References of "Barth, Alexander"
     in
Bookmark and Share    
Full Text
Peer Reviewed
See detailFiltering inertia-gravity waves from the initial conditions of the linear shallow water equations
Barth, Alexander ULg; Beckers, Jean-Marie ULg; Alvera Azcarate, Aïda ULg et al

in Ocean Modelling (2007), 19(3-4), 204-218

A method for filtering inertia-gravity waves from elevation and depth-averaged velocity is described. This filtering scheme is derived from the linear shallow water equations for constant depth and ... [more ▼]

A method for filtering inertia-gravity waves from elevation and depth-averaged velocity is described. This filtering scheme is derived from the linear shallow water equations for constant depth and constant Coriolis frequency. The filtered solution is obtained by retaining only the eigenvectors corresponding to the geostrophic equilibrium and by discarding explicitly the eigenvectors corresponding to the fast moving inertia-gravity waves. An alternative formulation is derived using a variational approach. Both filtering methods are tested numerically for a periodic domain with constant depth and the variational approach is implemented for a closed domain with large topographic variations. The filtering methods significantly reduce the amplitudes of the inertia-gravity waves while preserving the mean flow. The variational method is compared to the Incremental Analysis Update technique and the benefits of the variational filter are presented. (C) 2007 Elsevier Ltd. All rights reserved. [less ▲]

Detailed reference viewed: 69 (13 ULg)
See detailAssimilation of High-Frequency Radar Currents in a Nested Model of the West Florida Shelf
Barth, Alexander ULg; Alvera Azcarate, Aïda ULg; Weisberg, R. H.

Conference (2007)

High-frequency radar currents are assimilated in a West Florida Shelf (WFS) model based on the Regional Ocean Model System (ROMS), which is nested in the Atlantic Hybrid Coordinate Ocean Model (HYCOM) for ... [more ▼]

High-frequency radar currents are assimilated in a West Florida Shelf (WFS) model based on the Regional Ocean Model System (ROMS), which is nested in the Atlantic Hybrid Coordinate Ocean Model (HYCOM) for the purpose of including both local and deep-ocean forcing, particularly the Gulf of Mexico Loop Current. Tides are not included in this model. An ensemble simulation of the WFS model is carried out under different wind-forcings in order to estimate the error covariance of the model state vector and the covariance between ocean currents and winds. Radial currents measured by high-frequency radar antennas near Saint Petersburg and Venice, Florida, USA, are assimilated using this ensemble-based error covariance. Different assimilation techniques using a time-average ensemble, a filter to reduce surface-gravity waves and an extended state vector including wind stress were tested. Results of the WFS model assimilating surface currents show an improvement of the model currents not only at the surface but also at depth. [less ▲]

Detailed reference viewed: 19 (3 ULg)
Full Text
Peer Reviewed
See detailStudy of the combined effects of data assimilation and grid nesting in ocean models – application to the Gulf of Lions
Vandenbulcke, Luc ULg; Barth, Alexander ULg; Rixen, Michel et al

in Ocean Science (2006), 2

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 ... [more ▼]

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. [less ▲]

Detailed reference viewed: 54 (16 ULg)
See detailData assimilation as a tool for upscaling
Vandenbulcke, Luc ULg; Barth, Alexander ULg; 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: 30 (0 ULg)
See detailA baroclinic, regional West Florida Shelf model nested in a 1/12 degree North Atlantic HYCOM model, inclusive of tides
Barth, Alexander ULg; Alvera Azcarate, Aïda ULg; 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: 21 (3 ULg)
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 ULg; Barth, Alexander ULg; 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: 30 (1 ULg)
Full Text
Peer Reviewed
See detailDINEOF reconstruction of clouded images including error maps. Application to the Sea-Surface Temperature around Corsican Island
Beckers, Jean-Marie ULg; Barth, Alexander ULg; Alvera Azcarate, Aïda ULg

in Ocean Science (2006), 2

We present an extension to the Data INterpolating Empirical Orthogonal Functions (DINEOF) technique which allows not only to fill in clouded images but also to provide an estimation of the error ... [more ▼]

We present an extension to the Data INterpolating Empirical Orthogonal Functions (DINEOF) technique which allows not only to fill in clouded images but also to provide an estimation of the error covariance of the reconstruction. This additional information is obtained by an analogy with optimal interpolation. It is shown that the error fields can be obtained with a clever rearrangement of calculations at a cost comparable to that of the interpolation itself. The method is presented on the reconstruction of sea-surface temperature in the Ligurian Sea and around the Corsican Island (Mediterranean Sea), including the calculation of inter-annual variability of average surface values and their expected errors. The application shows that the error fields are not only able to reflect the data-coverage structure but also the covariances of the physical fields. [less ▲]

Detailed reference viewed: 93 (12 ULg)
Full Text
Peer Reviewed
See detailCoupling a two-way nested primitive equation model and a statistical SST predictor of the Ligurian Sea via data assimilation
Barth, Alexander ULg; Alvera Azcarate, Aïda ULg; Beckers, Jean-Marie ULg et al

in Ocean Modelling (2006), 13(3-4), 255-270

A primitive equation model and a statistical predictor are coupled by data assimilation in order to combine the strength of both approaches. In this work, the system of two-way nested models centred in ... [more ▼]

A primitive equation model and a statistical predictor are coupled by data assimilation in order to combine the strength of both approaches. In this work, the system of two-way nested models centred in the Ligurian Sea and the satellite-based ocean forecasting (SOFT) system predicting the sea surface temperature (SST) are used. The data assimilation scheme is a simplified reduced order Kalman filter based on a constant error space. The assimilation of predicted SST improves the forecast of the hydrodynamic model compared to the forecast obtained by assimilating past SST observations used by the statistical predictor. This study shows that the SST of the SOFT predictor can be used to correct atmospheric heat fluxes. Traditionally this is done by relaxing the model SST towards the climatological SST. Therefore, the assimilation of SOFT SST and climatological SST are also compared. (c) 2006 Elsevier Ltd. All rights reserved. [less ▲]

Detailed reference viewed: 71 (12 ULg)
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 ULg; Barth, Alexander ULg; 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: 28 (2 ULg)
See detailDerivation of high-resolution ocean surface fields for regional and coastal models
Alvera Azcarate, Aïda ULg; Barth, Alexander ULg; 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: 12 (1 ULg)
Full Text
Peer Reviewed
See detailTwo-way nested model of mesoscale circulation features in the Ligurian Sea
Barth, Alexander ULg; Alvera Azcarate, Aïda ULg; Rixen, Michel et al

in Progress in Oceanography (2005), 66(2-4), 171-189

A coarse resolution primitive equation model of 1/4 degrees resolution is implemented covering the whole Mediterranea Sea. Within this grid a 1/20 degrees resolution model of the Liguro-Provencal basin ... [more ▼]

A coarse resolution primitive equation model of 1/4 degrees resolution is implemented covering the whole Mediterranea Sea. Within this grid a 1/20 degrees resolution model of the Liguro-Provencal basin and the northern part of the Tyrrhenian Sea is embedded. A third fine resolution model of 1/60 degrees is nested in the latter one and simulates the dynamics of the Ligurian Sea. Comparisons between one-way and two-way nesting in simulating the Northern Current (NC) are made. The properties of the Eastern and Western Corsican Current and the Northern Current are investigated with this nesting system. Special attention is given to the variability of the NC. Meanders and interactions with Winter Intermediate Water lenses are shown. Topographic features also lead to a highly variable NC. (c) 2005 Elsevier Ltd. All rights reserved. [less ▲]

Detailed reference viewed: 53 (14 ULg)
Full Text
Peer Reviewed
See detailReconstruction of incomplete oceanographic data sets using empirical orthogonal functions: application to the Adriatic Sea surface temperature
Alvera Azcarate, Aïda ULg; Barth, Alexander ULg; Rixen, Michel et al

in Ocean Modelling (2005), 9(4), 325-346

A method for the reconstruction of missing data based on an EOF decomposition has been applied to a large data set, a test case of Sea Surface Temperature satellite images of the Adriatic Sea. The EOF ... [more ▼]

A method for the reconstruction of missing data based on an EOF decomposition has been applied to a large data set, a test case of Sea Surface Temperature satellite images of the Adriatic Sea. The EOF decomposition is realised with a Lanczos method, which allows optimising computational time for large matrices. The results show that the reconstruction method leads to accurate reconstructions as well as a low cpu time when dealing with realistic cases. The method has been tested with different amounts of missing data, artificially adding clouds ranging from 40% to 80% of data loss, and then compared to the same data set with no missing data. A comparison with in situ data has also been made. These validation studies show that results are robust, even when the amount of missing data is very high. The reconstruction of the data from the Adriatic Sea shows realistic features and a reliable temperature distribution. In addition, the method is compared to an Optimal Interpolation reconstruction. The results obtained with both methods are very similar. The main difference is the computational time, which is reduced nearly 30 times with the method presented here. Once the reconstruction has been performed, the EOF decomposition is analysed to show the method's reliability, and a cold event on the Albanian coast is studied. The reconstructed data reflect the effect of wind on the Albanian coast, that led to a cold-water episode in this zone for a 6-day period. (c) 2004 Elsevier Ltd. All rights reserved. [less ▲]

Detailed reference viewed: 169 (34 ULg)
See detailWavelets in the forecast verification of an assimilation experiment in the Ligurian Sea
Alvera Azcarate, Aïda ULg; Barth, Alexander ULg; Ben Bouallegue, Z. et al

Conference (2004, June)

Detailed reference viewed: 12 (1 ULg)
See detailData assimilation in nested-grid models
Vandenbulcke, Luc ULg; Barth, Alexander ULg; Ben Bouallegue, Z. et al

Conference (2004, May)

Detailed reference viewed: 16 (2 ULg)
See detailForecast assessment in the mediterranean sea : A structure oriented approach
Ben Bouallegue, Z.; Alvera Azcarate, Aïda ULg; Vandenbulcke, Luc ULg 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: 43 (6 ULg)