What is the altitude reference: sea level? I'm asking this because it's not clear to me what "surface" does mean. I'm looking at wind over Mongolia, where the mean ground level is at 1500m of altitude...
Other question: how does the GFS data works in such country: does "surface" actually means "mathematic modelisation if we where at sea level" or really "ground level"?
If anyone could give me some clarifications that would be nice ;-)