Compute pressure (dbars) exerted at T cell grid point by weight of water
column between z=0 and grid point
ro = density in kg/m^3
psurf = surface pressure in kg/m/sec^2 = hydrostatic pressure
at z=0 associated with fluid between z=0 and z=eta_t
as well as atmospheric pressure patm.
hydrostatic_pressure
DESCRIPTION
Hydrostatic pressure [newton/m^2] at T cell grid points.
Integration here is from z=0 to depth of grid point. This is
the so-called "baroclinic" pressure. If input density "rho" is an anomoly,
p will be a hydrostatic pressure anomoly. If "rho" is full density,
p will be a full hydrostatic pressure.
pressure_gradient
DESCRIPTION
Gradient of hydrostatic pressure excluding the surface and atmospheric
pressures (i.e., we are computing here the gradient of the baroclinic
pressure). Account is taken of variable partial cell thickness.
1 = dp/dx; 2 = dp/dy
Thickness weight since this is what we wish to use in update of
the velocity.