[production/RRFS.v1] saSAS sigmab initialization changes to reduce large initial reflectivity in RRFS

[JiliDong-NOAA]

Description

This PR follows Jongil's suggestion and aims to reduce the large convective reflectivity caused by saSAS adjustment in the first timestep during a warm start. The issue is likely related to the inconsistency when DA updates the moisture at t but not the moisture from the previous timestep (t-36s). The moisture from the previous timestep is needed for initializing sigmab (updraft area fraction) when calculating qadv (q advection or tendency term).

The PR forces qadv to zero in the first timestep when a namelist parameter sigmab_coldstart is set to .true. It also reduces the lower limit of sigmab from 0.01 to 0.0 in the first timestep.

dependent on ccpp-physics PR:

ufs-community/ccpp-physics#225

Issue(s) addressed

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• fixes #<issue_number>
• fixes noaa-emc/fv3atm/issues/<issue_number>

Testing

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• Will the code updates change regression test baseline? If yes, why? Please show the baseline directory below.
• Please commit the regression test log files in your ufs-weather-model branch

Dependencies

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• waiting on noaa-emc/nems/pull/<pr_number>
• waiting on noaa-emc/fv3atm/pull/<pr_number>

Requirements before merging

• All new code in this PR is tested by at least one unit test
• All new code in this PR includes Doxygen documentation
• All new code in this PR does not add new compilation warnings (check CI output)