Protect a possible divide by zero in w3src4

[JessicaMeixner-NOAA]

Pull Request Summary

Bug fix for protecting a possible divide by zero in w3src4

Description

When using ST4 physics, it's possible to attempt to divide by a zero ustar. This update protects this possibility and defines a z0 value for the case if ustar=0.

Issue(s) addressed

• fixes Divide by zero errors in w3src4md  #497

Check list

• Is your feature branch up to date with the authoritative repository (NOAA/develop)? yes

• Make sure you have checked the checklist for a developer submitting to develop and updating version number no version number update

• Please list appropriate labels code managers should add for this PR:
  bug

• Reviewers: @aliabdolali @MatthewMasarik-NOAA @ukmo-ccbunney @mickaelaccensi (anyone/everyone)

Commit Message

• Protect a possible divide by zero ustar in w3src4

Testing

• How were these changes tested? We are now able to run ww3_ufs_01 (and the fully coupled model) with debug flags (which is how this bug was originally found).
• Are the changes covered by regression tests? No, we don't run w/debug flags every time.
• If a new feature was added, was a new regression test added? No new feature
• Have regression tests been run? Yes (w/an earlier develop)
• Which compiler / HPC you used to run the regression tests in the PR? NCEP hera.intel
• Please provide the summary output of matrix.comp (matrix.Diff.txt, matrixCompFull.txt and matrixCompSummary.txt):

matrixCompSummary.txt
matrixCompFull.txt
matrixDiff.txt

Expected not b4b diffs:

**********************************************************************
********************* non-identical cases ****************************
**********************************************************************
mww3_test_03/./work_PR2_UQ_MPI_d2                     (8 files differ)
mww3_test_03/./work_PR3_UNO_MPI_d2_c                     (6 files differ)
mww3_test_03/./work_PR3_UNO_MPI_d2                     (8 files differ)
mww3_test_03/./work_PR2_UNO_MPI_d2                     (8 files differ)
mww3_test_03/./work_PR1_MPI_d2                     (8 files differ)
mww3_test_03/./work_PR3_UQ_MPI_d2_c                     (8 files differ)
mww3_test_03/./work_PR3_UQ_MPI_d2                     (8 files differ)
mww3_test_07/./work_PR3_UQ                     (3 files differ)
ww3_tp2.10/./work_MPI_OMPH                     (4 files differ)
ww3_tp2.16/./work_MPI_OMPH                     (3 files differ)
ww3_ufs1.1/./work_d                     (0 files differ)
ww3_ufs1.2/./work_b                     (0 files differ)
ww3_ufs1.3/./work_a                     (1 files differ)

• Please list which labels code managers should add to indicate code changes:
  no expected code changes

[aliabdolali]

The test ran successfully on NOAA RDHPC with pre-known non-b4b tests:

**********************************************************************
********************* non-identical cases ****************************
**********************************************************************
mww3_test_03/./work_PR2_UQ_MPI_d2                     (6 files differ)
mww3_test_03/./work_PR2_UNO_MPI_d2                     (6 files differ)
mww3_test_03/./work_PR3_UQ_MPI_d2_c                     (8 files differ)
mww3_test_03/./work_PR1_MPI_d2                     (9 files differ)
mww3_test_03/./work_PR3_UNO_MPI_d2_c                     (10 files differ)
mww3_test_03/./work_PR3_UNO_MPI_d2                     (8 files differ)
mww3_test_03/./work_PR3_UQ_MPI_d2                     (8 files differ)
mww3_test_07/./work_PR3_UQ                     (3 files differ)
ww3_tp2.10/./work_MPI_OMPH                     (6 files differ)
ww3_tp2.16/./work_MPI_OMPH                     (5 files differ)
ww3_ufs1.1/./work_d                     (0 files differ)
ww3_ufs1.2/./work_b                     (0 files differ)
ww3_ufs1.3/./work_a                     (1 files differ)
 

matrixCompFull.txt
matrixCompSummary.txt
matrixDiff.txt