For work (car testing - vehicle dynamics), we typically use only water - much easier to fill the water truck up with!
When we need slipperier, we use "jennite" - which basically seems to be asphalt with sealer on it, which gets pretty slippery when wet, about equivalent to snow, maybe 0.4 mue. Of course, it doesn't really behave like snow, since it's not deformable, but that's probably far more information than you need.
So see if they have an area of the ramp which has sealer on it, and hose that down.
HTH...