Assimilation studies of open‐ocean flows: 1. Estimation of initial and boundary conditions

Gunson, James Reginald; Malanotte‐Rizzoli, Paola

doi:10.1029/96jc02781

Cited by 19 publications

(4 citation statements)

References 39 publications

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“…Moreover, the time dependency of the open-boundary conditions is well adapted to variational analysis, which considers the space and time evolutions of the system as a whole. Such an approach has been applied in quasigeostrophic models of regional basins (Seiler 1993;Gunson and Malanotte-Rizzoli 1996). Continuing on these latter works, a data-assimilation method based on the rigid-lid OPA primitive-equation model is developed in order to estimate the initial and time-dependent open-boundary conditions relative to a remote influence.…”

Section: Introductionmentioning

confidence: 99%

Controlling boundary conditions with a four-dimensional variational data-assimilation method in a non-stratified open coastal model

et al. 2004

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An innovative way to take the large-scale circulation influence into account in coastal primitiveequation models is explored by an inverse modelling approach. Restricted to barotropic external forcing, this work is a first step in the development of a fourdimensional variational (4DVAR) data-assimilation approach to estimate the best initial and open-boundary conditions that force a coastal model according to interior observations. This development is founded on the OPA modelling system which representation of barotropic coastal dynamics is restricted to motions of long time scales ($ a day) due to its ''rigid lid'' approximation. Twin experiments are performed in an academic configuration of the Gulf of Lions (located in the northwestern Mediterranean Sea) to study the sensitivity of a remote barotropic forcing to different observational networks measuring surface currents deployed in this area. Three monitoring designs are tested for a large-scale barotropic perturbation in the hindcast mode. It is shown that the space and time distribution of observations acts on the efficiency of the 4DVAR method and then allows coarser datasets.

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Section: Introductionmentioning

confidence: 99%

Controlling boundary conditions with a four-dimensional variational data-assimilation method in a non-stratified open coastal model

et al. 2004

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“…The feasibility of estimating open‐boundary conditions in quasi‐geostrophic (QG) models was shown by Moore [1991], Seiler [1993], and Nechaev and Yaremchuk [1994]. A number of investigators [e.g., Schröter et al , 1993; Gunson and Malanotte‐Rizzoli , 1996; Cong et al , 1998; Yaremchuk and Maximenko , 2002] used these QG models with resolution varying from 1/4° [ Schröter et al , 1993] to roughly 1/8° [ Cong et al , 1998] to estimate the eddy‐scale circulation over monthly timescales. In contrast, our goal here is to attempt regional state estimation with a general circulation model (GCM) and real observations over 1–2 years.…”

Section: Introductionmentioning

confidence: 99%

Strategies for nested and eddy‐permitting state estimation

2006

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[1] Both ocean process and prediction studies increasingly rely on state estimation (i.e., data assimilation) to provide the most complete representation of how the ocean circulates. This study applies the formalism and methodology of state estimation, recently developed for the global, coarse-resolution problem, to eddy-permitting state estimation in regional domains. Two major challenges exist for a state estimate that is nested inside a global state estimate: (1) estimation of open-boundary conditions consistent with information interior and exterior to the regional domain and (2) estimation with the higher-resolution models of regional studies. Here a least squares cost function defines the problem of minimizing the misfit between a North Atlantic regional general circulation model and actual observations, including those of the Subduction Experiment. A first experiment, using a novel ''multiscale'' method to constrain the large-scale regional circulation, shows that the use of the adjoint of both an eddy-permitting model and its coarse-resolution twin leads to a solution of the least squares problem in a computationally practical way. Therefore no fundamental obstacle exists to constraining the large-scale regional circulation nested within a global circulation. A second experiment in the North Atlantic shows that the model circulation can be constrained to the full observational signal, including eddy variability, as observed at selected point locations. Both experiments in this study produce eddy-permitting state estimates which are exactly self-consistent with the equations of motion as embodied by a general circulation model. Therefore dynamical balances can be diagnosed and easily interpreted; in particular, a companion paper uses the state estimates to determine eddy subduction rates in the North Atlantic.

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“…If we had postulated a normal distribution, we could have determined an a posterior covariance matrix from a discrete Hessian matrix, which can be calculated with the adjoint model (Marotzke and Wunsch, 1993;Schiller, 1995;Gunson and Malanotte-Rizzoli, 1996). But, from the mean parameter vector and the extensions of the acceptable parameter space we can see that the parameter solutions are not normally or equally distributed.…”

Section: Optimizationmentioning

confidence: 99%

Parameter estimates of a zero-dimensional ecosystem model applying the adjoint method

Schartau

Oschlies

Willebrand

2001

Deep Sea Research Part II: Topical Studies in Oceanography

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Assimilation experiments with data from the Bermuda Atlantic Time-series Study (BATS, 1989}1993) were performed with a simple mixed-layer ecosystem model of dissolved inorganic nitrogen (N), phytoplankton (P) and herbivorous zooplankton (H). Our aim is to optimize the biological model parameters, such that the mis"ts between model results and observations are minimized. The utilized assimilation method is the variational adjoint technique, starting from a wide range of "rst-parameter guesses. A twin experiment displayed two kinds of solutions, when Gaussian noise was added to the model-generated data. The expected solution refers to the global minimum of the mis"t model-data function, whereas the other solution is biologically implausible and is associated with a local minimum. Experiments with real data showed either bottom-up or top-down controlled ecosystem dynamics, depending on the deep nutrient availability. To con"ne the solutions, an additional constraint on zooplankton biomass was added to the optimization procedure. This inclusion did not produce optimal model results that were consistent with observations. The modelled zooplankton biomass still exceeded the observations. From the model-data discrepancies systematic model errors could be determined, in particular when the chlorophyll concentration started to decline before primary production reached its maximum. A direct comparision of measured C-production data with modelled phytoplankton production rates is inadequate at BATS, at least when a constant carbon to nitrogen C : N ratio is assumed for data assimilation.

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Assimilation studies of open‐ocean flows: 1. Estimation of initial and boundary conditions

Cited by 19 publications

References 39 publications

Controlling boundary conditions with a four-dimensional variational data-assimilation method in a non-stratified open coastal model

Controlling boundary conditions with a four-dimensional variational data-assimilation method in a non-stratified open coastal model

Strategies for nested and eddy‐permitting state estimation

Parameter estimates of a zero-dimensional ecosystem model applying the adjoint method

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