[MITgcm-support] unexpected behavior of advection scheme 81

[Martin Losch]

Hi all,

Longjiang found a problem with the advection scheme 81 (prather with limiting). Apparently, this scheme only works adequately for tracers>0. As an illustration see the attached figure, where I advect, similarly to the figures in the documentation <http://mitgcm.org/public/r2_manual/latest/online_documents/node72.html> a rectangle (black line: initial conditions) with advection scheme 80 and 81 in a periodic domain. Advection scheme 80 does not seem to care if the rectangle ranges between 0 and 1 (upper blue curves) or -1 and 0 (lower orange curves). Advection scheme 81 does something very funny when the rectangle is between (-1,0) (lower read curve). The bottom panel of the attached figure shows the same after going around 10 times, after which nothing is left of the signal for advection schem 81. I am attaching this little experiment, in case anyone feels like reproducing it.

Longjiang has traced this back to line 159/160 in gad_som_adv_x.F (and similarly in the other directions), where
         slpmax = 0.
         IF ( sm_o(i,j).GT.zeroRL ) slpmax = sm_o(i,j)
In case of sm_o<0 (e.g. negative temperatures), this would mean slpmax=0. etc.

Is this intentional? It would mean that one should not use advection scheme 81 for, say, temperature (in polar regions). One could advection THETA+273.15 ...

Martin
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