NSSL-mp bug fix for (obsolete) droplet nucleation (#2195)

TYPE: bug fix

KEYWORDS: NSSL microphysics scheme

SOURCE: Ted Mansell (NOAA/NSSL) (@MicroTed )

DESCRIPTION OF CHANGES:
Problem:
Other updates resulted in the old droplet nucleation scheme (irenuc=2) producing too few cloud droplets. This option is no longer the default, but ought to work the same as it did before in case anybody wanted to use it.

Solution:
Restored use of background value for this particular option.

LIST OF MODIFIED FILES: 
phys/module_mp_nssl_2mom.F

TESTS CONDUCTED: 
The Jenkins tests are all passing.

Author: MicroTed

phys/module_mp_nssl_2mom.F

@@ -10334,7 +10334,11 @@ SUBROUTINE NUCOND    &
        DO mgs = 1,ngscnt
        ! default value of renucfrac is 0.0
         IF ( irenuc /= 6 ) THEN
-        cnuc(mgs) = ccnc(mgs)*(1. - renucfrac) + ccnc(mgs)*renucfrac
+          IF ( irenuc == 2 ) THEN
+            cnuc(mgs) = Max(ccnc(mgs),cwnccn(mgs))*(1. - renucfrac) + ccnc(mgs)*renucfrac
+          ELSE
+            cnuc(mgs) = ccnc(mgs)*(1. - renucfrac) + ccnc(mgs)*renucfrac
+          ENDIF
         ELSE
         cnuc(mgs) = ccnc(mgs)*(1. - renucfrac) + Max(0.0,ccnc(mgs) - ccna(mgs))*renucfrac
         ENDIF