Advanced Ocean Modelling: Using Open-Source Software

136 5 3D Level ModellingFig. 5.8 Example of the implementation of no-slip conditions for flow parallel to coastlinesA field of Eulerian tracer concentration is added for visualisation of the fluiddynamics. To this end, concentrations of unity are allocated initially on one side ofthe dashed line in Fig. 5.7, whereas zero values are assigned on the other side. Patternsevolving in this tracer field are indicative of possible sea ice patterns formingin this region. The assumption here is that sea ice operates as a passive tracer whichis only valid for young stages of the ice-formation process; that is, before a solidice sheet has been formed. The total simulation time is 10 days with a numericaltime step of Δt = 45 s. The pressure accuracy for the S.O.R. simulation is set toɛ =0.01Pa.5.4.4 Creation of Variable BathymetryVariable bathymetry can be created via prescription of initial coastlines and blocktyperegions of certain water depths and the use of a diffusion equation for subsequentsmoothing. The diffusion equation is given by:∂h∂t( ∂ 2 )h= κ∂x + ∂2 h2 ∂y 2(5.15)where the diffusion coefficient κ and the duration of smoothing are adjusted suchthat the result is acceptable. Coastlines and land should not disappear during the process.This can be implemented in the code via the choice of zero-gradient conditionsat the borders between dry and wet grid cells.5.4.5 ResultsThe inflow passes through the Soya Strait and continues as a coastal boundary currentalong the coast of Hokkaido (Fig. 5.9). The current approaches a maximumspeed of 1.0–1.3 m/s at a distance of 20–25 km from the coast, which agrees withobservational evidence. The barotropic flow (baroclinic effects are eliminated viachoice of uniform density) becomes dynamically unstable and forms meanders in