   tau_u(i+1) = tau_u(i or i-1) + delt*dtau_du*dt_u(:,:,N)
                 where dt_u is the final (output) tendency and 
                 N is the lowest model level; similarly for tau_v

         flux_tr -- real, dimension(:,:,:) 
                   with size(flux,tr_3) = number of tracers
                   (although listed as inout, the implicit treatment of 
                    tracer fluxes is not implemented here, so the 
                    ouput tracer flux will be identical to the input flux
    