index out of range in foggy conditions

[fjansson]

In rrtmg_lw_rtrnmr.f90, subroutine rtrnmr, there are several suspect-looking accesses to cldfrac(lev-1) in the loop over lev ending at lev=1. I'm hitting this one in a DALES run which happens to have cloud in the lowest model layer. A debug build stops with the message

At line 499 of file /home/jansson/dales-tester/dales/external/RRTMG/RRTMG_LW/src/rrtmg_lw_rtrnmr.f90
Fortran runtime error: Index '0' of dimension 1 of array 'cldfrac' below lower bound of 1

It's of course possible we're doing something wrong on the DALES side. But looking at the RRTMG_LW code, it seems if the lowest level is cloudy there is an access out of bounds.

[fjansson]

And another issue in the same routine: the arrays faccmb1d, faccmb2d, faccmb1, faccmb2 are not always initialized. I found this when compiling DALES in debug mode with -finit-real=nan. These arrays are then initialized to nan, and these nans propagates to the output of rrtmg_lw

My fix, for now is to initialize these four arrays to 0, hoping that 0 is the correct value for the elements that are not touched in the two loops. I will submit a PR.