In order to ensure consistency between the fine- and coarse-mesh
model it should be aimed to embed the smaller model inside the
model, which has been used for initialization. Beside the fact that
rays leave via the uppermost boundary, it might happen that very
low elevation observations cross the model boundary laterally
and enter the coarser model much below the upper height of the
fine-mesh model. Such occurrences might not be a big problem if
refractivity values are already close to zero.