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Cloud evaluation

The IFS predicts the three-dimensional cloud field with three variables for each grid box; cloud fraction, cloud liquid water and cloud ice.  Cloud processes such as condensation, evaporation, glaciation and precipitation formation in convective and stratiform clouds are all taken into account with physically-based equations.

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* strictly pressure levels are not used, but actually the IFS model levels that correspond to the given pressure levels in a standard atmosphere.  This means that one can get low cloud over the Tibetan plateau, for example, because we are using there the same IFS model levels to divide up cloud layers that we use the over open ocean.

Cloud base height

Cloud base height is derived by upwardly searching the structure of the model atmosphere to find the altitude where the cloud fraction with a covering of more than 1% of the model grid box and condensate content  greater than 1.0E-6 Kg Kg^-1.  

Ceiling

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

Ceiling is derived by upwardly searching the structure of the model atmosphere to find the altitude where the base of the lowest layer of cloud with a covering of more than 50% of the model grid box and condensate content greater than 1.0E-6 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.  

Convective cloud top height

 Convective cloud top height is derived by upwardly searching the structure of the model atmosphere to find the altitude where free convection ceases.

(FUG Associated with Cy49r1)

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