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The evolution of subgrid‐scale humidity fluctuations in the presence of homogeneous cooling
Author(s) -
Stiller Olaf,
Gregory David
Publication year - 2003
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1256/qj.02.95
Subject(s) - advection , adiabatic process , mixing (physics) , moisture , turbulence , humidity , mechanics , lapse rate , scale (ratio) , environmental science , field (mathematics) , meteorology , atmospheric sciences , statistical physics , physics , thermodynamics , mathematics , pure mathematics , quantum mechanics
The evolution of the moisture field is studied for the situation where clouds are formed through homogeneous cooling. Large‐eddy simulations (LES) which include only the most intrinsic physical mechanisms show that the moisture distribution may change considerably even for this idealized case. The reason is a circulation driven by the latent heat which is released inhomogeneously due to the inhomogeneity (fluctuations) of the moisture field. This circulation in turn affects the moisture field through two mechanisms: (i) turbulent mixing and (ii) vertical advection. The parameters which govern the relative strengths of these mechanisms are expressed as characteristic length‐scales related to the humidity and moist‐static‐energy field. The LES results demonstrate the usefulness of this framework. They also exhibit an interesting (square root) relationship between the height of vertical displacements and the corresponding length‐scale obtained from adiabatic theory (i.e. neglecting mixing processes). A simplified mathematical model for the mixing is presented which links this behaviour to a possible self similarity of the mixing efficiency of moist static energy. Some implications for cloud parametrizations are discussed. © Crown copyright, 2003. Royal Meteorological Society