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See detailUsing monovariate and multivariate EOFs to reconstruct missing data with DINEOF
Alvera Azcarate, Aïda ULg; Barth, Alexander ULg; Sirjacobs, Damien ULg et al

Conference (2008)

DINEOF (Data Interpolating Empirical Orthogonal Functions) is an EOF-based method to reconstruct missing data in geophysical data sets. DINEOF can be used to reconstruct monovariate data sets (as sea ... [more ▼]

DINEOF (Data Interpolating Empirical Orthogonal Functions) is an EOF-based method to reconstruct missing data in geophysical data sets. DINEOF can be used to reconstruct monovariate data sets (as sea surface temperature (SST), chlorophyll, etc), and multivariate data sets with little increase in complexity. For multivariate reconstructions, extended EOFs are used, which take into account the interrelationships between related variables to infer data at missing locations. Spatial maps of the standard deviation of the reconstruction error can be also calculated. In the past, DINEOF has been compared to Optimal Interpolation (OI) techniques for the Adriatic Sea SST. The results showed that DINEOF was faster than OI, making it very suitable for operational applications. DINEOF was also more accurate when compared to in situ data. Another advantage of DINEOF is that there is no need for a priori knowledge of the statistics of the reconstructed data set (such as covariance or correlation length), thus reducing the subjectivity of the analysis. DINEOF has been successfully used to reconstruct a large variety of domains over the world ocean, mostly at the regional scale. In addition to an overview of the technique's capabilities, limitations and future developments, recent work aimed to improve the quality of the reconstructions at the global and local scales will be presented. [less ▲]

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See detailAn analysis of the error space of a high-resolution implementation of the GHER hydrodynamic model in the Mediterranean Sea
Vandenbulcke, Luc ULg; Rixen, M.; Alvera Azcarate, Aïda ULg et al

in Ocean Modelling (2008), 24(1-2), 46-64

An ensemble of 250 model setups covering the Mediterranean Sea is built by perturbing various parameters: the bathymetry, the initial conditions, atmospheric forcing fields (air temperature, cloud ... [more ▼]

An ensemble of 250 model setups covering the Mediterranean Sea is built by perturbing various parameters: the bathymetry, the initial conditions, atmospheric forcing fields (air temperature, cloud coverage, wind), and internal model parameters (diffusion coefficients). The ensemble is then forwarded in time using the GHER hydrodynamic model, allowing to obtain information about the expected error associated with the forecast in a natural way. The evolution of this error is analyzed. In particular, we examine the time evolution and stationarity of its spatial average, and the spatial distribution of the error at different instants, by means of its first to fourth order moments, and of empirical orthogonal functions. We verify whether the a posteriori error distribution is Gaussian using the Anderson-Darling test. From these results, we are able to assess what parameters and forcing fields are most critical for the forecast. Qualitative conclusions are obtained throughout the text, in accordance with our expectations. Moreover, quantitative estimations of the expected error are also given. (C) 2008 Elsevier Ltd. All rights reserved. [less ▲]

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See detailApplication of a SEEK filter to a 1D biogeochemical model of the Ligurian Sea: Twin experiments and real in-situ data assimilation
Raick, Caroline ULg; Alvera Azcarate, Aïda ULg; Barth, Alexander ULg et al

in Journal of Marine Systems (2007), 65(1-4), 561-583

The Singular Evolutive Extended Kalman (SEEK) filter has been implemented to assimilate in-situ data in a 1D coupled physical-ecosystem model of the Ligurian Sea. The biogeochemical model describes the ... [more ▼]

The Singular Evolutive Extended Kalman (SEEK) filter has been implemented to assimilate in-situ data in a 1D coupled physical-ecosystem model of the Ligurian Sea. The biogeochemical model describes the partly decoupled nitrogen and carbon cycles of the pelagic food web. The GHER hydrodynamic model (1D version) is used to represent the physical forcings. The data assimilation scheme (SEEK filter) parameterizes the error statistics by means of a set of empirical orthogonal functions (EOFs). Twin experiments are first performed with the aim to choose the suitable experimental protocol (observation and estimation vectors, number of EOFs, frequency of the assimilation,...) and to assess the SEEK filter performances. This protocol is then applied to perform real data assimilation experiments using the DYFAMED data base. By assimilating phytoplankton observations, the method has allowed to improve not only the representation of the phytoplankton community, but also of other variables such as zooplankton and bacteria that evolve with model dynamics and that are not corrected by the data assimilation scheme. The validation of the assimilation method and the improvement of model results are studied by means of suitable error measurements. (c) 2006 Elsevier B.V. All rights reserved. [less ▲]

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See detailMultigrid state vector for data assimilation in a two-way nested model of the Ligurian Sea
Barth, Alexander ULg; Alvera Azcarate, Aïda ULg; Beckers, Jean-Marie ULg et al

in Journal of Marine Systems (2007), 65(1-4), 41-59

A system of two nested models composed by a coarse resolution model of the Mediterranean Sea, an intermediate resolution model of the Provencal Basin and a high resolution model of the Ligurian Sea is ... [more ▼]

A system of two nested models composed by a coarse resolution model of the Mediterranean Sea, an intermediate resolution model of the Provencal Basin and a high resolution model of the Ligurian Sea is coupled with a Kalman-filter based assimilation method. The state vector for the data assimilation is composed by the temperature, salinity and elevation of the three models. The forecast error is estimated by an ensemble run of 200 members by perturbing initial condition and atmospheric forcings. The 50 dominant empirical orthogonal functions (EOF) are taken as the error covariance of the model forecast. This error covariance is assumed to be constant in time. Sea surface temperature (SST) and sea surface height (SSH) are assimilated in this system. (c) 2006 Elsevier B.V. All rights reserved. [less ▲]

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See detailForecast verification of a 3D model of the Mediterranean Sea. The use of discrete wavelet transforms and EOFs in the skill assessment of spatial forecasts
Alvera Azcarate, Aïda ULg; Barth, Alexander ULg; Ben Bouallegue, Zied et al

in Journal of Marine Systems (2007), 65(1-4), 460-483

The quality assessment of a nested model system of the Mediterranean Sea is realised. The model has two zooms in the Provencal Basin and in the Ligurian Sea, realised with a two-way nesting approach. The ... [more ▼]

The quality assessment of a nested model system of the Mediterranean Sea is realised. The model has two zooms in the Provencal Basin and in the Ligurian Sea, realised with a two-way nesting approach. The experiment lasts for nine weeks, and at each week sea surface temperature (SST) and sea level anomaly are assimilated. The quality assessment of the surface temperature is done in a spatio-temporal approach, to take into account the high complexity of the SST distribution. We focus on the multi-scale nature of oceanic processes using two powerful tools for spatio-temporal analysis, wavelets and Empirical Orthogonal Functions (EOFs). We apply two-dimensional wavelets to decompose the high-resolution model and observed SST into different spatial scales. The Ligurian Sea model results are compared to observations at each of those spatial scales, with special attention on how the assimilation affects the model behaviour. We also use EOFs to assess the similarities between the Mediterranean Sea model and the observed SST. The results show that the assimilation mainly affects the model large-scale features, whereas the small scales show little or no improvement and sometimes, even a decrease in their skill. The multiresolution analysis reveals the connection between large- and small-scale errors, and how the choice of the maximum correlation length of the assimilation scheme affects the distribution of the model error among the different spatial scales. (c) 2006 Elsevier B.V. All rights reserved. [less ▲]

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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 ULg; Alvera Azcarate, Aïda ULg; 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 ▲]

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See detailMultivariate reconstruction of missing data in sea surface temperature, chlorophyll, and wind satellite fields
Alvera Azcarate, Aïda ULg; Barth, Alexander ULg; Beckers, Jean-Marie ULg et al

in Journal of Geophysical Research. Oceans (2007), 112(C3), 03008

An empirical orthogonal function–based technique called Data Interpolating Empirical Orthogonal Functions (DINEOF) is used in a multivariate approach to reconstruct missing data. Sea surface temperature ... [more ▼]

An empirical orthogonal function–based technique called Data Interpolating Empirical Orthogonal Functions (DINEOF) is used in a multivariate approach to reconstruct missing data. Sea surface temperature (SST), chlorophyll a concentration, and QuikSCAT winds are used to assess the benefit of a multivariate reconstruction. In particular, the combination of SST plus chlorophyll, SST plus lagged SST plus chlorophyll, and SST plus lagged winds have been studied. To assess the quality of the reconstructions, the reconstructed SST and winds have been compared to in situ data. The combination of SST plus chlorophyll, as well as SST plus lagged SST plus chlorophyll, significantly improves the results obtained by the reconstruction of SST alone. All the experiments correctly represent the SST, and an upwelling/downwelling event in the West Florida Shelf reproduced by the reconstructed data is studied. [less ▲]

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See detailError analysis of a high-resolution physical model of the Mediterranean Sea
Vandenbulcke, Luc ULg; Barth, Alexander ULg; Alvera Azcarate, Aïda ULg 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 ▲]

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See detailA Nested Model of the Cariaco Basin: Study of the Hydrography and Interactions with the Open Ocean
Alvera Azcarate, Aïda ULg; Barth, Alexander ULg; 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 ▲]

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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 ULg; Barth, Alexander ULg; 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 ▲]

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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 ▲]

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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 ▲]

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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 ▲]

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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 ▲]

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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 ▲]

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