     (units arbitrary)
                                        size(tr,4) = number of tracers
         
              input fields to be diffused, 
                  values at time step i   for   two-time level schemes
                  at time step i-1 for three-time level (leapfrog) schemes
              (t and q also used to compute density)

         diff_m -- real, dimension(:,:,:) 
             kinematic diffusivity for momentum (m*2/s)
         diff_t -- real, dimension(:,:,:) 
             kinematic