fix units of evaporation for imp_scheme==2 in slucm

[einaraz]

Fix the conversion of kinematic water vapor flux to evaporation in m/s for water budget in slucm

TYPE: bug fix

KEYWORDS: slucm, urban evaporation, urban physics

SOURCE: Einara Zahn, Princeton University

DESCRIPTION OF CHANGES:
Problem:
Evaporation rate over impervious surfaces was incorrectly computed in SLUCM for option IMP_SCHEME==2. The kinematic vapor flux (kg/kg x m/s) was not converted to evaporation in m/s before entering the water budget equation, leading to wrong update of water storage.

Solution:
Multiplied the kinematic water vapor flux by air density and divided by water density in module_sf_urban.f90, only inside the code block for IMP_SCHEME == 2. The conversion does not affect the default version IMP_SCHEME==1.

ISSUE: For use when this PR closes an issue.
Fixes #2186

LIST OF MODIFIED FILES: phys/module_sf_urban.F

TESTS CONDUCTED:

1. Running urban simulations on rainy days, with and without the unit conversion.
2. Jenkins tests have passed.

RELEASE NOTE: Corrected evaporation rate calculation for water budget over impervious surfaces in SLUCM when IMP_SCHEME == 2. Affects available water in impervious surfaces and latent heat flux output in urban simulations with impervious scheme enabled.

[weiwangncar]

The regression test results:

Test Type              | Expected  | Received |  Failed
= = = = = = = = = = = = = = = = = = = = = = = =  = = = =
Number of Tests        : 23           24
Number of Builds       : 60           57
Number of Simulations  : 158           150        0
Number of Comparisons  : 95           86        0

Failed Simulations are: 
None
Which comparisons are not bit-for-bit: 
None

[weiwangncar]

@cenlinhe Can you review this PR? Thanks!

[cenlinhe]

@einaraz Do we also need to correct the unit for FLXHUMB, FLXHUMG, and FLXHUMR? Looks like these three quantities also have the units of kg/kg * m/s instead of m/s.

[cenlinhe]

@tslin2 Could you please also take a look?

[einaraz]

@einaraz Do we also need to correct the unit for FLXHUMB, FLXHUMG, and FLXHUMR? Looks like these three quantities also have the units of kg/kg * m/s instead of m/s.

@cenlinhe We don't need to. I'm converting only evaporation from the previous time step (all end in 'P') directly in the water budget, where all terms are in m/s. FLXHUMB, FLXHUMG, and FLXHUMR are still kinematic fluxes, and at the end of the code all aggregated and converted to W/m2.

[cenlinhe]

@einaraz Do we also need to correct the unit for FLXHUMB, FLXHUMG, and FLXHUMR? Looks like these three quantities also have the units of kg/kg * m/s instead of m/s.

@cenlinhe We don't need to. I'm converting only evaporation from the previous time step (all end in 'P') directly in the water budget, where all terms are in m/s. FLXHUMB, FLXHUMG, and FLXHUMR are still kinematic fluxes, and at the end of the code all aggregated and converted to W/m2.

OK, I see. The fix looks good to me.

The base branch was changed.

[tslin2]

@tslin2 Could you please also take a look?

changes look good to me, thanks.