Contact: Baptiste Mourre

The use of altimetry for the observation of dynamical processes over continental shelves has up to now been very limited, notably because of the small time and space scales of the phenomena in these areas, which are not sufficiently sampled from space.

Looking to the future, the main objective of the study is to assess the capability of different altimeter scenarios to observe these processes. New altimeter technologies such as the Wide Swath Ocean Altimeter are considered, as well as projects of satellite constellations.

Due to the specificities of the coastal ocean, a combination of altimeter observations with numerical modeling seems necessary to get the best estimate of the state of the ocean in these areas. This is the reason why the approach is based on data assimilation. The model used in this work is the barotropic, finite-element MOG2D model, implemented over the entire European continental shelf. Moreover, our focus is on the specific high-frequency response of the ocean to meteorological forcing (wind and pressure).

A special attention is paid to properly specify model error statistics, which are required for assimilation experiments.

In particular, a detailed exploration of the model error subspace due to uncertainties in bathymetry is carried out via the use of an ensemble method. Model error covariances are shown to be neither homogeneous over shelves, nor isotropic when approaching the coast. They are not even stationary, since they are very dependent on the meteorological forcing.

An Ensemble Kalman Filter assimilation scheme is then implemented to assimilate sea level data in the model. Multiple twin experiments are performed to estimate the capabiliy of different observing scenarios to reduce model errors. The diagnostic is based on the reduction of the ensemble spread thanks to the assimilation. In particular, we investigate the contributions of the Wide Swath Ocean Altimeter and different satellite constellations. The complementarity of altimetry and tide gauges for controling the model trajectory in this particular context concerning the response of the ocean to meteorological forcing in the presence of bathymetric errors is also demonstrated.