de of this module, then used to start upward
               tridiagonal sweep,
               see  <A HREF="vert_diff.tech.ps">vert_diff.tech.ps</A>

     delta_q = similarly for the increment in specific humidity 
                (non-dimensional  = Kg/Kg)

 
==========================================================================

 subroutine gcm_vert_diff_down (is, js, delt,                              &
                          u, v, t, q, tr,                                  &
                     