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

Abstract

Six surface drifters (drogued at about 1 m depth) deployed in the inner German Bight (North Sea) were tracked for between 14 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 (0.6 % of 10 m wind) was needed for successful simulations based on BSHcmod currents archived for a 5 m depth surface layer. Adding 50 % of surface Stokes drift simulated with the third generation wave model WAM was tested as an alternative approach. Results resembled each other during most of the 5 time. Successful simulations based on TRIM surface currents (1 m depth) suggest that both approaches were mainly needed to compensate insufficient vertical resolution of hydrodynamic currents.The study suggests that 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 3-5 km per day) was estimated from two drifters that were initially spaced 20 km apart but converged quickly and 10 diverged again after having stayed at a distance of 2 km and less for about 10 days. In most cases, errors in simulated 25 h drifter displacements were of similar order of magnitude.

Supplementary material to "Surface drifters in the German Bight: Model validation considering windage and Stokes drift"

“…Hourly fields of surface Stokes drift were simulated with the third generation spectral wave model WAM (WAMDI-Group, 1988;Komen et al, 1996), extending an existing wind-wave hindcast for the years 1949-2014 (Groll and Weisse, 2016) and including surface Stokes drift as a new element of archived model output. Wave simulations were driven with the same COSMO-CLM hindcast also used for simulations with TRIM.…”

“…Simulated absolute dispersion being too low was also reported by Kjellsson and Döös (2012) evaluating drifters deployed in the Baltic Sea. Referring to global ocean data, Döös et al (2011) tuned random turbulent velocity in their drift model to achieve better agreement between relative dispersion of simulated trajectories and corresponding observations. However, they found this approach being too simple for a reasonable reproduction of Lagrangian properties.…”

“…The minimum distance of only 800 m qualified the two drifters as a 'chance pair' (e.g. (Döös et al, 2011)). Note that drifters #6 and #8 were of different types (cf.…”

“…According to Koszalka et al (2009) and Döös et al (2011), 'chance pairs' should possibly be distinguished from pairs of drifters intentionally launched together, because their behaviour may depend on specific hydrodynamic conditions. An interesting question is what characterizes the 10 day period when drifters #6 and #8 stayed close together.…”

Response to Referee #2

“…The minimum distance of only 800 m qualified the two drifters as a 'chance pair' (e.g. (Döös et al, 2011)). Note that drifters #6 and #8…”