Incorrect comparison to global 'nsplt' instead of local ksplt(k) in fv_tracer_2d

[kgerheiser]

I believe I found a bug in fv_tracer2d.F90 when filling the optional dpA in the subroutine tracer_2d. A comparison is made to the global nsplt when it should be compared to the local ksplt(k) value. This leads to dpA not being fully filled if ksplt for that level is less than nsplt

This: https://github.com/GEOS-ESM/GEOSgcm_GridComp/blob/f5fd1e30f4c6ee376c6f799b32cb71e606335dd4/GEOSagcm_GridComp/GEOSsuperdyn_GridComp/FVdycoreCubed_GridComp/fvdycore/model/fv_tracer2d.F90#L546

Should be:

if ( it /= ksplt(k)) then 

because if ksplt(k) == 1 for that level, and nsplt == 2, it will never be 2 because the outer if ( it .le. ksplt(k) ) will fail and therefore dpA won't be assigned.

[tclune]

Assigned to @wmputman .

[lizziel]

Kyle, how would this bug manifest itself in a simulation? The version of FV3 in GEOS-Chem High Performance (GCHP) may need the fix but I am uncertain of the priority without assessing impact. Do you have test results you could share?

[kgerheiser]

I ran into this when running the GEOS-CTM at resolutions greater than C48. The code would just crash in offline_tracer_advection with floating invalid when using dpA to calculate PE because it was half-filled with NaNs.

[wmputman]

It's the correct change, i don't test this case very often if ever since we most exclusively run GEOS with z_tracer = T so we always use the tracer_2d_1L routine above this.

[lizziel]

This fix appears to correct the issue I was having in GEOS-Chem with this version of FV3 where concentrations were blowing up at our default dynamic timestep. I was getting around it by reducing the timestep but now this is no longer necessary. Thank you!!