Lagrangian dispersion of Argos surface drifters and model simulated trajectories
Kristofer Döös
Stockholm University
(Abstract received 06/24/2009 for session B)
ABSTRACT
The Lagrangian dispersion from model trajectories and Argos surface drifters are computed. The model trajectories are based on the TRACMASS trajectory code and driven by the ocean general circulation model NEMO. Pairs of Argos drifters are selected and the corresponding average dispersion has been calculated. Different model resolutions are used in order to evaluate to what extent the model resolution affects the Lagrangian dispersion. The model trajectory solutions always include implicit large scale diffusion due to along-trajectory changes of temperature and salinity/humidity and by the models parameterisation of turbulent mixing in the momentum equations. These trajectories do not, however, explicitly include sub-grid scale turbulence since they are passively advected by the simulated currents on the model grid. There are however two ways to incorporate sub-grid scale turbulence in TRACMASS, which are here tested to investigate the possibility of tuning the dispersion rate of the model trajectories to that of the Argos surface drifters. One consists of adding a random horizontal turbulent velocity (u',v') to the horizontal velocity (U,V) from the GCM. The other adds a random displacement to the trajectory position in order to incorporate a sub-grid parameterisation of the non resolved scales. Both methods enable an increase of the dispersion rate that can be adjusted by changing the diffusion coefficients of the trajectory sub-grid parameterisations.
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