Observation, parameterization and simulation of turbulent mixing in the Gulf of Finland, the Baltic Sea

Three time series of shear microstructure measurements (duration 13, 24 and 14 h respectively) were performed in 3 different wind forcing regimes as well as in 3 different background density stratification and current velocity shear situations at the entrance to the Gulf of Finland, in July 1998. Vertical shear of current velocity was enhanced by near-inertial waves during the first (A1) and third (A3) time series. We compared a Richardson number based parameterization and an estimation using the two equation k-ɛ turbulence closure (General Ocean Turbulence Model, GOTM) with “measured” eddy diffusivities. For two out of the three time series eddy diffusivities calculated via a Richardson number parameterization and via simulation using the k-ɛ model agreed well with the experimental data. However, in the case of relatively high current velocity shear and weak background density stratification (time series A3) both applied methods resulted in a remarkable discrepancy against the measured eddy diffusivity. On the contrary, calculations with a new parameterization scheme, which considers the internal wave kinetic energy fitted well for all three time series. Similarly the modified k-ɛ simulations which considered the internal wave energy level better matched the measured profiles.