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Cloud-droplet growth due to supersaturation fluctuations in stratiform clouds

Cloud-droplet growth due to supersaturation fluctuations in stratiform clouds

Category
Academic article
Abstract
Condensational growth of cloud droplets due to supersaturation fluctuations is investigated by solving the hydrodynamic and thermodynamic equations using direct numerical simulations (DNS) with droplets being modeled as Lagrangian particles. The supersaturation field is calculated directly by simulating the temperature and water vapor fields instead of being treated as a passive scalar. Thermodynamic feedbacks to the fields due to condensation are also included for completeness. 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, for the first time, we explicitly demonstrate that the time evolution of the size distribution is sensitive to the Reynolds number, but insensitive to the mean energy dissipation rate. This is shown to be due to the fact that temperature fluctuations and water vapor mixing ratio fluctuations increase 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 mean updraft velocity is almost zero. © 2019 Author(s).
Client
  • Notur/NorStore / NN9405K
  • Research Council of Norway (RCN) / 231444
Language
English
Author(s)
Affiliation
  • Sweden
  • Stockholm University
  • Nordic Institute for Theoretical Physics
  • USA
  • University of Colorado at Boulder
  • National Center for Atmospheric Research
  • Norwegian University of Science and Technology
  • SINTEF Energy Research / Termisk energi
Year
2019
Published in
Atmospheric Chemistry and Physics
ISSN
1680-7316
Volume
19
Issue
1
Page(s)
639 - 648