Compositing z0 for fractional grid

[benwgreen]

In GFS_surface_composites.F90, the momentum roughness length zorl is currently composited (for frac_grid = .true.) by taking the arithmetic mean of zorl_lnd, zorl_wat, zorl_ice. A more physically realistic method is to composite based on the logarithms of the component zorls, which can be done in sfc_diff.f to produce self-consistent composite values of ustar, cd, cdq, rb, stress, fm, fh, fm10, and fh2.

This will require modification to GFS_surface_composites(.F90,.meta) and sfc_diff(.f,.meta)

@JongilHan66 @shansun6 @SMoorthi-emc @junwang-noaa @yangfanglin ([email protected])

[climbfuji]

@benwgreen I believe this issue was addressed in #627, can you please confirm?

[benwgreen]

@benwgreen I believe this issue was addressed in #627, can you please confirm?

@climbfuji Yes this was covered in #627
Edit: Although the broader science behind compositing may need to be revisited in the future, the immediate issue which was causing runaway skin temperatures and model crashes has been resolved.