An error decomposition method: Application to mediterranean sst simulations assessment

Fields composed of daily simulations provided by an OGCM of the Mediterranean Sea are compared to weekly satellite observations. The method used is inspired of the object-oriented verification procedure introduced in meteorological forecast assessment by Ebert and al. 2000. The Error Decomposition Method presented here aims to identify error sources.
The method is carried out within the framework of the MFSTEP hindcasts. The MFSTEP project is an international scientific collaboration program which aims to create an operational forecasting system for the Mediterranean Sea. The simulations provided at the basin scale are 10 days forecasting fields in a 3-D ocean. The hydrodynamic model primitive equations are combined with the data assimilation scheme SOFA applied every week. The set of data used for the comparison are weekly SST satellite observations and means of seven daily MFSTEP simulations (analyzed fields) for the equivalent weeks.
The original simulation is transformed until the total squared difference between the observed and hindcast fields is minimized. Successively, a new combination of seven consecutive daily simulations is produced, the new SST field is displaced horizontally and the bias suppressed. This allows a decomposition of the total error in 4 parts: a temporal shift error, a position error, an intensity error and a pattern error. This last element is the remaining error after simulation field transformation and corresponds to the unexplained error.

The method is applied at different restricted areas of the Mediterranean basin. The predominant displacements in time and in space minimizing error are discussed through the physical processes taking places at each location. More over, ratio between the different error components is analysed in term of scale effect: the role of the application domain size is pointed out. Finally, the seasonal impact on the different results is commented.