Using the SKAGEX dataset for evaluation of ocean model skills

Numerical ocean models are now being applied in numerous oceanographic stud ies. However, the qualities of the model results are often uncertain and th ere is a great need for standards and procedures for evaluation of the skil ls of numerical general circulation models. In this paper measurements from repeated hydrographical sections across Skagerrak taken in 1990, the SKAGE X dataset, are used to evaluate the skills of two cr-coordinate ocean model s and to study the sensitivity of these models to model parameters. A metho dology for quantification of model skills based on observations from repeat ed hydrographical sections in general is suggested. Area averages of absolu te differences are for Skagerrak completely dominated by the discrepancies in the upper few meters of the ocean and may not be used to assess models' abilities to reproduce the fields in the larger and deeper part of the ocea n. Therefore, discrepancies between average values in time from the observe d fields and time averaged values from model outputs are related to the nat ural variability of the fields. The numbers produced with the suggested mea sure are relative numbers that will be specific for each section and for ea ch series of observation. Ideally we would therefore like to see the measur es computed for a number of sections for various models and choices of mode l parameters in order to assess model skills. The value of the SKAGEX datas et as a tool for model improvements is demonstrated. Evidence to support th e importance of applying non-oscillatory, gradient preserving advection sch emes in areas with sharp density fronts is given. The method is used to ide ntify that the forcing/initial values/boundary values for the temperature f ield are inferior to the corresponding values for the salinity field. With the present coarse resolution, 11 layers in the vertical, it is shown that it is far from obvious that the quality of the model results improve when r eplacing simple Richardson number formulations for vertical mixing processe s with higher order turbulence closure in the Skagerrak. area. (C) 1999 Els evier Science B.V. All rights reserved.