Is it time to revise how we handle `vegetation_area_fraction` in MPAS-Workflow?

[byoung-joo]

There are differences between mpas-jedi and MPAS-Workflow on how we convert vegetation_area_fraction for GeoVars.

1. In develop branch of MPAS-Workflow, vegetation_area_fraction goes through the nonlinear variable change in a form of vegfra / 100.0_kind_real. This is the existing way of handling this variable change.

• https://github.com/NCAR/MPAS-Workflow/blob/develop/config/mpas/geovars.yaml#L299-L305
• https://github.com/JCSDA-internal/mpas-jedi/blob/develop/src/mpasjedi/VariableChange/Model2GeoVars/mpasjedi_vc_model2geovars_mod.F90#L506

2. In develop branch of mpas-jedi, a simple scaling factor is applied when reading the variable from the NetCDF file: vegetation_area_fraction = vegfra * 0.01_RKIND. This was introduced in https://github.com/JCSDA-internal/mpas-jedi/pull/1040 .

• https://github.com/JCSDA-internal/mpas-jedi/blob/develop/test/testinput/namelists/geovars.yaml#L301-L305
• https://github.com/JCSDA-internal/mpas-jedi/blob/develop/src/mpasjedi/Utilities/mpas4da_mod.F90#L361

This variable is used for simulating radiance observations (i.e., CRTM). In the 15-30km 3dhybrid test (w/ clear-sky amsua & clear-sky mhs), the following differences are observed from two analysis files, which seems to be very small.

nccmp -dfSq mem002/an.2018-04-15_00.00.00.nc  ../an/an.2018-04-15_00.00.00.nc 
Variable               Group     Count          Sum     AbsSum          Min         Max       Range         Mean      StdDev
pressure_p             /        934434     -11.6284    1463.42    -0.015625    0.015625     0.03125 -1.24443e-05   0.0027559
rho                    /       1232444    0.0003087  0.0405438 -2.38419e-07 2.38419e-07 4.76837e-07  2.50478e-10 4.95966e-08
qv                     /      25132225 -0.000109099 0.00320373 -1.11759e-08 1.76951e-08  2.8871e-08   -4.341e-12 2.95991e-10
surface_pressure       /          5378     0.933594    41.8242   -0.0078125    0.015625   0.0234375  0.000173595  0.00778564
theta                  /        856906      0.59079    31.4592  -0.00012207  0.00012207 0.000244141  6.89445e-07 3.87349e-05
u                      /     144066240   0.00944458    81.5267 -6.48499e-05 6.38962e-05 0.000128746  6.55572e-11 1.04004e-06
uReconstructZonal      /      41167469    0.0197222    28.8168 -4.62532e-05 6.96182e-05 0.000115871  4.79073e-10 1.23066e-06
uReconstructMeridional /      61389990     0.123765    26.7134 -4.19617e-05 4.29153e-05  8.4877e-05  2.01604e-09 8.81693e-07