41 
where r signifies the bottom friction coefficient (r= 0.0025). 
HYDROSTATICS AND PRESSURE 
As a remnant of the equation of motion for the vertical component, there is hydrostatic 
equilibrium along the vertical 
The (hydrostatic) pressure is obtained by integrating this equation. With air pressure as 
boundary condition at the surface, one obtains 
where 
atmospheric pressure at water surface, 
water level elevation w.r.t. zero reference level (<^=0), 
gravity acceleration. 
As in many other models, pressure is split into a barotropic and a baroclinic part by the 
approximate decomposition 
0 
where p 0 is a fixed density, typical of surface water. 
CONTINUITY EQUATION 
The equation of continuity reflects the conservation of mass. At this point water is 
assumed to be incompressible, i.e. sound waves are filtered out. 
1 du 1 divcos (o) dw n 
+ _— ' — +— = 0 
RcostpdA R cos (p dcp dz 
Various integral forms can be derived. In our finite volume model, horizontal integration 
is performed over a rectangle in the x-cp plane while, in the vertical, segments are 
bounded by either z-levels or (inclined) bottom or moving surface. For the uppermost and 
lowermost cells, kinematic boundary conditions are taken into account as the water 
surface and bottom are flow surfaces. Summing up these finite expressions over a water 
column gives a finite form of the vertically integrated continuity equation 
It is this form which is used to determine the sea surface elevation.