To help keep the model numerically stable, an explicit viscosity is
added to the system through variables viscx and viscy
(set in subroutine DriveInit). This viscosity is equivalent to the
coefficient in the advection-diffusion operators (e.g. equation
5.11 and 5.12 in Neelin and Zeng [1]), with one
subtle difference. In the model, instead of making isotropic,
is specified separately for the x and y-directions. Thus,
the diffusion term 
is actually represented as
in the model.