PAR fraction calculation causes model to crash when using Tree Recruitment Scheme with seedling dynamics

[r-ward]

Describe the issue

The SeedlingParPatch subroutine uses absolute canopy area (m2) instead of fractional area when calculating par_high_frac and par_low_frac, causing negative values that produce NaNs in seedling light calculations.

fates/main/FatesInterfaceMod.F90

Lines 2411 to 2414 in 3b58472

	! Start with the assumption that there is a single canopy layer
	seedling_par_high = atm_par_dir+atm_par_dif
	par_high_frac = 1._r8-cpatch%total_canopy_area
	par_low_frac = cpatch%total_canopy_area

Solution: normalize by patch area
par_high_frac = 1._r8 - (cpatch%total_canopy_area / cpatch%area)
par_low_frac = cpatch%total_canopy_area / cpatch%area

Relevant log output

--WARNING-- skipping CN balance check for first timesteps after startup or data assimilation
 --WARNING-- skipping CN balance check for first timesteps after startup or data assimilation
 --WARNING-- skipping CN balance check for first timesteps after startup or data assimilation
 mass balance error detected
 element type (see PRTGenericMod.F90):            1
 error fraction relative to biomass stock:    0.0000000000000000     
 absolut error (flux in - change):                        NaN
 call index:            1
 Element index (PARTEH global):           1
 net:                        NaN
 dstock:                        NaN
 seed_in:    2.7397260273972605E-002
 net_root_uptake:    0.0000000000000000     
 gpp_acc:    1.8857599694259481     
 flux_generic_in:    0.0000000000000000     
 wood_product_harvest:    0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000     
 wood_product_landusechange:    0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000        0.0000000000000000     
 error from patch resizing:    0.0000000000000000     
 burn_flux_to_atm:    0.0000000000000000     
 seed_out:    0.0000000000000000     
 flux_generic_out:    0.0000000000000000     
 frag_out:                        NaN
 aresp_acc:   0.81214024114323924     
 herbivory_flux_out:    0.0000000000000000     
 error=net_flux-dstock:                       NaN
 biomass   178.67897611883575     
 litter   9.9332023308671769     
 seeds                       NaN
 total stock                       NaN
 previous total   189.34740446594373     
 lat lon   9.1530000000000005        280.15390000000002  




 === DEBUG PAR CALCULATION Day:   1.0208333333333333     
 Site:           1 Patch:           1
 Input atm_par_dir:   288.47089003109909     
 Input atm_par_dif:   88.529109968900912     
 seedling_par_high:   377.00000000000000     
 par_high_frac:  -514.12037051582422     
 seedling_par_low:   224.36420292270481     
 par_low_frac:   515.12037051582422     
 new_seedling_layer_par:  -78248.808344434467

FATES tag

sci.1.87.5_api.41.0.0-12-gf2c22336

Host land model tag

ctsm5.3.077

Machine

derecho

Other supported machine name

No response

Additional context

I have fixes for this and couple of additional bugs (which I'll make separate issues), on my branch debug_trs

[rgknox]

Thanks for submitting @r-ward .

I was digging through the calculations in this subroutine just to wrap my head around things. I remembered I created some visualizations of the these canopy area variables, which might be handy. See here:

https://fates-users-guide.readthedocs.io/en/latest/user/Canopy-Layering-Convention.html

[r-ward]

Oh cool, these are so helpful! Thanks for sharing