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Insights into cloud parametrization provided by a prognostic approach

2002; Wiley; Volume: 128; Issue: 583 Linguagem: Inglês

10.1002/qj.200212858305

1477-870X

David Gregory, Damian R. Wilson, Andrew C. Bushell,

Meteorological Phenomena and Simulations

Abstract Starting from an assumption that within an area there is a distribution of temperature and humidity, equations are derived for the prognostic evolution of cloud fraction and area‐mean condensed water under the action of uniform forcing. These are contrasted to alternative equation sets which are based upon consideration of humidity variations alone, and approximations involved in the derivations of these are highlighted. The properties of, and relationships between, current diagnostic and prognostic cloud schemes are examined using the prognostic formulation, and assumptions concerning the nature of the subgrid distribution of thermodynamic variables are discussed. While the approaches of Smith and Tiedtke provide a consistent description of cloud formation, the Sundqvist approach is found to implicitly include changes in the character of the distribution as cloud cover increases, inconsistent with an assumption of uniform forcing. Additional properties of the Tiedtke scheme are explored and extensions suggested to improve the treatment of cloud dissipation. © Crown Copyright 2002.