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Discussion papers
https://doi.org/10.5194/acp-2018-644
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-644
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 28 Aug 2018

Research article | 28 Aug 2018

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).

Cloud-droplet growth due to supersaturation fluctuations in stratiform clouds

Xiang-Yu Li1,2,3,4,5, Gunilla Svensson1,3,7, Axel Brandenburg2,4,5,6, and Nils E. L. Haugen8,9 Xiang-Yu Li et al.
  • 1Department of Meteorology and Bolin Centre for Climate Rese arch, Stockholm University, Stockholm, Sweden
  • 2Nordita, KTH Royal Institute of Technology and Stockholm Un iversity, 10691 Stockholm, Sweden
  • 3Swedish e-Science Research Centre, www.e-science.se, Stockholm, Sweden
  • 4Laboratory for Atmospheric and Space Physics, University o f Colorado, Boulder, CO 80303, USA
  • 5JILA, Box 440, University of Colorado, Boulder, CO 80303, USA
  • 6Department of Astronomy, Stockholm University, 10691 Stockholm, Sweden
  • 7Global & Climate Dynamics, National Center for Atmospheric Research, Boulder, CO 80305, USA
  • 8SINTEF Energy Research, 7465 Trondheim, Norway
  • 9Department of Energy and Process Engineering, NTNU, 7491 Trondheim, Norway

Abstract. Condensational growth of cloud droplets due to supersaturation fluctuations is investigated by solving the hydrodynamic and thermodynamic equations using direct numerical simulations with droplets being modeled as Lagrangian particles. We find that the width of droplet size distributions increases with time, which is contrary to the classical theory without supersaturation fluctuations, where condensational growth leads to progressively narrower size distributions. Nevertheless, in agreement with earlier Lagrangian stochastic models of the condensational growth, the standard deviation of the surface area of droplets increases as t1/2. Also, we numerically confirm that the time evolution of the size distribution depends strongly on the Reynolds number and only weakly on the mean energy dissipation rate. This is shown to be due to the fact that temperature fluctuations and water vapor mixing ratio fluctuations increases with increasing Reynolds number, therefore the resulting supersaturation fluctuations are enhanced with increasing Reynolds number. Our simulations may explain the broadening of the size distribution in stratiform clouds qualitatively, where the updraft velocity is almost zero.

Xiang-Yu Li et al.
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Short summary
The broadening of droplet-size distributions in statiform clouds is puzzling, where the updraft velocity is almost zero. Without turbulence, the classical treatment of condensational growth of cloud droplets fails to explain this broadening. We investigated the time evolution of droplet-size distributions using direct numerical simulations, where turbulence is resolved into the smallest scales. We found that the broadening is due to the turbulence-faciliated supersaturation fluctuations.
The broadening of droplet-size distributions in statiform clouds is puzzling, where the updraft...
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