Ceiling is a new output parameter introduced with cycle 43r1. It gives the height in metres (m), above the level of the model orography, of the base of the lowest cloud layer covering more than 50% of the sky (more than 4 oktas). This follows the definition of Ceiling used in aviation (ICAO), except that we also represent situations of >6000 m (~20000 ft). Plots below show an example of a Ceiling forecast for the whole Globe and for Europe.
Please note that Ceiling generally differs from Cloud Base Height, sometimes substantially so, because of the requirement, for Ceiling, of >50% cover (compared to >1% for Cloud Base Height). This partly explains why, on the example plots below, high Ceiling values are focussed on tropical areas, where the cloud cover from the convection is often below the 50% threshold at lower levels. Related documentation, on Convective Cloud Top (and Cloud Base Height) can be found here.