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Local ensemble assimilation scheme with global constraints and conservation

机译：具有全局约束和保护的局部集成同化方案

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摘要

Ensemble assimilation schemes applied in their original, global formulation respect linear conservation properties if the ensemble perturbations are set up accordingly. For realistic ocean systems, only a relatively small number of ensemble members can be calculated. A localization of the ensemble increment is therefore necessary to filter out spurious long-range correlations. The conservation of the global properties will be lost if the assimilation is performed locally, since the conservation requires a coupling between all model grid points which is removed by the localization. The distribution of ocean observations is often highly inhomogeneous. Systematic errors of the observed parts of the ocean state can lead to spurious adjustment of the non-observed parts via data assimilation and thus to a spurious increase or decrease in long-term simulations of global properties which should be conserved. In this paper, we propose a local assimilation scheme (with different variants and assumptions) which can satisfy global conservation properties. The proposed scheme can also be used for non-local observation operators. Different variants of the proposed scheme are tested in an idealized model and compared to the traditional covariance localization with an ad-hoc step enforcing conservation. It is shown that the inclusion of the conservation property reduces the total RMS error and that the presented stochastic and deterministic schemes avoiding error space rotation provide better results than the traditional covariance localization.
机译：如果相应地设置整体扰动，则在其原始的全局公式中应用的集合同化方案将遵循线性守恒性质。对于现实的海洋系统，只能计算相对较少的集合成员。因此，必须对整体增量进行定位，以滤除虚假的远程相关性。如果局部进行同化，则全局属性的守恒将丢失，因为守恒需要所有模型网格点之间的耦合，而该耦合会被定位移除。海洋观测资料的分布通常高度不均匀。海洋状态的观测部分的系统误差可能导致通过数据同化对未观测部分进行虚假调整，从而导致长期应模拟的全球特性虚假增加或减少，应予以保留。在本文中，我们提出了一种可以满足全球保护性质的局部同化方案（具有不同的变体和假设）。所提出的方案也可以用于非本地观测员。在理想化的模型中测试了所提出方案的不同变体，并与传统的协方差本地化（采用强制保留的临时步骤）进行了比较。结果表明，包含守恒属性可以减少总RMS误差，并且所提出的避免误差空间旋转的随机和确定性方案比传统的协方差局部化效果更好。

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