.0002 



Figure 1. The region of application (shaded area) of 

 the System 21 Mark 8 model for short waves. 



interest so that the boundary effects will not influence results. However, it 

 should not be so large that excessive calculations are required. Certain 

 estimates can be made to determine an appropriate region size. One estimate 

 is related to the finite-difference grid that subdivides the region. This 

 grid is a set of equally spaced straight orthogonal lines with spacing (AX) 

 of at least 10 gridlines per wavelength in the region of interest. This sets 

 the approximate distance between the gridlines and helps in estimating the 

 region size that can be accommodated. 



A second estimate approximates the time step (At) of the model, expressed 



At = (10 to 15) 



AX 



(2gh) 



0.5 



(1) 



where g is gravity and h the mean depth. This states that the time step 

 could be about 10 to 15 times the Courant stability criteria for an explicit 

 model. However, it may not be practical to have a time step this large since 

 the time step might be larger than the wave period; therefore, the wave may 

 not be resolved in time . 



An estimate of the computer time needed for a System 21 Mark 8 model run 

 can be made using the estimates of the AX, At, size of the grid region, and 

 the length of the simulation. This is expressed as 



D ,. M X N X (T/At) , , . 

 Run time = (seconds) 



(2) 



