An explicit viscosity diffusion is included in the model
through variables viscx and viscy
(set in subroutine DriveInit). As in GCMs this crudely parameterizes
non-resolved eddy motion and helps keep numerics stable.
For moisture q, the viscosity set separately
through variables viscxq and viscyq, which are larger than
viscx and viscy.
The larger coefficient for moisture is partly a numerical consideration,
since the model sometimes produces very sharp convergence zones. Physically,
we note that the model has less high frequency motion that might act like
a diffusion in moisture, while temperature is in any case spread horizontally
by wave dynamics.
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.