the value within the lowest atmospheric
              layer of the field being diffused)

  gcm_vert_diff  : 
              adds tendency due to vertical diffusion to tendencies
              of temperature, specific humidity, two components of 
              momentum, and mixing ratios of any # of other tracers,
              to be used only if surface fluxes are not implicitly 
              dependent on surface parameters (the surface heat flux and
              evaporation can be implicit functions of a