This section was contributed by Martin Schmidt ( mschmidt@paula.io-warnemuende.de).
Option explicit_free_surface, along with one of the two
options explicit_eb or explicit_efb enables the method
introduced by Killworth, Stainforth, Webb and Paterson (1991). The
free sea surface elevation is treated as an additional prognostic
variable calculated together with the vertically integrated barotropic
velocity. One major difference between the implicit free surface and
explicit free surface code is a small timestep used to resolve the
linear components of the barotropic equations. The reason for the
smaller time steps in the explicit free surface is due to the
allowance of external gravity waves (with wave speeds on the order of
). Note that the method has been found suitable
also for open boundary conditions where measured sea levels are
prescribed.