id summary reporter owner description type status priority milestone component version severity resolution keywords cc 1646 pit falls of choosing parameters in the F-S sigma code jamesharle deazer "Having had a look at the NOCL set up of a high resolution AMM60 configuraton I've come across a possible pit fall concerning the choice of parameters in the Furner-Siddorn vertical coordinates. See the attached figures of the top grid box depth (e3t) using the Siddorn-Furner sigma coords for hc=150 and hc=50 (50 being the default shipped with NEMO for the AMM12 domain). Spot the discontinuity! {{{#!div style='text-align: center' [[Image(fs_hc_150.png)]][[Image(fs_hc_50.png)]] }}} From reading the code - it looks like the fixed surface grid box depth (rn_zs=1m in AMM60) and stretched coordinates come into play at depths > hc. Anything less than the hc depth and we're in pure sigma - so as AMM60 has only 50 wet vertical levels - top grid box (e3t) is going to just under 3m at depths approaching 150m (i.e. hc=150). And that's why hc = 50 looks as it should! If hc < jpk you'd have the reverse scenario: a very sudden shallowing of the depth of the top grid box. So I guess the guidance for this is to make hc = jpk (for rn_zs=1m)! Or more generally hc = jpk * rn_zs To avoid users selecting inappropriate combinations of these parameters, perhaps 'hc' should be set in the furner-siddorn subroutine and a WARNING issued if it differs from the one in the namelist? " Defect assigned low OCE v4.0.* minor DOM SCO jamesharle