A mass drift term can be defined as
Generally s (i.e. a relaxation time of 6 hours). The over-bars in Eq. 121 represent spatial averages over the entire domain. Thus this relaxation has no impact on the pressure gradients, but the average value of the mass stays close to that of the large scale model. Note that this term is turned off (i.e. ) in the global grid application. Integrating the mass drift term in the vertical gives Note that again since the flux form equations are being used, for consistency, all of the other prognostic equations should include the relaxation term thus the mass diffusion/filter term is expressed (defined in Eq. 123) as Note again that this mass drift term can be turned off by setting . If the mass drift term is off, then (vanishes) in the prognostic equations.