Repository: Freie Universit├Ąt Berlin, Math Department

Cloud microphysical effects of turbulent mixing and entrainment

Kumar, B. and Schumacher, J. and Shaw, R.A. (2013) Cloud microphysical effects of turbulent mixing and entrainment. Theoretical and Computational Fluid Dynamics , 27 (3-4). pp. 361-376. ISSN Print: 0935-4964; Online: 1432-2250

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Official URL: http://link.springer.com/article/10.1007%2Fs00162-...

Abstract

Turbulent mixing and entrainment at the boundary of a cloud is studied by means of direct numerical simulations that couple the Eulerian description of the turbulent velocity and water vapor fields with a Lagrangian ensemble of cloud water droplets that can grow and shrink by condensation and evaporation, respectively. The focus is on detailed analysis of the relaxation process of the droplet ensemble during the entrainment of subsaturated air, in particular the dependence on turbulence timescales, droplet number density, initial droplet radius and particle inertia. We find that the droplet evolution during the entrainment process is captured best by a phase relaxation time that is based on the droplet number density with respect to the entire simulation domain and the initial droplet radius. Even under conditions favoring homogeneous mixing, the probability density function of supersaturation at droplet locations exhibits initially strong negative skewness, consistent with droplets near the cloud boundary being suddenly mixed into clear air, but rapidly approaches a narrower, symmetric shape. The droplet size distribution, which is initialized as perfectly monodisperse, broadens and also becomes somewhat negatively skewed. Particle inertia and gravitational settling lead to a more rapid initial evaporation, but ultimately only to slight depletion of both tails of the droplet size distribution. The Reynolds number dependence of the mixing process remained weak over the parameter range studied, most probably due to the fact that the inhomogeneous mixing regime could not be fully accessed when phase relaxation times based on global number density are considered.

Item Type:Article
Uncontrolled Keywords:Turbulent mixing, Entrainment, Cloud droplets, Phase relaxation
Subjects:Mathematical and Computer Sciences > Mathematics > Applied Mathematics
ID Code:1210
Deposited By: Ulrike Eickers
Deposited On:08 Mar 2013 14:00
Last Modified:17 Jun 2013 09:13

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