Brief summary of bug

In recent surface datasets, including the ones being used for CMIP6 runs, Antarctica's ice shelves are being treated as wetlands rather than glaciers.

General bug information

CTSM version you are using: As far as I can tell, affects all versions for the last year or two.

Does this bug cause significantly incorrect results in the model's science? No

Although the landunit type is incorrect over the ice shelves, at least in the one run I checked (b.e21.BHIST.f09_g17.CMIP6-historical.001), the affected regions are still covered in deep snow year-round, so the impact on land-atmosphere fluxes should be minimal.

Configurations affected: All (I think)

Details of bug

General description

Jan Lenaerts noticed that there are no glaciers over Antarctica's ice shelves in the CMIP6 runs (for example, 80 S, 180 E/W (Ross Ice Shelf)). This is very apparent if you look at the 4th element (glc_mec) of PCT_LANDUNIT in a history file from one of these runs.

I think I have traced the problem back to a change in the LANDMASK field on the new vegetation raw data files that we've been using for the last year or two: This field is 0 over the ice shelves in /glade/p/cesm/cseg/inputdata/lnd/clm2/rawdata/pftcftdynharv.0.25x0.25.LUH2.histsimyr1850-2015.c170629/mksrf_landuse_histclm50_LUH2_1850.c170629.nc, whereas it is 1 over ice shelves in the files we used before a couple of years ago (/glade/p/cesmdata/cseg/inputdata/lnd/clm2/rawdata/pftlandusedyn.0.5x0.5.simyr1850-2005.c090630/mksrf_landuse_rc1850_c090630.nc).

Probable cause

For better or worse (probably worse), mksurfdata_map uses the landmask on the PFT rawdata file to determine where to set landcover to 100% wetland:

https://github.com/ESCOMP/ctsm/blob/a34404419aaa81f5021602963222c50daa10c0f6/tools/mksurfdata_map/src/mksurfdat.F90#L707-L724

My understanding of the code is that the LANDMASK field isn't used directly in the mksurfdata_map code, but (if I remember correctly) I think it's used indirectly: I think the LANDMASK field on that file is used to set the mask field on the mapping files, and the mask field on the mapping files determines the mask in mksurfdata_map.

What this does and doesn't impact in CMIP6 runs

Fortunately, since we're using virtual glacier columns over Antarctica in the non-WACCM CMIP6 runs, I think this bug doesn't cause any loss of information from the land ice perspective – but some diagnostics may require more post-processing than they otherwise would have.

In principle, using the wrong landunit here could lead to scientifically incorrect results. However, at least in the one run I checked (b.e21.BHIST.f09_g17.CMIP6-historical.001), the affected regions are still covered in deep snow year-round, so the impact on land-atmosphere fluxes should be minimal.

Suggested fix(es)

I can see two ways to fix this issue:

(1) Fix the LANDMASK field on the raw vegetation files. This would require putting valid data on that file for the Antarctic ice shelves (this could be called bare ground everywhere in that region).

(2) Remove the code referenced above which uses the PFT data's LANDMASK to 0 out everything. I really don't like that code, and would like us to remove it, but some analysis is required to determine if its removal would have unforeseen consequences, and whether it needs to be replaced by other, similar code.

@whlipscomb @gunterl @Katetc @dlawrenncar @lawrencepj1 @ekluzek @olyson