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Condensational and collisional growth of cloud droplets in a turbulent environment
Stockholm University, Faculty of Science, Department of Meteorology . Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Swedish e-Science Research Centre, Sweden; University of Colorado, USA.ORCID iD: 0000-0002-5722-0018
Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy. University of Colorado, USA.
Stockholm University, Faculty of Science, Department of Meteorology . Swedish e-Science Research Centre, Sweden.
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(English)In: Article in journal (Refereed) Submitted
Abstract [en]

The effect of turbulence on combined condensational and collisional growth of cloud droplets is investigated using high-resolution direct numerical simulations. The motion of droplets is subjected to both turbulence and gravity. We solve the thermodynamic equations that govern the supersaturation field together with the hydrodynamic equations describing the turbulence. The collision-coalescence process is approximated by a superparticle approach assuming unit collision and coalescence efficiency, i.e., droplet coalesce upon collision. Condensational growth of cloud droplets due to supersaturation fluctuations depends on the Reynolds number, while the collisional growth was previously found to depend on the mean energy dissipation rate. Here we show that the combined processes depend on both Reynolds number and the mean energy dissipation rate. Droplet size distributions broaden either with increasing Reynolds number or mean energy dissipation rate in the range explored here. Even though collisional growth alone is insensitive to Reynolds number, it is indirectly affected by the large scales of turbulence through condensation. This is argued to be due to the fact that condensational growth results in wider droplet-size distributions, which triggers collisional growth. Since turbulence in warm clouds has a relatively small mean energy dissipation rate, but a large Reynolds number, turbulence mainly affects the condensational growth and thus influences the collisional growth indirectly through condensation. Thus, the combined condensational and collisional growth of cloud droplets is mostly dominated by Reynolds number. This work, for the first time, numerically demonstrates that supersaturation fluctuations enhance the collisional growth. It supports the findings from laboratory experiments and the observations that supersaturation fluctuations are important for precipitation.

National Category
Climate Research
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-158535OAI: oai:DiVA.org:su-158535DiVA, id: diva2:1237404
Funder
Knut and Alice Wallenberg Foundation, Dnr. KAW 2014.0048Swedish Research Council, 017-03865 and 2014-585The Research Council of Norway, FRINATEK grant 231444Available from: 2018-08-08 Created: 2018-08-08 Last updated: 2018-08-09
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arXiv:1807.11859

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