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Horizontal Viscosity

An explicit viscosity diffusion is included in the model through variables viscx and viscy (set in subroutine DriveInit). As in GCMs this crudely parameterizes non-resolved eddy motion and helps keep numerics stable. For moisture q, the viscosity set separately through variables viscxq and viscyq, which, by fault, are 3 times viscx and viscy. The larger coefficient for moisture is partly a numerical consideration, since the model sometimes produces very sharp convergence zones. Physically, we note that the model has less high frequency motion that might act like a diffusion in moisture, while temperature is in any case spread horizontally by wave dynamics. This viscosity is equivalent to the tex2html_wrap_inline1527 coefficient in the advection-diffusion operators (e.g. equation 5.11 and 5.12 in Neelin and Zeng [1]), with one subtle difference. In the model, instead of making tex2html_wrap_inline1527 isotropic, tex2html_wrap_inline1527 is specified separately for the x and y-directions. Thus, the diffusion term tex2html_wrap_inline1537 is actually represented as tex2html_wrap_inline1539 in the model.



Johnny Wei-Bing Lin
Thu Sep 9 13:11:12 PDT 1999