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See detailSuper-ensemble techniques applied to wave forecast: performance and limitations
Lenartz, Fabian ULg; Beckers, Jean-Marie ULg; Chiggiato, Jacopo et al

in Ocean Science (2010), 6(2), 595-604

Nowadays, several operational ocean wave forecasts are available for a same region. These predictions may considerably differ, and to choose the best one is generally a difficult task. The super-ensemble ... [more ▼]

Nowadays, several operational ocean wave forecasts are available for a same region. These predictions may considerably differ, and to choose the best one is generally a difficult task. The super-ensemble approach, which consists in merging different forecasts and past observations into a single multi-model prediction system, is evaluated in this study. During the DART06 campaigns organized by the NATO Undersea Research Centre, four wave forecasting systems were simultaneously run in the Adriatic Sea, and significant wave height was measured at six stations as well as along the tracks of two remote sensors. This effort provided the necessary data set to compare the skills of various multi-model combination techniques. Our results indicate that a super-ensemble based on the Kalman Filter improves the forecast skills: The bias during both the hindcast and forecast periods is reduced, and the correlation coefficient is similar to that of the best individual model. The spatial extrapolation of local results is not straightforward and requires further investigation to be properly implemented. [less ▲]

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See detailEnsemble perturbation smoother for optimizing tidal boundary conditions by assimilation of High-Frequency radar surface currents - application to the German Bight
Barth, Alexander ULg; Alvera Azcarate, Aïda ULg; Gurgel, Klaus-Werner et al

in Ocean Science (2010), 6(1), 161-178

High-Frequency (HF) radars measure the ocean surface currents at various spatial and temporal scales. These include tidal currents, wind-driven circulation, density-driven circulation and Stokes drift ... [more ▼]

High-Frequency (HF) radars measure the ocean surface currents at various spatial and temporal scales. These include tidal currents, wind-driven circulation, density-driven circulation and Stokes drift. Sequential assimilation methods updating the model state have been proven successful to correct the density-driven currents by assimilation of observations such as sea surface height, sea surface temperature and in-situ profiles. However, the situation is different for tides in coastal models since these are not generated within the domain, but are rather propagated inside the domain through the boundary conditions. For improving the modeled tidal variability it is therefore not sufficient to update the model state via data assimilation without updating the boundary conditions. The optimization of boundary conditions to match observations inside the domain is traditionally achieved through variational assimilation methods. In this work we present an ensemble smoother to improve the tidal boundary values so that the model represents more closely the observed currents. To create an ensemble of dynamically realistic boundary conditions, a cost function is formulated which is directly related to the probability of each boundary condition perturbation. This cost function ensures that the boundary condition perturbations are spatially smooth and that the structure of the perturbations satisfies approximately the harmonic linearized shallow water equations. Based on those perturbations an ensemble simulation is carried out using the full three-dimensional General Estuarine Ocean Model (GETM). Optimized boundary values are obtained by assimilating all observations using the covariances of the ensemble simulation. [less ▲]

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See detailSpatiotemporal variations of fCO(2) in the North Sea
Omar, A. M.; Olsen, A.; Johannessen, T. et al

in Ocean Science (2010), 6(1), 77-89

Data from two Voluntary Observing Ship (VOS) (2005-2007) augmented with data subsets from ten cruises (1987-2005) were used to investigate the spatiotemporal variations of the CO2 fugacity in seawater ... [more ▼]

Data from two Voluntary Observing Ship (VOS) (2005-2007) augmented with data subsets from ten cruises (1987-2005) were used to investigate the spatiotemporal variations of the CO2 fugacity in seawater (fCO(2)(sw)) in the North Sea at seasonal and inter-annual time scales. The observed seasonal fCO(2)(sw) variations were related to variations in sea surface temperature (SST), biology plus mixing, and air-sea CO2 exchange. Over the study period, the seasonal amplitude in fCO(2)(sw) induced by SST changes was 0.4-0.75 times those resulting from variations in biology plus mixing. Along a meridional transect, fCO(2)(sw) normally decreased northwards (-12 mu atm per degree latitude), but the gradient disappeared/reversed during spring as a consequence of an enhanced seasonal amplitude of fCO(2)(sw) in southern parts of the North Sea. Along a zonal transect, a weak gradient (-0.8 mu atm per degree longitude) was observed in the annual mean fCO(2)(sw). Annually and averaged over the study area, surface waters of the North Sea were CO2 undersaturated and, thus, a sink of atmospheric CO2. However, during summer, surface waters in the region 55.5-54.5 degrees N were CO2 supersaturated and, hence, a source for atmospheric CO2. Comparison of fCO(2)(sw) data acquired within two 1 degrees x1 degrees regions in the northern and southern North Sea during different years (1987, 2001, 2002, and 2005-2007) revealed large interannual variations, especially during spring and summer when year-to-year fCO(2)(sw) differences (approximate to 160-200 mu atm) approached seasonal changes (approximate to 200-250 mu atm). The springtime variations resulted from changes in magnitude and timing of the phytoplankton bloom, whereas changes in SST, wind speed and total alkalinity may have contributed to the summertime interannual fCO(2)(sw) differences. The lowest interannual variation (10-50 mu atm) was observed during fall and early winter. Comparison with data reported in October 1967 suggests that the fCO(2)(sw) growth rate in the central North Sea was similar to that in the atmosphere. [less ▲]

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See detailEnhancing temporal correlations in EOF expansions for the reconstruction of missing data using DINEOF
Alvera Azcarate, Aïda ULg; Barth, Alexander ULg; Sirjacobs, Damien ULg et al

in Ocean Science (2009), 5(4), 475-485

DINEOF (Data Interpolating Empirical Orthogonal Functions) is an EOF-based technique for the reconstruction of missing data in geophysical fields, such as those produced by clouds in sea surface ... [more ▼]

DINEOF (Data Interpolating Empirical Orthogonal Functions) is an EOF-based technique for the reconstruction of missing data in geophysical fields, such as those produced by clouds in sea surface temperature satellite images. A technique to reduce spurious time variability in DINEOF reconstructions is presented. The reconstruction of these images within a long time series using DINEOF can lead to large discontinuities in the reconstruction. Filtering the temporal covariance matrix allows to reduce this spurious variability and therefore more realistic reconstructions are obtained. The approach is tested in a three years sea surface temperature data set over the Black Sea. The effect of the filter in the temporal EOFs is presented, as well as some examples of the improvement achieved with the filtering in the SST reconstruction, both compared to the DINEOF approach without filtering. [less ▲]

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See detailDynamically constrained ensemble perturbations - application to tides on the West Florida Shelf
Barth, Alexander ULg; Alvera Azcarate, Aïda ULg; Beckers, Jean-Marie ULg et al

in Ocean Science (2009), 5(3), 259-270

A method is presented to create an ensemble of perturbations that satisfies linear dynamical constraints. A cost function is formulated defining the probability of each perturbation. It is shown that the ... [more ▼]

A method is presented to create an ensemble of perturbations that satisfies linear dynamical constraints. A cost function is formulated defining the probability of each perturbation. It is shown that the perturbations created with this approach take the land-sea mask into account in a similar way as variational analysis techniques. The impact of the land-sea mask is illustrated with an idealized configuration of a barrier island. Perturbations with a spatially variable correlation length can be also created by this approach. The method is applied to a realistic configuration of the West Florida Shelf to create perturbations of the M2 tidal parameters for elevation and depth-averaged currents. The perturbations are weakly constrained to satisfy the linear shallow-water equations. Despite that the constraint is derived from an idealized assumption, it is shown that this approach is applicable to a non-linear and baroclinic model. The amplitude of spurious transient motions created by constrained perturbations of initial and boundary conditions is significantly lower compared to perturbing the variables independently or to using only the momentum equation to compute the velocity perturbations from the elevation. [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 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 ▲]

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