What are the clouds bases referenced to? AGL or AMSL? Thanks!
If you look to a mountainous area (e.g. The Alps), you'll see cloud bases 350ft.
It can't be AMSL, isn't it?
So, cloud bases are AGL.
It is "above the level of the model orography". So principially it is AGL, but be aware of ECMWF model resolution which is 9km, i.e. terrain is sampled with 9km step. So if you are in mountainous terrain, you can not rely on it, do not use it for planning in such cases.
More about Cloud bases: Can you tell us what do you use it for? Are you a pilot? We are thinking of adding new ECMWF parameter called Ceiling, which is similar to present Cloud base parameter. It may be interesting for you. If you want to know more, you can read this short description: https://software.ecmwf.int/wiki/display/FCST/43r1+new+parameters%3A+Ceiling
What do you think about this parameter? Would it be more useful for you than Cloud base?