Rothermel model bug fix

[XiulinGao]

Description:

This PR is to correct Rothermel surface fire model in FATES-SPITFIRE by fixing

1. the calculation of verage fuel bed SA:V, bulk density, and fuel load of a heterogenous fuel bed (e.g., a fuel bed that is made of different sizes and category of fuel) by switching to a SA:V based weighting factor

2. calculate reaction intensity, fuel load, fuel moisture, and fuel moisture of extinction for dead vs live fuel separately instead of calculating a single value based on the averaged fuel traits.

3. I also correct some minor unit conversion issue. For instance, the orginal EQ. for calculating max. reaction velocity in Rothermel is SAV^1.5*(495 + 0.0594SAV^1.5)^-1, in which unit of SAV is ft2/ft3; as we are using cm2/cm3 for SAV, we need to convert cm2/cm3 to ft2/ft3 by multiplying 30.48 in order to use this EQ, which leads to 1.0_r8/(0.0591_r8 + 2.942_r8(SAV**(-1.5_r8))) (need to do a simple transformation by dividing the orginal equation by SAV^1.5); however in SPITFIRE this is 1.0_r8/(0.0591_r8 + 2.926_r8*(SAV**(-1.5_r8))), a relatively large difference in the third constant.

Note: if you see a large file diff that's because I used code formatting tool, which added a lot white space and I don't know how to revert that without quitting all the essential code change. So please view file change by selecting 'hide whitespace' in setting next to the 'conversation' dropdown.

This is to fix #1494

Collaborators:

@rosiealice

Expectation of Answer Changes:

Yes, we expect to see changes in surface fuel characteristics and fire behavior when fire is turned on, which will also result in different vegetation response.

Checklist

If this is your first time contributing, please read the CONTRIBUTING document.

All checklist items must be checked to enable merging this pull request:

Contributor

• The in-code documentation has been updated with descriptive comments
• The documentation has been assessed to determine if updates are necessary

Integrator

• FATES PASS/FAIL regression tests were run
• Evaluation of test results for answer changes was performed and results provided
• FATES-CLM6 Code Freeze: satellite phenology regression tests are b4b

If satellite phenology regressions are not b4b, please hold merge and notify the FATES development team.

Documentation

• Technical Note update:
• User's Guide update:

Test Results:

CTSM (or) E3SM (specify which) test hash-tag:

CTSM (or) E3SM (specify which) baseline hash-tag:

FATES baseline hash-tag:

Test Output:

[samsrabin]

Unsubscribing for now, but let me know when this is ready for review.