Sion, Huang and Cheng are developing a dynamic model for white dwarf-disk interaction in the boundary layers, which is urgently needed to quantitatively understand the properties of the observed accretion belts. By applying the energy criteria characterized by the Richardson number, , we found that in a differentially rotating envelope of the white dwarf, the mixing caused by the forced convection is most efficient on equi-potential surfaces, along which . To simulate these mixing processes, we use the equipotential surfaces as one family of our coordinate surfaces. Since the rotational speed distribution as well as the shape of the surfaces in the white dwarf envelope varies with time, so do our coordinate surfaces. Further results from our code using this time-dependent zoning technique will be reported in due time.