Multigrid state vector for data assimilation in a two-way nested model of the Ligurian Sea

Barth, Alexander; Alvera‐Azcárate, Aida; Beckers, Jean-Marie; Rixen, M.; Vandenbulcke, Luc

doi:10.1016/j.jmarsys.2005.07.006

Cited by 35 publications

(25 citation statements)

References 38 publications

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“…The geostrophic equilibrium is often used to model error covariances in data assimilation (e.g. Dee, 1991;Brankart et al, 2003) and also to compute perturbations of the velocity based on the perturbation of the density field (Barth et al, 2007a). Equations (25)- (26) are more complete than the geostrophic equilibrium since the former do not assume small accelerations.…”

Section: Methodsmentioning

confidence: 99%

“…This method is computationally very efficient since it can be implemented using the Fast Fourier Transform. Indeed, it can be shown that for any translation-invariant covariance matrix, its eigenfunctions are the Fourier modes (Barth et al, 2007a). In some circumstances, the approach might be appropriate for atmospheric fields and also for oceanographic fields far from the coastline.…”

Section: Impact Of the Land-sea Mask And Correlation Lengthmentioning

confidence: 99%

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Dynamically constrained ensemble perturbations – application to tides on the West Florida Shelf

et al. 2009

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Abstract.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 shallowwater 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.

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Section: Methodsmentioning

confidence: 99%

Section: Impact Of the Land-sea Mask And Correlation Lengthmentioning

confidence: 99%

Dynamically constrained ensemble perturbations – application to tides on the West Florida Shelf

et al. 2009

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“…We then face however a) the problem of specifying the correlation functions and b) the problem of inverting the covariance matrices. In some cases (e.g., Barth et al, 2006), when errors are correlated at a prescribed scale L, an intermediate complexity approach can be implemented: first, OI is performed with the full error-covariance matrix to serve as a reference solution; then the analysis is repeated with a diagonal error-covariance matrix where the diagonal is, compared to the non-diagonal version, inflated by a factor r . It turns out numerically that when the inflation factor L 2 / x/ y is used, the analysis is closest to the reference solution.…”

Section: Least-square Fits and Optimal Interpolationmentioning

confidence: 99%

“…increase the error variance) of the observations. It can be shown (e.g., Barth et al, 2006), that the error variance must be multiplied by the number r of redundant (or strongly correlated) observations:…”

Section: Error Estimationmentioning

confidence: 99%

DINEOF reconstruction of clouded images including error maps – application to the Sea-Surface Temperature around Corsican Island

2006

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Abstract. 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.

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“…Let's note that since the observations assimilated in the coarse grid are still physically located in the Gulf of Lions, information should be transported to the intermediate-resolution model through the boundary conditions, and hereafter transported to the high-resolution model. -Moreover, we will define a fifth case, based on the work of Barth et al (2006). In this case, all 3 state vectors from the 3 grids are assembled in a unique state vector.…”

Section: Comparison Of Different Setupsmentioning

confidence: 99%

Study of the combined effects of data assimilation and grid nesting in ocean models – application to the Gulf of Lions

Vandenbulcke¹,

Barth

Rixen

et al. 2006

Ocean Sci.

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Abstract. 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 opensea 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.

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Multigrid state vector for data assimilation in a two-way nested model of the Ligurian Sea

Cited by 35 publications

References 38 publications

Dynamically constrained ensemble perturbations – application to tides on the West Florida Shelf

Dynamically constrained ensemble perturbations – application to tides on the West Florida Shelf

DINEOF reconstruction of clouded images including error maps – application to the Sea-Surface Temperature around Corsican Island

Study of the combined effects of data assimilation and grid nesting in ocean models – application to the Gulf of Lions

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