enforce minimum snow grain radius (#552)

If the snow grain radius is set to zero, possibly because of zapping small ice or if ice disappears mid-timestep, then updates of snow grain radius will produce NaNs. Snow grain radius is usually bounded between a min and max so this generally doesn't happen, but a recent coupled E3SM bgc run crashed with this error. While the error seems to be relatively rare, this bug fix changes answers when the snow grain radius is nonzero but still less than the minimum.

Author: eclare108213

columnphysics/icepack_snow.F90

@@ -360,6 +360,9 @@ subroutine icepack_step_snow(dt,                   &
                hsn(n)   = vsnon(n)/aicen(n)
                hin(n)   = vicen(n)/aicen(n)
             endif
+            do k = 1, nslyr
+               rsnw (k,n) = max(rsnw_fall, rsnw(k,n))
+            enddo
          enddo
 
          call update_snow_radius (dt,                &