megan coefs fix

[fvitt]

Description of changes

Corrections on how MEGAN coefficients are applied.

Depends on ESCOMP/CMEPS#557

Specific notes

Contributors other than yourself, if any:

CTSM Issues Fixed (include github issue #):
Fixes #3016

Are answers expected to change (and if so in what way)? Yes, for MEGAN fields

Any User Interface Changes (namelist or namelist defaults changes)? No

Does this create a need to change or add documentation? Did you do so? No

Testing performed, if any: scientific testing is underway

[wwieder]

@ekluzek is this something your can review and merge?

@fvitt what timeline is this needed on the for the CAM-Chem group?

[wwieder]

Seems like lots of trailing white space cleanup with a few answer changing modifications. Thanks for opening up this PR.

[wwieder]

One more question, in #3016 there was a suggestion to set gamma_sm = 1., but I asked for one more science test to evaluate BVOC emissions with an updated CLM parameter file that reduces water stress in tropical forests.

A few questions / options to consider:

• Do we want to add as a namelist option for this to be under user control at run time? Alternatively,
• Should we go ahead and manually turn off the soil moisture control on isoprene emissions with this PR?
• Should either option above be linked with this PR to make a single MEGAN_updates tag?

@tilmes, @HuiWangWanderInGitHub, @lkemmons, @rosiealice can you weigh in here?

[ekluzek]

@wwieder yes, I'm planning on doing the testing and the merge, when we get the go ahead from @fvitt and Simone and Louisa.

[lkemmons]

I think it would be great to have the soil moisture (drought) impact on MEGAN emissions be a namelist option (on/off).
Maybe also with the option to provide an overall scaling factor to the new gamma_sm calculation, which I think Rosie suggested.

[wwieder]

I like that Idea too, @lkemmons, but would ask if @fvitt could create this capability, either on this PR or by opening a new one when it's ready?

[fvitt]

I like that Idea too, @lkemmons, but would ask if @fvitt could create this capability, either on this PR or by opening a new one when it's ready?

I will look into it.

[fvitt]

I think it would be great to have the soil moisture (drought) impact on MEGAN emissions be a namelist option (on/off). Maybe also with the option to provide an overall scaling factor to the new gamma_sm calculation, which I think Rosie suggested.

@lkemmons Do you have source mods where these changes at least hacked in?

[lkemmons]

No, sorry, I have not implemented a namelist option.

…

On Tue, Apr 15, 2025 at 4:16 PM Francis Vitt ***@***.***> wrote: I think it would be great to have the soil moisture (drought) impact on MEGAN emissions be a namelist option (on/off). Maybe also with the option to provide an overall scaling factor to the new gamma_sm calculation, which I think Rosie suggested. @lkemmons <https://github.com/lkemmons> Do you have source mods where these changes at least hacked in? — Reply to this email directly, view it on GitHub <#3065 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AH5BH7O35PL5WSQHCXEHVID2ZWANJAVCNFSM6AAAAAB24YUFXWVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDQMBXGY2TENZWHE> . You are receiving this because you were mentioned.Message ID: ***@***.***> *fvitt* left a comment (ESCOMP/CTSM#3065) <#3065 (comment)> I think it would be great to have the soil moisture (drought) impact on MEGAN emissions be a namelist option (on/off). Maybe also with the option to provide an overall scaling factor to the new gamma_sm calculation, which I think Rosie suggested. @lkemmons <https://github.com/lkemmons> Do you have source mods where these changes at least hacked in? — Reply to this email directly, view it on GitHub <#3065 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AH5BH7O35PL5WSQHCXEHVID2ZWANJAVCNFSM6AAAAAB24YUFXWVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDQMBXGY2TENZWHE> . You are receiving this because you were mentioned.Message ID: ***@***.***>

[fvitt]

No, sorry, I have not implemented a namelist option.

I was referring to changes to the code for "the soil moisture (drought) impact on MEGAN emissions" and "overall scaling factor to the new gamma_sm calculation"

[lkemmons]

The new code for gamma_sm is already in. We want to include the option to have that be =1, or use the new calculation. An overall scale factor would also have to be added.

…

On Tue, Apr 15, 2025 at 4:36 PM Francis Vitt ***@***.***> wrote: No, sorry, I have not implemented a namelist option. I was referring to changes to the code for "the soil moisture (drought) impact on MEGAN emissions" and "overall scaling factor to the new gamma_sm calculation" — Reply to this email directly, view it on GitHub <#3065 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AH5BH7LRAZV5NEQU24RKKWD2ZWCWXAVCNFSM6AAAAAB24YUFXWVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDQMBXGY4DIMZYGA> . You are receiving this because you were mentioned.Message ID: ***@***.***> *fvitt* left a comment (ESCOMP/CTSM#3065) <#3065 (comment)> No, sorry, I have not implemented a namelist option. I was referring to changes to the code for "the soil moisture (drought) impact on MEGAN emissions" and "overall scaling factor to the new gamma_sm calculation" — Reply to this email directly, view it on GitHub <#3065 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AH5BH7LRAZV5NEQU24RKKWD2ZWCWXAVCNFSM6AAAAAB24YUFXWVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDQMBXGY4DIMZYGA> . You are receiving this because you were mentioned.Message ID: ***@***.***>

[wwieder]

As @HuiWangWanderInGitHub noted in #3016 around line 568 in clm/src/biogeochem/VOCEmissionMod.F90

if (use_megan_gamma_sm = true):
    gamma_sm = get_gamma_SM(btran(p)) 
else:
   gamma_sm = 1.0_r8.

where use_megan_gamma_sm is a namelist parameter.

I'm less confident in how to apply the idea of gamma_sm_scale_factor, but suggest this could be handled in the get_gamma_SM subroutine? Maybe @HuiWangWanderInGitHub or @rosiealice has an idea here? Simple ideas are below:

• blue = default parameterization for gamma_sm
• orange = add a min. value to gamma_sm (here 0.6 for example).
• green = modify the parameter values some other way, (here with btran_threshold) con: this changes the shape of the function.

If we add a modifier into gamma_sm I suggest it go on the CLM parameter file.

my hacky python code for this example is below:

# Default parameterization
a1 = -7.4463
b1 = 3.2552
btran_threshold = 0.2
btran_in = np.arange(0, 1.01, 0.01)
get_gamma_SM = 1. / (1. + b1**(a1 * (btran_in - btran_threshold)))
plt.plot(btran_in, get_gamma_SM, label='btran_threshold = 0.2')

# add a min value, keep function the same
get_gamma_SM = np.where(get_gamma_SM>0.6,get_gamma_SM,0.6)
plt.plot(btran_in, get_gamma_SM, label='gamma_SM min = 0.6')

# modify the function, here with btran_threshold = 0.1
btran_threshold = 0.1
get_gamma_SM = 1. / (1. + b1**(a1 * (btran_in - btran_threshold)))
plt.plot(btran_in, get_gamma_SM, label = 'btran_threshold = 0.1')
plt.xlabel('soil moisture (btran)')
plt.ylabel('gamma SM')
plt.legend()

Hope this helps.

[wwieder]

I guess if we like the used of min_megan_gamma_sm (orange line above) this could be the single namelist change that goes into the get_gamma_SM function, where min_megan_gamm_sm = 1 effectively shuts off gamma_sm and no changes to the paramter file needs to occur? e.g.

get_gamma_SM = max(get_gamma_SM, min_megan_gamma_sm)