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Condensational and Collisional Growth of Cloud Droplets in a Turbulent Environment
KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.ORCID iD: 0000-0002-7304-021X
Stockholm Univ, Dept Meteorol, Stockholm, Sweden.;Stockholm Univ, Bolin Ctr Climate Res, Stockholm, Sweden.;Swedish E Sci Res Ctr, Stockholm, Sweden..
SINTEF Energy Res, Trondheim, Norway.;Norwegian Univ Sci & Technol NTNU, Dept Energy & Proc Engn, Trondheim, Norway..
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2020 (English)In: Journal of the Atmospheric Sciences, ISSN 0022-4928, E-ISSN 1520-0469, Vol. 77, no 1, p. 337-353Article in journal (Refereed) Published
Abstract [en]

We investigate the effect of turbulence on the combined condensational and collisional growth of cloud droplets by means of high-resolution direct numerical simulations of turbulence and a superparticle approximation for droplet dynamics and collisions. The droplets are subject to turbulence as well as gravity, and their collision and coalescence efficiencies are taken to be unity. We solve the thermodynamic equations governing temperature, water vapor mixing ratio, and the resulting supersaturation fields together with the Navier-Stokes equation. We find that the droplet size distribution broadens with increasing Reynolds number and/or mean energy dissipation rate. Turbulence affects the condensational growth directly through supersaturation fluctuations, and it influences collisional growth indirectly through condensation. Our simulations show for the first time that, in the absence of the mean updraft cooling, supersaturation-fluctuation-induced broadening of droplet size distributions enhances the collisional growth. This is contrary to classical (nonturbulent) condensational growth, which leads to a growing mean droplet size, but a narrower droplet size distribution. Our findings, instead, show that condensational growth facilitates collisional growth by broadening the size distribution in the tails at an early stage of rain formation. With increasing Reynolds numbers, evaporation becomes stronger. This counteracts the broadening effect due to condensation at late stages of rain formation. Our conclusions are consistent with results of laboratory experiments and field observations, and show that supersaturation fluctuations are important for precipitation.

Place, publisher, year, edition, pages
AMER METEOROLOGICAL SOC , 2020. Vol. 77, no 1, p. 337-353
Keywords [en]
Cloud droplets, Cloud microphysics
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-266532DOI: 10.1175/JAS-D-19-0107.1ISI: 000504707800001OAI: oai:DiVA.org:kth-266532DiVA, id: diva2:1391450
Note

QC 20200204

Available from: 2020-02-04 Created: 2020-02-04 Last updated: 2020-02-04Bibliographically approved

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Li, Xiang-YungBrandenburg, Axel

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