Consistency of residence time calculations based on Lagrangian trajectory analysis in ocean modeling

The consistency of residence time calculations for Lagrangian particles is investigated based on simulations of near-stationary flow within an idealized model area consisting of a channel with a semicircular bay incursion. Variation in residence time is analyzed with respect to two modeling parameters; the horizontal grid resolution of the hydrodynamic model and the representation of particle diffusion in the Lagrangian trajectory model. It is found that residence time calculations are highly sensitive to grid resolution effects if the dominating flow features are insufficiently resolved. Particle diffusion contribute to decrease the residence time in under-resolved model simulations. The residence time calculations for non-diffusive and diffusive trajectory simulations converge when the grid resolution is sufficiently high to represent the dominating flow features, but the gap between predictions for different diffusion models remains significant even for the best resolved model simulations in this study. However, analysis of 5 day aggregated results show some consistent trends, particularly for the rate of particle loss at the initial stage of the simulations.