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See detailA West Florida Shelf ROMS Nested into HYCOM: Ensemble-based Assimilation of HF-Radar Surface Currents and a 2005 Red Tide Case Study with Simulated Drifters
Barth, Alexander ULiege; Alvera Azcarate, Aïda ULiege; Weisberg, R. H.

Conference (2007)

A West Florida Shelf (WFS) model is constructed by nesting the Regional Ocean Model System (ROMS) in the Atlantic Hybrid Coordinate Ocean Model (HYCOM) to include both local and deep-ocean forcing ... [more ▼]

A West Florida Shelf (WFS) model is constructed by nesting the Regional Ocean Model System (ROMS) in the Atlantic Hybrid Coordinate Ocean Model (HYCOM) to include both local and deep-ocean forcing, particularly the Gulf of Mexico Loop Current (LC). Hindcast experiments from 2004 to 2006 are presented and compared to observed temperature (moorings and BSOP profiling floats), ADCP velocity time series and HF-Radar surface currents. Two different mixing schemes (Mellor Yamada level 2.5 and K-Profile Parameterization, KPP) are tested and the importance of the vertical resolution for mixing is addressed. The model results of those different configurations are compared to temperature observations on the shelf. Results obtained with the Mellor Yamada scheme are closer to observations during winter (negative buoyancy flux and strong winds) while in summer (positive buoyancy flux and in general weaker wind) the KPP scheme produces more realistic results. Given the present HYCOM configuration we assessed the benefit of nesting ROMS in HYCOM compared to nesting ROMS in climatology. The model solutions on the shelf were compared to various in situ data. The model performed best when using the HYCOM boundary values. Simulated trajectories for drifters deployed off Tampa Bay and Sarasota were used to address the evolution of Karenia brevis concentrations during the 2005 red tide. Near surface drifters were advected offshore, whereas drifters deployed in the bottom Ekman layer matched the subsequently observed Karenia brevis distributions, showing the importance of the 3D structure of coastal ocean currents for red tide on the WFS. As a first attempt at assimilating CODAR surface currents we used an ensemble simulation carried out under different wind forcings to estimate the error covariance of the model state vector and the covariance between the ocean currents and the wind. Improvements were obtained for the modeled currents, not only at the surface, but also at depth. [less ▲]

Detailed reference viewed: 47 (4 ULiège)
See detailA Nested Model of the Cariaco Basin: Study of the Hydrography and Interactions with the Open Ocean
Alvera Azcarate, Aïda ULiege; Barth, Alexander ULiege; Virmani, J. I. et al

Conference (2007)

The circulation of the Cariaco Basin (Venezuela) is modeled using the Regional Ocean Model System (ROMS) nested in the global 1/12 degree Hybrid Coordinate Ocean Model (HYCOM). The objective of this work ... [more ▼]

The circulation of the Cariaco Basin (Venezuela) is modeled using the Regional Ocean Model System (ROMS) nested in the global 1/12 degree Hybrid Coordinate Ocean Model (HYCOM). The objective of this work is to obtain a better understanding of the Cariaco Basin circulation by studying the processes that link the basin with the Caribbean Sea. To our knowledge, this is the first attempt to model the circulation in the Cariaco Basin with a nested high resolution hydrodynamical model. In particular, we examined the interaction of the Cariaco Basin with the large-scale, open-ocean processes, as the westward Caribbean Current and the eastward subsurface counter-current flowing along the South America Caribbean coast. These two current systems connect the Cariaco Basin with the Caribbean Sea waters, and therefore are directly related to the ventilation of the basin. By studying the kinematics and dynamics of the Cariaco Basin we anticipate gaining a better understanding on how the past conditions affected the basin characteristics and hence the geological records obtained from the basin sediments. We will report on several years of observations from the continuous monitoring of currents within the basin, plus analyses of year-long model runs that provide a basin-wide, three-dimensional context for the circulation. [less ▲]

Detailed reference viewed: 15 (1 ULiège)
See detailIAS Mesoscale Surface Circulation Observed Through Satellite Altimetry and its Influence in a Small Scale, Coastal Domain, Studied with a ROMS Model of the Cariaco Basin
Alvera Azcarate, Aïda ULiege; Barth, Alexander ULiege; Virmani, J. I. et al

Conference (2007)

The Intra-Americas Sea (IAS) surface circulation is characterized by large scale currents. The Caribbean current, which originates in the Lesser Antilles, travels westwards through the Caribbean Sea and ... [more ▼]

The Intra-Americas Sea (IAS) surface circulation is characterized by large scale currents. The Caribbean current, which originates in the Lesser Antilles, travels westwards through the Caribbean Sea and eastern Mexico and passes through the Gulf of Mexico to finally form the Gulf Stream. This complex system of currents is also characterized by a high mesoscale variability, such as eddies and meanders. The objectives of this work are twofold: first, the multi-scale surface circulation of the IAS is described using satellite altimetry. The topographic influence of the different basins forming the IAS, the characteristic time and spatial scales, and the time variability of the surface circulation will be addressed. The second objective is to analyze the influence of this large scale circulation on a small scale coastal domain with a ROMS-based model of the Cariaco basin (Venezuela). Cariaco is a deep (1400 m), semi-enclosed basin connected to the open ocean by two shallow channels (Tortuga and Centinela Channels). Its connection with the open sea, and therefore the ventilation of the basin, occurs in the surface layers. The Cariaco ROMS model will be used to study the exchanges of mass, heat and salt through the channels. A 1/60 degree ROMS model nested in the global 1/12 degree HYCOM model from the Naval Research Laboratory will be used for this study. In addition, a series of observations (satellite altimetry and in situ temperature, salinity and velocity data), will be used to assess the influence of the Caribbean circulation on the basin. [less ▲]

Detailed reference viewed: 29 (2 ULiège)
See detailError analysis of a high-resolution physical model of the Mediterranean Sea
Vandenbulcke, Luc ULiege; Barth, Alexander ULiege; Alvera Azcarate, Aïda ULiege et al

Conference (2007)

We analyze the errors that are inevitably associated to hydrodynamic models, in a realistic case. The error of the GHER model in the Mediterranean Sea has already been studied in e.g. Beckers et al. (2000 ... [more ▼]

We analyze the errors that are inevitably associated to hydrodynamic models, in a realistic case. The error of the GHER model in the Mediterranean Sea has already been studied in e.g. Beckers et al. (2000) by comparing it with other primitive equation models, or in Alvera (2004) by comparing the model with observations and with the climatology, using usual statistical methods and also wavelet decompositions. In this study, we rather study the sensitivity of the model to various variables using an ensemble of models. We chose a relatively high resolution, 1/16°, corresponding to the resolution now used in operational OGCMs covering the Mediterranean, such as the MFS system (http://www.bo.ingv.it/mfs). We explain how we generated an ensemble of model simulations, where various more-or-less well known inputs are allowed to vary according to the uncertainty affecting them. Statistics calculated on this ensemble are, in fact, the response of the non-linear hydrodynamic system to errors on the forcing terms. When those statistics are calculated at a certain timestep, they allow us to provide a spatial analysis of the model error; statistics calculated over the time dimension will show whether errors are intensified by the system, or rather disappear. The model error is interesting as such. However, it can also be used for different purposes. For example, it allows using data assimilation techniques without needing the usual assumptions of reduced-rank Kalman Filters. It also allows studying the sensitivity of coupled model (biological, oil spill, search-and-rescue, …) to physical forcings. [less ▲]

Detailed reference viewed: 19 (2 ULiège)
See detailData assimilation as a tool for upscaling
Vandenbulcke, Luc ULiege; Barth, Alexander ULiege; Ben Bouallegue, Z. et al

Conference (2006, April)

In ocean and atmospheric sciences, grid nesting is a common procedure in order to achieve (very) high resolution model outputs in regions of particular interest, at an acceptable computational cost ... [more ▼]

In ocean and atmospheric sciences, grid nesting is a common procedure in order to achieve (very) high resolution model outputs in regions of particular interest, at an acceptable computational cost. Nesting of grids can be passive (one-way nesting) or active (two-way nesting, with feedback from the high resolution to the low resolution grid). The benefits of active nesting have been shown multiple times in the litterature (see e.g. [1]). The positive effect of the feedback is visible inside the nested grid, but also outside of it, as corrections are advected with the flow. It must be noted however that in many operationnal implementations, only passive nesting is used, usually because active nesting requires too much data exchange between models, which should then wait for each other during their run. Data assimilation techniques are also widespread in oceanic and atmospheric models. They are usually applied in order to merge observations in models, but also e.g. to merge different outputs from ensemble runs of a model, to merge outputs from different models, or to replace downscaling between nested grids (see [3]). In our work, we present an alternative to active nesting (for implementations currently using passive nesting). First, the low-resolution model is run, followed by the local model. Afterwards, the low-resolution model is run once more, assimilating outputs from the local model as pseudo-data. The benefits of this approach over simple passive nesting are shown using a twin experiment. The GHER model (see [2]) is configured with a 0.25 resolution of the Mediterranean Sea, and with a 0.05 resoluion of the North-Western part; a twin experiment is then set. The reference run uses full two-way nesting, another run uses one-way nesting, and in a third run the assimilation procedure described above is implemented.Conclusions from this experiment are that our "upscaling" has positive impacts on the forecasts, provided a fair amount of EOFs is used during (reduced-rank) assimilation cycles. Finally, the set-up of ongoing work to implement our upscaling procedure in a realistic, operationnal system (the MFS system) is presented. [less ▲]

Detailed reference viewed: 43 (2 ULiège)
See detailA baroclinic, regional West Florida Shelf model nested in a 1/12 degree North Atlantic HYCOM model, inclusive of tides
Barth, Alexander ULiege; Alvera Azcarate, Aïda ULiege; Weisberg, R. H.

Conference (2006)

A West Florida Shelf (WFS) model based on the Regional Ocean Model System (ROMS) is nested in the 1/12° North Atlantic Hybrid Coordinate Ocean Model (NAT HYCOM). The nesting procedure is based on a flow ... [more ▼]

A West Florida Shelf (WFS) model based on the Regional Ocean Model System (ROMS) is nested in the 1/12° North Atlantic Hybrid Coordinate Ocean Model (NAT HYCOM). The nesting procedure is based on a flow relaxation scheme, and the model implementation is tested against in situ data over a one-year hindcast simulation. While the focus of NAT HYCOM is the large-scale circulation, the aim of this study is to show that the NAT HYCOM results can be improved by nesting a regional model with increased resolution. Results are compared qualitatively to sea surface height and quantitatively to in situ temperature and velocity measurements on the shelf. The nesting of a regional model improves the performance on the shelf. The inclusion of tides is a new addition, and preliminary results are shown. [less ▲]

Detailed reference viewed: 23 (3 ULiège)
See detailMapped fields of surface geostrophic currents based on altimetry, and fields of sea surface winds, cloud-free sea surface temperature and chlorophyll concentration using monovariate OI and a multivariate EOF technique
Alvera Azcarate, Aïda ULiege; Barth, Alexander ULiege; Helber, R. W. et al

Conference (2006)

There is an increasing demand for regional oceanic models capable of simulating the regional ocean circulation. Accurate surface forcing functions are necessary to achieve this goal. Here we present ... [more ▼]

There is an increasing demand for regional oceanic models capable of simulating the regional ocean circulation. Accurate surface forcing functions are necessary to achieve this goal. Here we present analyses of several data sets covering the contiguous eastern Gulf of Mexico and southeast Atlantic: a) Wind fields resulting from the blending by optimal interpolation (OI) of NCEP, in situ and QuikSCAT winds. These winds show improvements in the coastal region, where orography and coastal boundary layer effects are important and under-resolved. b) Cloud-free SST, created by merging several SST sources using OI. c) Cloud-free chlorophyll, also created using OI. d) Surface drifter trajectories, generated from geostrophic currents and used to track water masses, with application to the Mississippi River outflow subsequent to Hurricane Katrina. e) Multivariate cloud-free products, using SST and chlorophyll, and SST and QuikSCAT winds, to obtain more accurate reconstructions than the monovariate equivalents. We use an EOF-based method, called DINEOF, which has proven to give similar results to OI-based reconstruction but up to 30 times faster, making it very suitable for operational applications. These data sets, originally created for the West Florida Shelf, have been expanded for the Southeast Atlantic Coastal Ocean Observing System (SEACOOS) and for broader environmental applications. [less ▲]

Detailed reference viewed: 33 (1 ULiège)
See detailA nested West Florida Shelf hydrodynamic model. Application to the 2005 red tide
Barth, Alexander ULiege; Alvera Azcarate, Aïda ULiege; Zheng, L. et al

Conference (2006)

Detailed reference viewed: 16 (1 ULiège)
See detailLocal assimilation of sea surface temperature and elevation in a two-way nested model of the Gulf of Lions, using a single multigrid state vector
Vandenbulcke, Luc ULiege; Barth, Alexander ULiege; Ben Bouallegue, Z. et al

Conference (2005, April)

A three fold nested model is built, covering (a) the Mediterranean Sea (resolution 1/4 degree) (b) its North-Western part (resolution 1/20 degree), and (c) the Gulf of Lions (resolution 1/100 degree). The ... [more ▼]

A three fold nested model is built, covering (a) the Mediterranean Sea (resolution 1/4 degree) (b) its North-Western part (resolution 1/20 degree), and (c) the Gulf of Lions (resolution 1/100 degree). The GHER hydrodynamic model (see e.g. [1]) is used for a simulation of the springs of 1997 and 1998. As the model allows mode splitting, the timestep in each grid is 3 seconds for the barotropic modes, and 3 minutes for the baroclinic modes. ECMWF atmospheric forcings and MODB4/MEDAR climatic data are used. This simulation is run with one-directionnal and bi-directionnal nesting (i.e. without and with statevector feedback), and results are compared. The output of the 1997 and 1998 simulations (3D temperature and salinity fields, and sea surface elevation field) are then used to build 3D multivariate EOFs over the 3 grids alltogether. This guarantees perfect correlations between points from different grids, that are physically at the same location. The following twin experiment is then set up. The simulation from 1998 serves as a control run. A delayed state of this run, serves as initial conditions for the perturbed run. The first 40 EOFs are used to build a reduced-rank model errorspace. Sea surface temperature and sea surface elevation from the reference run, physically located in the Gulf of Lions, are then assimilated in the perturbed run, using a reduced-rank optimal interpolation assimilation scheme. A previous experiment showed non-physical long-range corrections (far outside the Gulf of Lions); these corrections are removed by multiplying the corrections with a radial Gaussian function centered on the corresponding observations. The multivariate statevector ensures corrections are made to temperature, salinity and sea surface elevation fields. Using the corrected fields, the geostrophic equilibrium is used to calculate corrections to the velocity field. In this above twin experiment, observations are assimilated all at once in the 3 grids since a single statevector is used. The results are compared to classic approaches where each grid has a corresponding statevector, and observations are assimilated in a single grid (or in different grids separately). Finally, ongoing research about statistical predictors is presented. Indeed, primitive equation models are too costly to evolve the errorspace in time, even when reducedrank assimilation schemes are used. Statistical methods aim to replace the hydrodynamic model by a much faster method, that would then be used to evolve in time each of the directions of the errorspace, or alternately, the members of an ensemble method. Statistical methods need to be trained on real results; they are thus first tested on the model itself rather than on the errorspace. Preliminary results are presented. [less ▲]

Detailed reference viewed: 33 (2 ULiège)
See detailDerivation of high-resolution ocean surface fields for regional and coastal models
Alvera Azcarate, Aïda ULiege; Barth, Alexander ULiege; He, R. et al

Conference (2005)

Coastal ocean circulation models need high-resolution input fields, such as winds, sea surface height and heat fluxes, to represent the variability of coastal systems. Atmosphere model outputs and ... [more ▼]

Coastal ocean circulation models need high-resolution input fields, such as winds, sea surface height and heat fluxes, to represent the variability of coastal systems. Atmosphere model outputs and satellite data are usually used. However, atmosphere models are usually too coarse and do not represent the high variability of coastal systems, and satellite data do not present a complete coverage, mainly due to cloudiness. In situ observations can accurately represent the complex temporal variability of coastal regions, but usually their spatial coverage is far from optimal. Several products derived from atmosphere models, satellite images and in situ observations are prepared to use as high-resolution input fields suitable for coastal models. An optimally interpolated (OI) wind field has been realized by merging atmosphere model winds, satellite-derived winds (from quikSCAT) and in situ buoy measurements. Other fields, such as geostrophic currents, are derived from Sea Surface Height anomaly obtained from the Topex/Poseidon, Jason, ERS 1/2 and Envisat altimeter product of the CLS center, plus a MICOM mean dynamic topography. Sea Surface Temperature (SST) is also needed to correct surface heat fluxes, but satellite SST is often gappy due to clouds. Two different approaches are investigated in order to obtain complete fields, one using OI and the other using Empirical Orthogonal Functions (EOF) for the reconstruction of missing data. The EOF-based method can reconstruct different variables together, such as SST and surface chlorophyll, by using the correlation between them. This multi-variate approach is used here, and compared to the mono-variate OI product. [less ▲]

Detailed reference viewed: 14 (3 ULiège)
See detailWavelets in the forecast verification of an assimilation experiment in the Ligurian Sea
Alvera Azcarate, Aïda ULiege; Barth, Alexander ULiege; Ben Bouallegue, Z. et al

Conference (2004, June)

Detailed reference viewed: 13 (1 ULiège)
See detailData assimilation in nested-grid models
Vandenbulcke, Luc ULiege; Barth, Alexander ULiege; Ben Bouallegue, Z. et al

Conference (2004, May)

Detailed reference viewed: 21 (3 ULiège)
See detailForecast verification in the Ligurian Sea. Multiresolution analysis and study of the thermocline
Alvera Azcarate, Aïda ULiege; Barth, Alexander ULiege; Rixen, M. et al

Conference (2004, April)

The results of the GHER 3D model are analysed, in order to establish the benefits of a Sea Surface Temperature (SST) assimilation experiment. The influence of the assimilation into the results of the ... [more ▼]

The results of the GHER 3D model are analysed, in order to establish the benefits of a Sea Surface Temperature (SST) assimilation experiment. The influence of the assimilation into the results of the model is examined first in the studied domain, the Ligurian Sea. Then, the benefit of the assimilation outside this domain (in the nearby zones at the surface, and in the Ligurian Sea at depth) is also studied. Finally, the effect of the SST assimilation on the other variables is examined. The procedure for the skill assessment of the model is as follows. First, the classical verification tools are applied: Root Mean Square Error (RMSE), Anomaly Correlation Coefficient (ACC), and Mean Square Error Skill Score (MSESS). Climatology and a free run of the model are used as reference systems. After this, a multiresolution analysis is carried out, to decompose the model results into different spatial scales. At each scale, the error measures mentioned above are applied. This allows to establish which scales are mainly contributing to the error. For this multiresolution analysis, a Discrete Wavelet Transform is used. The study of the assimilation benefits at depth is made by comparison with CTDs. The aim is to study the position and strength of the thermocline, as this zone presents high variability and it has an important impact into the system. A good representation of the thermocline is thus interesting. [less ▲]

Detailed reference viewed: 18 (1 ULiège)
See detailA reduced order data assimilation scheme coupled with a two-way nested model. Application to the Ligurian Sea
Barth, Alexander ULiege; Alvera Azcarate, Aïda ULiege; Beckers, Jean-Marie ULiege et al

Conference (2004, April)

A system of nested models is coupled with a data assimilation module. The system is composed by a low resolution model (1/4 ) covering the whole Mediterranean Sea, an intermediate resolution model (1/20 ... [more ▼]

A system of nested models is coupled with a data assimilation module. The system is composed by a low resolution model (1/4 ) covering the whole Mediterranean Sea, an intermediate resolution model (1/20 ) of the Liguro-Provençal basin and a high resolution model (1/60 ) simulating the fine mesoscale structures in the Ligurian Sea. Boundary conditions and the averaged fields (feedback) are exchanged between two successive nesting levels. A reduced order, optimal interpolation data assimilation scheme was implemented. The state vector is composed by temperature, salinity and sea surface elevation. Novel in the present approach is that these variables from the three nested model grids are assembled to one multigrid state vector. This implementation allows to take into account the correlation of the variables across the nested model grids in order to avoid for example artificial gradients after an assimilation cycle. The eigenvectors of the covariance matrix are constructed by an EOF analysis of the free model run. Cross-grid correlations especially in the overlapping domains are thus consistently represented. Horizontal correlations over long distances are suppressed by multiplying each error mode with a set of radial Gaussian functions. This procedure increases considerably the rank of the covariance matrix but ensures the local impact of each observation. Sea surface temperature (SST, from the DLR EOWEB), sea surface height (SSH, from the CLS) and CTD profiles (SIRENA cruise from SACLANT Center and cruises from the MEDAR/Medatlas database) are assimilated into the model. In overlapping model grids the measurements are related to the highest resolution grid. Since the SSH has a resolution of 1/8 , the surface elevation of the Ligurian Sea and the Liguro-Provençal model are filtered in order to be coherent with the space scales present in the observations. 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 SST, SSH and the CTD profiles are assimilated. The results are compared with a free model run. In particular the model forecast just before the assimilation step are compared with the observations. The model forecast and the measurements are then independent and the difference is a measure of the model forecast skill and the impact of the previous assimilation cycles. [less ▲]

Detailed reference viewed: 39 (1 ULiège)
See detailForecast assessment in the mediterranean sea : A structure oriented approach
Ben Bouallegue, Z.; Alvera Azcarate, Aïda ULiege; Vandenbulcke, Luc ULiege et al

Conference (2004, April)

The MFSTEP1 project is an international scientific collaboration program which aims to create an operational forecasting system for the Mediterranean sea. The simulations provided at the basin scale are ... [more ▼]

The MFSTEP1 project is an international scientific collaboration program which aims to create an operational forecasting system for the Mediterranean sea. The simulations provided at the basin scale are 10 days forecasting fields in a 3-D ocean. The hydrodynamic model primitive equations are combined with the data assimilation scheme SOFA2. The data collection is done in a near real time process and the set of XBT and SLA observations are used in one week assimilation cycle. The forecast assessment is traditionally realised using classical statistic tools like RMSE or the bias and the assimilation benefit is estimated by skill scores using as reference the free model, persistence or also climatology. The process is essentially based on the comparison of two fields at a fixed time, one corresponding to the simulations and the other one to the observations. The interest of such statistical methods comes in the quick and sensitive appreciation they provide about the quality, accuracy and consistency of the simulation. However this kind of assessment procedure brings in it self a conceptual contradiction: performances of a dynamical process are measured using a snap shot view of the ocean state. A system evolution assessment procedure is carried out within the framework of the MFSTEP hindcast. The hindcast system is intrinsically analysed (without independent informations) comparing the background forecast evolution with the abrupt variation which occurs at the observations assimilation time steps. The system evolution between two consecutive days is analysed using a decomposition method. The temperature and salinity fields evolution in a sub-region of theWestern Mediterranean basin is seen in a structural point of view and decomposed in three elements : a global spatial(2D) displacement which conserves the internal features, a global intensity variation which expresses the system energy changes, and an internal pattern changes ensemble. The index of evolution used is a mean squared difference between the two consecutive simulations. The displacement contribution is estimated after the determination of the shift (field translation) which minimises the local mean squared difference between the translated field and the next simulation. The intensity variation contribution is calculated as the difference of the squared mean fields. The remaining difference after manipulations is considered as the internal pattern changes contribution to the system evolution. [less ▲]

Detailed reference viewed: 64 (6 ULiège)
See detailA nested-grid model with data assimilation in the Gulf of Lions
Vandenbulcke, Luc ULiege; Barth, Alexander ULiege; Alvera Azcarate, Aïda ULiege et al

Conference (2004, April)

When a model combines the use of nested grids and data assimilation, a preliminary, simple, 1D test case showed the interest of combining the different state vectors coming from the different grids, into ... [more ▼]

When a model combines the use of nested grids and data assimilation, a preliminary, simple, 1D test case showed the interest of combining the different state vectors coming from the different grids, into one single vector, and using global error matrices covering all the grids at once. In this case, the assimilation procedure provides errorspace feedback from the fine grid to the coarser grid, which proves to be even more important than the statevector feedback. For data located in the fine grid, assimilation of the same data in the coarse grids is not necessary anymore, as both model and errorspace feedback is performed during assimilation. Large data transfers from local to basin-scale models can be avoided. The GHER hydrodynamic model (for a full description, see e.g. [1]) is applied to a three times nested model covering (a) the Mediterrannean Sea at 1/4 degree, (b) the Liguro-Provencal Bassin at 1/20 degree, and (c) the Gulf of Lions at 1/100 degree. The simulation starts on Januari 1st, 1998, using ECMWF atmospheric forcings and MODB4/MEDAR climatic data. As the model allows mode splitting, the simulation uses 2D timesteps of 3 seconds, and 3D timesteps of 3 minutes, on each grid. A twin experiment is performed. The perturbed initial condition is a delayed model state of the reference run. An initial reduced-rank model errorspace is constructed from 20 EOFs, themselves built from the reference run, over all three grids at the same time. Surface temperature and salinity from the reference run are assimilated in the model every 24 hours, using reduced-rank optimal interpolation (see [2]). Different simulations are implemented, using different ways to combine grid nesting and data assimilation: with or without state vector feedback, with data assimilation only in the local grid, or in the coarser grids, or both, and with or without errorspace feedback (i.e. with 3 separated statevectors or with one global statevector). The comparison of those experiments comfirms that using one global statevector reduces the error in the coarser grids much faster. The effect of data assimilation, and the performances of the different methods, can be examined by calculating RMS errors between the perturbed runs and the reference run. They can also be observed by following the model state trajectory in the EOFspace (for example, using the first three EOFs). In the context of the twin experiment described above, the first assimilation cycle clearly brings the model back in time. This is consistent with the choice of the perturbed initial conditions, being a delayed state of the reference run. The following assimilation cycles have little effect, as the trajectory is already almost brought back on the reference trajectory. If other parameters are modified too (e.g. the atmospheric fluxes), each assimilation cycle has an important effect on the modelstate trajectory. A new experiment performs assimilation in the Gulf of Lions in the spring of 1998 using real observations. Different variables can be assimilated, using data collected during the FETCH campaign: NOAA/AVHRR SST, temperature and salinity from Atalante CTDs, or altimetric data from the ERS2 or TOPEX satellites. [less ▲]

Detailed reference viewed: 64 (0 ULiège)