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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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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.
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.
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
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