use tke_seed_value only when necessary

[matzegoebel]

TYPE: bug fix

KEYWORDS: tke, isfflx, tke_heat_flux, tke_drag_coefficient, tke_seed

SOURCE: Matthias Göbel (University of Innsbruck)

DESCRIPTION OF CHANGES:
Problem:
The tke seed value is used in the absence of surface drag or a surface heat flux, since there is no way to generate tke without pre-existing tke. So far the usage of the seed value is only tied to the namelist variables tke_drag_coefficient and tke_heat_flux being greater than 0. However, these namelist variables are only applied if no PBL scheme is used, diff_opt=2, and isfflx=0.

If a PBL scheme is used or diff_opt=1, then isfflx=0 is a sufficient condition to get zero fluxes (see also technical notes).

Solution:
Fixed the if statements to enforce a correct setting of the tke seed value that fits to the purpose of the seed.

LIST OF MODIFIED FILES:
dyn_em/module_diffusion_em.F
wrftladj/module_diffusion_em_ad.F
wrftladj/module_diffusion_em_tl.F

TESTS CONDUCTED:

1. Jenkins testing is all pass.

RELEASE NOTE: corrected the tke seed value to be non-zero if and only if the surface heat and momentum fluxes are zero depending on isfflx, diff_opt, and bl_pbl_physics

[dudhia]

I would note that diff_opt=1 would not be a normal choice without a PBL because its vertical diffusion is fixed and would not use tke. Even mixing the tke scheme with a PBL scheme is a little hard to justify.

[matzegoebel]

You're definitely right on the first one. The diff_opt condition is only there for completeness, because diff_opt=1 + km_opt=2 is in principle allowed. Mixing a PBL scheme with a TKE based scheme for horizontal diffusion, on the other hand, is used in a number of studies. Whether that's appropriate is a different question.
Anyway, this code modification just aims at finding out whether the surface fluxes are zero, in which case you need a tke seed value, or not. I don't know if the seed does any harm, but in case you use an LES with isfflx=1 or 2, you don't need any seed, no matter the values of tke_heat_flux or tke_drag_coefficient. In case you use a PBL scheme and isfflx=0, on the other hand, you do need a seed, no matter the values of tke_heat_flux or tke_drag_coefficient. Or am I mistaken?

[weiwangncar]

@dudhia Can I say this for a release message:
Corrected the tke seed value to be non-zero for km_opt 2 and 5 if and only if the surface heat and momentum fluxes are zero depending on isfflx, diff_opt, and bl_pbl_physics.

[dudhia]

looks OK to me

…

On Tue, Dec 29, 2020 at 3:12 PM weiwangncar ***@***.***> wrote: @dudhia <https://github.com/dudhia> Can I say this for a release message: Corrected the tke seed value to be non-zero for km_opt 2 and 5 if and only if the surface heat and momentum fluxes are zero depending on isfflx, diff_opt, and bl_pbl_physics. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#1307 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AEIZ77CF5WZNKZU4YSSKWZTSXJH5RANCNFSM4TDJA45A> .