search

NorESMhub / CAM

Community Atmosphere Model including CAM6-Nor branches
1 stars 20 forks source link

Fixed test related to freezing of rain to produce ice if mean rain size is smaller than Dcs #33

Closed j34ni closed 3 years ago

j34ni commented 3 years ago

The Fortran standard does not specify the sequence in which compound logical expressions are evaluated in an if instruction.

When two conditions are combined and tested using .AND. as in:

       if (lamr(i,k) > qsmall .AND. 1._r8/lamr(i,k) < Dcs) then
          mnuccri(i,k)=mnuccr(i,k)
          nnuccri(i,k)=nnuccr(i,k)
          mnuccr(i,k)=0._r8
          nnuccr(i,k)=0._r8
       end if​

(from https://github.com/NorESMhub/CAM/blob/cam_cesm2_1_rel_05-Nor_v1.0.2/src/NorESM/micro_mg2_0.F90 at line 1736)

it is wrong to assume that the compiler has to treat them from left to right and stop the evaluation if the first condition becomes true: the second condition might be evaluated first which produces an erroneous arithmetic operation (floating-point exception) when lamr(i,k)=0:

The workaround is to use two consecutive if instructions:

       if (lamr(i,k) > qsmall) then 
               if (1._r8/lamr(i,k) < Dcs) then
                       mnuccri(i,k)=mnuccr(i,k)
                       nnuccri(i,k)=nnuccr(i,k)
                       mnuccr(i,k)=0._r8
                       nnuccr(i,k)=0._r8
               end if
       end if​​