Hi, Igor Polyakov <[EMAIL PROTECTED]> is interested in mom4 for global tides. He soon ran into the problem of having the surface height get large and negative, so that h = dz + eta became less than some fraction of dz (dz is the time independent surface cell thickness, whereas h = dz + eta is the time dependent cell thickness). Inside ocean_freesurf_mod, there is a subroutine "eta_check" that computes the minimu "h" in the model. If "h" is less than some fraction of dz, then the model is brought down with the thought that there may be a numerical or forcing problem. In particular, for some inland seas, unless the fresh water forcing is balanced, then there can be a problem with inland seas drying out, thus having the model loose its surface cell thickness. mom4 does not have logic that allows for vanishing cell thickness, hence the purpose of the "eta_check" routine is to alert the modeler to the potential for problems.
There are two approaches for those wishing to run with large amplitude tides and with small dz (for those wishing to have a mixed layer). 1. frac_crit_cell_height is a namelist variable. It can be set to an arbitrary positive number from zero to 1. If you anticipitate a tidal fluctuation no larger than some fraction of dz, then set frac_crit_cell_height accordingly. 2. If you are studying widely fluctuating tides, then mom4 is strictly not appropriate for cases where h = dz + eta becomes negative. As a "fudge", the modeler can go inside of "eta_check" and artificially keep eta_t from getting below some minimum value. Doing so, however, strictly breaks conservation of volume in cases where the cause of the small sea level is evaporation. If the cause is tidal forcing, and the modeler is sure that such artificial constraints will not be detrimental to the solution, then this approach may be suitable. Note that Igor says that the model ran for 4 months with M2 tidal forcing, and the solution looked quite nice in a global "2degree" simulation with the tripolar grid. Steve Griffies
begin:vcard n:Griffies;Stephen x-mozilla-html:FALSE org:DOC/NOAA/OAR/GFDL adr:;;;;;; version:2.1 email;internet:[EMAIL PROTECTED] title:Physical Scientist x-mozilla-cpt:;-11168 fn:Stephen Griffies end:vcard
