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

[摘要] Modern operational ocean forecasting systems routinely use data assimilationtechniques in order to take observations into account in the hydrodynamicmodel. Moreover, as end users require higher and higher resolutionpredictions, especially in coastal zones, it is now common to run nestedmodels, where the coastal model gets its open-sea boundary conditions from alow-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 runweekly, performing 1 week of hindcast and a 10-day forecast. Regional models,using different codes and covering different areas, then use this forecast toimplement boundary conditions. Local models in turn use the regional modelforecasts for their own boundary conditions. This nested system has proven tobe a viable and efficient system to achieve high-resolution weekly forecasts.However, when observations are available in some coastal zone, it remainsunclear whether it is better to assimilate them in the global or local model.We perform twin experiments and assimilate observations in the global or inthe local model, or in both of them together. We show that, when interestedin the local models forecast and provided the global model fields areapproximately correct, the best results are obtained when assimilatingobservations in the local model.

[学科分类] 海洋学与技术