Open AccessA Study of Cloud Mixing and Evolution Using PDF Methods. Part I: Cloud Front Propagation and Evaporation
Author(s) -
C. A. Jeffery,
Jon Reisner
Publication year - 2006
Publication title -
journal of the atmospheric sciences
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.853
H-Index - 173
eISSN - 1520-0469
pISSN - 0022-4928
DOI - 10.1175/jas3760.1
Subject(s) - mixing (physics) , evaporation , eddy diffusion , large eddy simulation , dissipation , spurious relationship , mechanics , physics , cloud physics , radius , statistical physics , probability density function , turbulence , meteorology , thermodynamics , cloud computing , mathematics , statistics , computer security , quantum mechanics , computer science , operating system
The evolution of mean relative humidity (RH) is studied in an isobaric system of clear and cloudy air mixed by an incompressible velocity field. A constant droplet radius assumption is employed that implies a simple dependence of the mixing time scale, τeddy, and the reaction (evaporation) time scale, τreact, on the specifics of the droplet size spectrum. A dilemma is found in the RH e-folding time, τefold, predicted by two common microphysical schemes: models that resolve supersaturation and ignore subgrid correlations, which gives τefold ∼ τreact, and PDF schemes that assume instantaneous evaporation and predict τefold ∼ τeddy. The resolution of this dilemma, Magnussen and Hjertager’s eddy dissipation concept (EDC) model τefold ∼ max(τeddy, τreact), is revealed in the results of 1D eddy diffusivity simulations and a new probability density function (PDF) approach to cloud mixing and evolution in which evaporation is explicitly resolved and the shape of the PDF is not specified a priori. The EDC model is shown to exactly solve the nonturbulent problem of spurious production of cloud-edge supersaturations described by Stevens et al. and produce good results in the more general turbulent case.