atmFlatTerrain precursor atmAmbientTurbSource setup

Hello,

The precursor setup of atmFlatTerrain is inconsistent with the successor and other atmospheric cases. The atmAmbientTurbSource produces turbulent mixing length of 72m which is way too high even compared to the setup Lmax = 41m. This should not exceed few meters 1~2m. I think, kAmb should drop at least one order of magnitude to match the successor. Unless there is a specific reason for such choice which I missed.