     q -- real, dimension(:,:,:)   specific humidity 
                                         (non-dimensional -- Kg water/Kg air)
         tr -- real, dimension(:,:,:,:) tracers
                                          (units arbitrary)
                                        size(tr,4) = number of tracers
         
              fields to be diffused, 
                  at time step i   for   two-time level schemes
                  at time step i-1 for three-time level (leapfrog) schemes
              