Surface drifters in the German Bight: Model validation considering windage and Stokes drift

Abstract

Six surface drifters (drogued at about 1g m depth) deployed in the inner German Bight (North Sea) were tracked for between 9 and 54 days. Corresponding simulations were conducted offline based on surface currents from two independent models (BSHcmod and TRIM). Inclusion of a direct wind drag (0.6g % of 10g m wind) was needed for successful simulations based on BSHcmod currents archived for a 5g m depth surface layer. Adding 50g % of surface Stokes drift simulated with a third-generation wave model (WAM) was tested as an alternative approach. Results resembled each other during most of the time. Successful simulations based on TRIM surface currents (1g m depth) suggest that both approaches were mainly needed to compensate insufficient vertical resolution of hydrodynamic currents. The study suggests that the main sources of simulation errors were inaccurate Eulerian currents and lacking representation of sub-grid-scale processes. Substantial model errors often occurred under low wind conditions. A lower limit of predictability (about 3-5g kmĝ€...dayg '1) was estimated from two drifters that were initially spaced 20g km apart but converged quickly and diverged again after having stayed at a distance of 2g km or less for about 10 days. In most cases, errors in simulated 25g h drifter displacements were of similar order of magnitude.