Abstract. A newly developed coastal model FESOM-C based on three-dimensional unstructured meshes and finite volume is applied to simulate dynamics of the southeastern part of the North Sea. Variable horizontal resolution enables using meshes that are coarse in the open sea but refined in the shallow areas (which include the Wadden Sea and the estuaries) to resolve important small-scale process (such as wetting and drying, sub-mesoscales eddies and dynamics of steep coastal fronts). Model results for the simulation for the period between January 2010 and December 2014 agree reasonably well with data from numerous autonomous observation stations with high temporal and spatial resolution, located in the region, data from ferry boxes and glider expeditions. The analysis of numerical solution convergence on meshes with different horizontal resolutions allows identifying areas where high mesh resolution (wetting and drying zones, shallow areas) and low mesh resolution (open boundary, open sea, and deep regions) are optimal for numerical simulations.