tmospheric
              temperature and specific humidity, respectively, in the 
              lowest atmospheric layer).  Diffusivities for 
              specific humidity, heat, and all tracers are assumed to be 
              equal.

  gcm_vert_diff_down :  Downward sweep of tridiagonal solver

  gcm_vert_diff_up   :  Upward sweep of tridiagonal solver

  surf_diff_type : type defined to hold fields required outside of this
                   module when the dependence of surface fluxes on surface 
   