Only reset snow north of 45N and between 100W - 0W

[billsacks]

Description of changes

Similar to #3036 , this is a draft that I'm opening to document this change that may be used in a CESM3 test run. This currently hard-codes a region for snow resetting. But, if desired, we could implement namelist variables to give a bounding box for snow resetting - though my feeling is that that probably is not worth doing.

Specific notes

Contributors other than yourself, if any:

CTSM Issues Fixed (include github issue #):

Are answers expected to change (and if so in what way)?

Any User Interface Changes (namelist or namelist defaults changes)?

Does this create a need to change or add documentation? Did you do so?

Testing performed, if any:
SMS_D_Ld3_P8x1.f10_f10_mg37.I2000Clm50BgcCropQianRs.green_gnu.clm-default with reset_snow = .true. and reset_snow_glc = .true.: passes; manual inspection of SNOW_DEPTH suggests this new code is working as intended

[wwieder]

Seems like we're going to have this featured enabled for future CESM runs. I wonder if we should merge it to main?

[billsacks]

This will take some more work before it can be merged. @ekluzek and I discussed a few possible levels of generality... in increasing order from easiest / least general to hardest / most general:

(1) Keep the bounding box hard-coded but introduce a new namelist flag that dictates whether to apply the reset_snow flags globally or over this hard-coded bounding box... ugly, but fairly quick to implement.

(2) Introduce namelist options that specify the bounds of this bounding box, for a single bounding box. We're lucky in this case that the bounding box doesn't cross the 0° longitude line, so we can get away with a single bounding box.

(3) Allow even more generality for the case where the desired bounding box crosses 0° longitude.

Some discussion is needed to decide which approach to take, and even within one of these approaches, exactly what the user interface should look like. For example, do we want a new flag that selects between applying these two reset_snow flags globally vs. over a bounding box but then still keep the existing flags that turn on/off reset_snow_glc and reset_snow individually? Or do we want separate control over global vs regional application of reset_snow_glc and reset_snow? etc.