NUMERICAL MODELING OF THE HALTEN BANK AREA - A VALIDATION-STUDY

From observations and simple model simulations, it has been establishe d that a proper model of the Halten Bank area must, as a minimum, (i) include the barotropic component and at least one baroclinic component , (ii) allow shear instabilities to properly grow into eddies, (iii) i nclude vertical mixing to allow for entrainment of lower layer water m asses into the upper layer, (iv) allow for advection of density (tempe rature and salinity) in the upper water masses, and (v) allow the incl usion of tides. All of these physics factors are included in the mathe matical formulation of fully three-dimensional numerical ocean models. Two such models, arbitrarily named the POM model and the SINMOD model , have been used to simulate the current, salinity and temperature str ucture in the Halten Bank area for March 1988 and then validated again st the Norwegian Continental Shelf Experiment 1988 (NORCSEX'88) data s et. Although the motion in the two models is governed by the same set of continuous equations, an inspection of the parameterizations and nu merical methods employed to solve the governing equations reveals that the SINMOD model should be expected to provide smoother and less ener getic results than the POM model. It is also speculated that eddy grow th is inhibited in both models due to (i) the tendency of level models to create fictitious diapycnal mixing in areas of sloping density sur faces, and (ii) too coarse resolution. These conclusions are partially supported by the validation analysis. In order to facilitate a meanin gful model-model comparison and validation. specific products were pro vided by careful analyses of both data and model results in accordance with a preselected hierarchic set of comparison criteria. The criteri a focus on mean circulation patterns and horizontal and vertical distr ibutions. The analyses reveal that the most probable limitation inhibi ting the models in reproducing the observed structures is the lack of horizontal resolution. The chosen grid mesh size (4 km) only barely re solves the eddy scale dictated by the Rossby radius of deformation, th us inhibiting eddy growth due to shear instabilities. Further, both th is and earlier studies conclude that it is an open question how comple x a model needs to be to be able to simulate the observed structure to a satisfactory degree.