Fix for divide-by-zero problem in Shin-Hong PBL (#1698)

TYPE: bug fix

KEYWORDS: Shin-Hong PBL divide by zero

SOURCE: internal, reported by CWB, Taiwan

DESCRIPTION OF CHANGES:
Problem:
Rare case of divide by zero.

Solution:
Avoided by use of if test. Note that an algorithm fix may be needed later.

LIST OF MODIFIED FILES:
M phys/module_bl_shinhong.F

TESTS CONDUCTED:
compile only (should fix problem)
The Jenkins tests are all passing.

RELEASE NOTE: minor bug-fix in Shin-Hong PBL to avoid rare divide by zero

Author: dudhia

phys/module_bl_shinhong.F

@@ -440,6 +440,7 @@ subroutine shinhong2d(j,ux,vx,tx,qx,p2d,p2di,pi2d,                          &
    real    ::  prnumfac,bfx0,hfx0,qfx0,delb,dux,dvx,                           &
                dsdzu,dsdzv,wm3,dthx,dqx,wspd10,ross,tem1,dsig,tvcon,conpr,     &
                prfac,prfac2,phim8z
+   real    ::  cenlfrac
 !
    integer,  dimension( its:ite )            ::                          kpbl
    real,     dimension( its:ite )            ::                           hol
@@ -972,7 +973,13 @@ subroutine shinhong2d(j,ux,vx,tx,qx,p2d,p2di,pi2d,                          &
          rigs(i) = rigsmax
        endif
        rigs(i)     = max(min(rigs(i), rigsmax),rimin)
-       enlfrac2(i) = max(min(wm3/wstar3(i)/(1.+cpent/rigs(i)),entfmax), entfmin)
+       if((rigs(i).gt.0) .and. (abs(rigs(i)+cpent) .le. 1.e-6))then
+          cenlfrac = entfmax
+       else
+          cenlfrac = rigs(i)/(rigs(i)+cpent)
+       endif
+       cenlfrac = min(cenlfrac,entfmax)
+       enlfrac2(i) = max(wm3/wstar3(i)*cenlfrac, entfmin)
        enlfrac2(i) = enlfrac2(i)*enlfrac
      endif
    enddo