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The evaporatively driven cloud-top mixing layer

Published online by Cambridge University Press:  27 July 2010

JUAN PEDRO MELLADO
JUAN PEDRO MELLADO*
Affiliation:
Institut für Technische Verbrennung, RWTH Aachen University, Templergraben 64, 52056 Aachen, Germany
*
†Present address: Max Planck Institute for Meteorology, Bundesstraße 53, 20146 Hamburg, Germany. Email address for correspondence: [email protected]
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Abstract

Direct numerical simulations of the turbulent temporally evolving cloud-top mixing layer are used to investigate the role of evaporative cooling by isobaric mixing locally at the stratocumulus top. It is shown that the system develops a horizontal layered structure whose evolution is determined by molecular transport. A relatively thin inversion with a constant thickness h = κ/we is formed on top and travels upwards at a mean velocity we ≃ 0.1(κ |bs|χc2)1/3, where κ is the mixture-fraction diffusivity, bs < 0 is the buoyancy anomaly at saturation conditions χs and χc is the cross-over mixture fraction defining the interval of buoyancy reversing mixtures. A turbulent convection layer develops below and continuously broadens into the cloud (the lower saturated fluid). This turbulent layer approaches a self-preserving state that is characterized by the convection scales constructed from a constant reference buoyancy flux Bs = |bs|we/χs. Right underneath the inversion base, a transition or buffer zone is defined based on a strong local conversion of vertical to horizontal motion that leads to a cellular pattern and sheet-like plumes, as observed in cloud measurements and reported in other free-convection problems. The fluctuating saturation surface (instantaneous cloud top) is contained inside this intermediate region. Results show that the inversion is not broken due to the turbulent convection generated by the evaporative cooling, and the upward mean entrainment velocity we is negligibly small compared to the convection velocity scale w* of the turbulent layer and the corresponding growth rate into the cloud.

JFM classification

Geophysical and Geological Flows: Atmospheric flows Convection: Moist convection Mixing: Turbulent mixing
Type
Papers
Information
Journal of Fluid Mechanics , Volume 660 , 10 October 2010 , pp. 5 - 36
Copyright
Copyright © Cambridge University Press 2010

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References

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