The item "freezing altitude" is:
a) Altitude from MSL ?
b) Height from grid point's elevation ?
c) Flight level ?
... By "grid point's elevation" I mean the level of model's orography.
b) is right (height above model orography)
Thank you for the answer.
I think it would be better (for pilots) to have the freezing level, expresed in Flight Levels (FL).
@meteo-GR Yes, FL makes sense. We will consider it.
I made some investigation about how the ECMWF freezing level is computed, when there are multiple "zero-degree levels" in one point, so for your information I post description here:
Freezing altitude shows height of the point where temperature goes from positive to negative values when scanning values from surface upwards. When there are multiple such points, then the second point (from surface) is displayed. Effect of this approach is that when there is inversion, then the "zero-crossing" above the inversion is shown.