                              

         dt_u, dt_v,  -- real, dimension(:,:,:)
             Tendency of u and v
             shape must conform to that of u, v etc
             input is the tendency due to all other processes
                 that have been added prior to this call
             output will be the tendency modified by the 
                 effects of diffusion 
             Units are [xi]/s where [xi] = [u], [v] etc

        dt_tr -- real, dimension(:,:,:,:)
             Tracer tendency (se