Abstract
Abstract. Traditionally, in order for lower-resolution, global- or basin-scale (regional) models to benefit from some of the improvements available in higher-resolution subregional or coastal models, two-way nesting has to be used. This implies that the parent and child models have to be run together and there is an online exchange of information between both models. This approach is often impossible in operational systems where different model codes are run by different institutions, often in different countries. Therefore, in practice, these systems use one-way nesting with data transfer only from the parent model to the child models. In this article, it is examined whether it is possible to replace the missing feedback (coming from the child model) by data assimilation, avoiding the need to run the models simultaneously. Selected variables from the high-resolution simulation will be used as pseudo-observations and assimilated into the low-resolution models. This method will be called “upscaling”. A realistic test case is set up with a model covering the Mediterranean Sea, and a nested model covering its north-western basin. Under the hypothesis that the nested model has better prediction skills than the parent model, the upscaling method is implemented. Two simulations of the parent model are then compared: the case of one-way nesting (or a stand-alone model) and a simulation using the upscaling technique on the temperature and salinity variables. It is shown that the representation of some processes, such as the Rhône River plume, is strongly improved in the upscaled model compared to the stand-alone model.
Highlights
In the present-day operational oceanography landscape, services are provided at different scales by different expert centres
There are large temperature differences at the shelf break of the Gulf of Lion, which extend all the way from the surface to the bottom of the Gulf of Lion
Other large differences appear in the Eastern and Western Corsican currents and their junction resulting in the Northern Current as well as at the southern open boundary
Summary
In the present-day operational oceanography landscape, services are provided at different scales by different expert centres. At the European Union level, the Copernicus Marine Environment Monitoring Service (CMEMS) provides reanalyses, analyses and forecasts at global and basin scales. The models for the different basins are run by different institutes and centres within the regional monitoring and forecasting centres. Various oceanographic centres use the CMEMS products to provide initial and/or boundary conditions to their respective models. These subregional and coastal models benefit from the specific knowledge of the local teams in their particular area of interest. Nested models usually run at higher resolution and may include more accurate data (bathymetry, river discharge data, etc.) and processes of smaller scales that cannot be included in basin-scale models. Highresolution observations of currents by high-frequency radars are expected to benefit models with a similar high resolution (i.e. nested models) most
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