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Cloud base height is derived by upwardly searching the structure of the model atmosphere to find the altitude where the base of the lowest layer of cloud fraction with a covering of more than 50% 1% of the the model grid box and condensate content greater than 10^-6 Kg  Kg Kg^-1.  .  The upward search starts from the surface.

If no cloud base is found, but the convection scheme diagnoses a convective cloud base, then the cloud base height is set to the convective cloud base.

Ceiling

Cloud ceiling is a measurement used in the aviation industry to indicate airport landing conditions.

Ceiling height is derived by upwardly searching the structure of the model atmosphere to find the altitude where the base of the lowest layer of cloud fraction with a covering of more than 1% 50% of the the model grid box and condensate content greater than 10^-6 Kg Kg Kg^-1.  

The upward search starts from the second lowest model level (Level 136, 31m).  Fog, or equivalently cloud in the lowest layer (Level 137, 10m), is not considered when deriving the ceiling.

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Fig2.1.5.-3: Schematic representation of difference assessing cloud base and ceiling.  Cloud base is derived by upwardly searching from the surface; the presence of fog implies cloud base at the surface, or possibly a base near 30m in the case of very low Stratus.  Ceiling is derived by upwardly searching from Level136 (about 31m) to avoid allocation of ceiling to the surface in case of fog. This can result in ceiling being quite high (or even there being no ceiling at all) even above surface fog. 

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Fig2.1.5.-4:Schematic representation of difference assessing cloud base and ceiling showing the difference between derivation of Ceiling and Cloud Base.  

Convective cloud top height

This can be derived from modelled vertical profiles of temperature throughout the troposphere.  At any given grid point The height of convective cloud top is output from the convection scheme inspects the temperature structure of the model atmosphere progressively from the surface to 300hPa.  If there exists a level of free convection (LFC) the scheme searches upward to the level as the height where the convective updraught (velocity) vanishes.  This is not necessarily the same as where the model parcel temperature becomes the same as the model environment temperature.  This is considered as the altitude where free convection ceases and is taken as the

convective cloud top height.    

 Image RemovedFig2.1.5.-5: Assessment of cloud top height from a schematic vertical profile.

(FUG Associated with Cy49r1)

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