resolution model (1/4^o) covering the whole Mediterranean Sea, an intermediate resolution model (1/20^o) of the Liguro-Provençal basin and a high resolution model (1/60^o) simulating the fine mesoscale structures in the Ligurian Sea. Boundary conditions and the averaged fields (feedback) are exchanged between two successive nesting levels. The model of the Ligurian Sea is also coupled with the assimilation package SESAM. It allows to assimilate satellite data and in situ observations using the local adaptative SEEK (Singular Evolutive Extended Kalman) filter. Instead of evolving the error space by the numerically expensive Lyapunov equation, a simplified algebraic equation depending on the misfit between observation and model forecast is used. Starting from the 1st January 1998 the low and intermediate resolution models are spun up for 18 months. The initial conditions for the Ligurian Sea are interpolated from the intermediate resolution model. The three models are then integrated until August 1999. During this period AVHRR Sea Surface Temperature of the Ligurian Sea is assimilated. The results are validated by using CTD and XBT profiles of the SIRENA cruise from the SACLANT Center. The overall objective of this study is pre-operational. It should help to identify limitations and weaknesses of forecasting methods and to suggest improvements of existing operational models. [less ▲]

Model intercomparison in the Mediterranean: MEDMEX simulations of the seasonal cycle

in Journal of Marine Systems (2002), 33

The simulation of the seasonal cycle in the Mediterranean by several primitive equation models is presented. All models were forced with the same atmospheric data, which consists in either a monthly ... [more ▼]

The simulation of the seasonal cycle in the Mediterranean by several primitive equation models is presented. All models were forced with the same atmospheric data, which consists in either a monthly averaged wind-stress with sea surface relaxation towards monthly mean sea surface temperature and salinity fields, or by daily variable European Centre for Medium Range Weather Forecast (ECMWF) reanalysed wind-stress and heat fluxes. In both situations models used the same grid resolution. Results of the modelling show that the model behaviour is similar when the most sensitive parameter, vertical diffusion, is calibrated properly. It is shown that an unrealistic climatic drift must be expected when using monthly averaged forcing functions. When using daily forcings, drifts are modified and more variability observed, but when performing an EOF analysis of the sea surface temperature, it is shown that the basic cycle, represented similarly by the models, consists of the seasonal cycle which accounts for more than 90% of its variability. (C) 2002 Elsevier Science B.V. All rights reserved. [less ▲]

Circulation of the western Mediterranean: From global to regional scales

in Deep-Sea Research Part II, Topical Studies in Oceanography (1997), 44(3-4), 531-549

A free-surface, three-dimensional, primitive equation model has been implemented with a horizontal resolution of 4.6 km to study the ocean circulation in the Gulf of Lions at time scales ranging from ... [more ▼]

A free-surface, three-dimensional, primitive equation model has been implemented with a horizontal resolution of 4.6 km to study the ocean circulation in the Gulf of Lions at time scales ranging from weeks to seasons. Numerical experiments have been conducted, in which the regional model is nested into a basin-scale model of the whole western Mediterranean. The global model is operated with a relatively coarse resolution (16 km) and provides boundary conditions at the open-sea boundaries of the regional domain. There is, however, no Feedback loop from the regional to the global model. The simulations are consistently driven with atmospheric fluxes computed from the output of the French PERIDOT meteorological forecasting system, between August 1988 and 1989. In addition to the initial conditions, in situ measurements of temperature and salinity are assimilated in the simulation of the general circulation, adopting a simple nudging technique to prevent an excessive drift of the model against climatology. The response of the regional model below and above the thermocline is discussed in the context of the prevailing meteorological situations. Some experiments give indications that a double-gyre system may develop from wind regimes that exhibit a cyclonic/anticyclonic wind stress curl. Advection-diffusion of passive tracers are also examined on the basis of the local hydrodynamic features, because this work has been conceived with the aim of determining the physical conditions in which ecological and biochemical processes develop at the interface between river mouths and the open ocean. (C) 1997 Elsevier Science Ltd. [less ▲]

Progress from 1989 to 1992 in understanding the circulation of the Western Mediterranean Sea

in Oceanologica Acta (1995), 18(2), 255-271

The present paper describes the major results obtained from 1989 to 1992 by the EUROMODEL group in studying the circulation in the Western Mediterranean Sea. Particular emphasis has been given to the ... [more ▼]

The present paper describes the major results obtained from 1989 to 1992 by the EUROMODEL group in studying the circulation in the Western Mediterranean Sea. Particular emphasis has been given to the physical processes responsible for seasonal and mesoscale variabilities. Observations (in situ and satellite), together with theoretical, physical and numerical models, have been widely used in the course of these studies. Attention has been focused on the dynamics of the northern basin (deep water formation, dynamics of the Northern Mediterranean Current, circulation and shelf/slope interaction in the Balearic Sea) and of the southern basin (Alboran Sea circulation, instabilities of the Algerian Current). The straits dynamics have been studied with particular reference to the Corsica channel. Preliminary runs of a basin-scale circulation model of the Western Mediterranean Sea forced by the straits are also presented. They are shown to be capable of generating a cyclonic circulation in the western basin. [less ▲]

Investigation of the Western Mediterranean's hydrodynamics with the GHER three-dimensional primitive equation model

in Coastal and Estuarine Studies (1994), 46

A three-dimensional, non-linear, primitive equation model with turbulent closure developed at the GeoHydrodynamics and Environment Research Laboratory (GHER), University of Liège, is designed to define ... [more ▼]

A three-dimensional, non-linear, primitive equation model with turbulent closure developed at the GeoHydrodynamics and Environment Research Laboratory (GHER), University of Liège, is designed to define marine weather-like processes ranging from mesoscale tides and storm surges, to synoptic frontal and eddy structures, to macroscale slowly-varying currents characteristic of the so-called “general circulation”. The model is applied here to the study of the hydrodynamics of the western Mediterranean as both a metagnostic (i.e. system-oriented) model and a diagnostic (i.e. process-oriented) model with both models allowed to run simultaneously and interactively. Critical processes such as deep water formation and the instabilities of the Algerian Current, are studied to provide a basic understanding of the physics needed to adequately parameterize sub-grid scale processes in the basin wide metagnostic model. This general circulation model provides boundary conditions for the finer grid diagnostic studies as well as the general overview of the basin's hydrodynamics. Further improvement is achieved by concurrent use of a variational inverse model to provide initial data fields and boundary conditions for the direct model. In turn, the metagnostic model provides hydrodynamic constraints which are imposed to the variational principle to ensure an interpolation/extrapolation of data compatible with the system's hydrodynamics. Results viz (i) a diagnostic study of the Algerian Current, (ii) a metagnostic study of the seasonal variability of the western Mediterranean Sea, are presented and compared with observations and with results of simpler (quasi-geostrophic, reduced gravity, etc.) models. Possible improved hydrodynamic forecasts of seasonal and interannual variability with the 3D primitive equation model is discussed with particular attention to the forecast's sensitivity to initial and boundary conditions. [less ▲]

Month-to-month variability of the general circulation fields in the western Mediterranean and the Gulf of Lions

in Water Pollution Research (1993), 30