Evaluation of two model versions of the Oslofjord with different grid resolutions

By Peter Isachsen

University of Oslo Department of Geoscience: Meterology and Oceanography Section

We consider results from a one year simulation in the Oslofjord area, produced by an 800- and 300- meter versions of the ROMS model. The two versions differ only in resolution and grid orientation. Our objective is to evaluate similarities and differences between the two versions. To this end we study current patterns, kinetic energy levels and last, but not least, particle trajectories. By doing this we quantify the effect of an increased grid resolution, enabling us to assess what size resolution is required for simulations of the fjord dynamics. We find that a higher skill regarding the representation of bathymetry and coastal geometry in the model grid enables resolution of a smaller, stronger and more confined eddies, causing the higher resolution model to be more energetic. Moreover, simulations in areas where the two model grids display diverging coastal geometry and bathymetry  produce diverging particle trajectories. In contrast simulations in more open regions where the relative difference in bathymetry is smaller yield more similar trajectories.

Increased grid resolution has several effects:

1. the model’s skill in representing the rugged coastal geometry and topograhy is increased
2. the model’s ability to represent smaller scale whirls and eddies behind island and promonteries is increased
3. current jets become narrower and eddies become more abundant and confined

Combined these effects have a major impact on particle trajectories. Hence drift and spreading of oil, drift of floating objects (search and rescue), dispersion of contaminants, etc., all appear to be highly sensitive to the model’s grid resolution.»

Les rapporten; 2014-06-02_Master_PI_final
Les presentasjon:master_pres_new.v2