FUN code logic to reduce or increase carbon allocation used for uptake is reversed

[olyson]

Brief summary of bug

This bug was first reported in Nov 2018 and subsequently investigated but an issue has not been filed until this point.
Carbon allocation to uptake responds to CN(uptake-cost) and CN(actual). The intended implementation is: For C:N less than the target C:N, C allocation is reduced with cost. For C:N greater than the target C:N, C allocation is increased with high C:N. However, the actual implementation is reversed.

General bug information

CTSM version you are using: release-clm5.0.01

Does this bug cause significantly incorrect results in the model's science? Yes

Configurations affected: All with FUN on

Details of bug

Important output or errors that show the problem

We've completed 40 year global 1850 runs (after AD and post-AD spinup) for a control and with the bug fix. Diagnostics are here:

http://webext.cgd.ucar.edu/I1850/clm50_release-clm5.0.15_delta_CN_FUNbug_2deg_GSWP3V1_1850/lnd/clm50_release-clm5.0.15_delta_CN_FUNbug_2deg_GSWP3V1_1850.21_40-clm50_release-clm5.0.15_2deg_GSWP3V1_1850.21_40/setsIndex.html

We've also completed a historical run. Diagnostics are here:

http://webext.cgd.ucar.edu/I20TR/clm50_release-clm5.0.15_delta_CN_FUNbug_2deg_GSWP3V1_hist/lnd/clm50_release-clm5.0.15_delta_CN_FUNbug_2deg_GSWP3V1_hist.1995_2014-clm50_release-clm5.0.15_2deg_GSWP3V1_hist.1995_2014/setsIndex.html

Our assessment is that the impact is relatively small, but would affect climate; however it doesn’t appear to strongly affect transient C response.

A branch with this fix is here:

https://github.com/olyson/ctsm/tree/delta_CN_FUNbug