Assimilation of future ocean-current measurements from satellites

在接下来的几年里，太空机构计划发射能够测量海洋表面洋流速度的卫星仪器。利用数据同化将这种测量结果吸收到海洋模型中，有可能提高对海洋本身和通过耦合建模对大气的预报的准确性。为了实现这一点，有必要建立一个同化系统，将速度的旋转部分和发散部分分开处理，因为这两个部分的误差可以假设是近似不相关的。这通常在大气预报系统中完成，其中风在同化方案中分解为流函数和速度势。然而，海洋模型中海岸线的存在使得这对海洋来说更加困难。将海流分解为旋转和发散部分需要解一个椭圆方程，这个问题在存在海岸边界条件时是不适定的。文献中用不同的假设提出了克服这一问题的方法，但没有一种方法在海洋资料同化系统中实现。在这个项目中，我们将研究以下问题：-如何以一种数值稳定的方式将海洋速度场分解为旋转和发散部分？-这种分解是否会导致错误不相关的字段？是否有更好的分解方法可以使用，例如基于潜在涡度？-如何利用这种分解来吸收未来的海流测量值？

Over the coming years space agencies plan to launch satellite instruments that can measure the ocean surface current velocity. Ingesting such measurements into ocean models using data assimilation has the potential to increase accuracy of forecasts of the ocean itself, and of the atmosphere through coupled modelling. In order to achieve this, it is necessary to build an assimilation system that treats the rotational and divergent parts of the velocity separately, since then the errors in these two components can be assumed to be approximately uncorrelated. This is commonly done in atmospheric forecasting systems, where the wind is decomposed into the streamfunction and velocity potential within the assimilation scheme. However, the presence of coastlines in ocean models makes this much harder to do for the ocean. The decomposition of the currents into their rotational and divergent parts requires the solution of an elliptic equation, a problem that is ill-posed in the presence of coastal boundary conditions. Methods to overcome this problem have been proposed in the literature with different assumptions, but none have been implemented within ocean data assimilation systems. In this project we will investigate the following questions: - How can ocean velocity fields be decomposed into their rotational and divergent parts in a way that is numerically stable?- Does this decomposition lead to fields whose errors are uncorrelated? Are there better decompositions to use, e.g. based on potential vorticity? - How can such decompositions be used to assimilate future ocean current measurements?