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T/S distortion where wind stress is low

asked 2020-05-12 17:26:05 +0200

updated 2020-05-22 16:21:06 +0200

Where the instantaneous wind stress (in this case a monthly climatology from ERA5) in the model is very low, there is distortion of surface (affecting the mixed layer) T/S fields, e.g. see this SST snapshot:

image description

and compare to the model output of wstr:

image description

These artefacts are also visible in the shflux and swflux diagnostics. I've seen artefacts using both a forcing file and a bulk file, but it seems to be worse with the bulk file. These artefacts are also visible in the vertical velocity W, and (to a much lesser extent) the horizontal velocities. Cpp settings are fairly similar to the Benguela example, this is a fairly standard realistic regional configuration - no coupling, nesting or tides.

There is a rather high temperature gradient in the upper water column, e.g. differences of several degrees between the surface cell and the cell below it (with a cell height of <3m). I tried activating SMFLUX_CFB which for some reason seems to have made these artefacts less significant, but the temperature gradient in the mixed layer is still too high and the artefacts are still there, e.g. here, you can see clearly nonphysical sharp gradients in SST in this same region where winds are low:

image description

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answered 2020-05-22 16:24:08 +0200

updated 2020-05-22 16:31:28 +0200

A small update - it appears to work with GLS (specifically using the Mellor-Yamada option). Using the SMFLUX_CFB option removed the artefacts in KPP, but it replaced it with the strange SST 'corona' that you can see in the last image above. In both KPP and GLS, the SMFLUX_CFB causes the planetary boundary layer depth to go to zero in regions of low wind stress which seems to be related to the strange SST above.

I am still confused as to why this is happening but in any case, GLS seems to work.

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Asked: 2020-05-12 17:26:05 +0200

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Last updated: May 22