ACP Atmospheric Chemistry and Physics ACP Atmos. Chem. Phys. 1680-7324 Copernicus Publications Göttingen, Germany 10.5194/acp-12-12155-2012 Numerical evidence for cloud droplet nucleation at the cloud-environment interface Sun J. 1 2 Leighton H. 2 Yau M. K. 2 Ariya P. 2 3 Key Laboratory of Cloud-Precipitation Physics and Severe Storms (LACS), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China Department of Atmospheric and Oceanic Sciences, McGill University, Montreal, Quebec H3A 0B9, Canada Department of Chemistry, McGill University, Montreal, Quebec H3A 2K6, Canada 21 12 2012 12 24 12155 12164 Copyright: © 2012 J. Sun et al. 2012 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://acp.copernicus.org/articles/12/12155/2012/acp-12-12155-2012.html The full text article is available as a PDF file from https://acp.copernicus.org/articles/12/12155/2012/acp-12-12155-2012.pdf

Cumulus clouds have long been recognized as being the results of ascending moist air from below the cloud base. Cloud droplet nucleation is understood to take place near the cloud base and inside accelerating rising cloudy air. Here we describe circumstances under which cloud droplet nucleation takes place at the interface of ascending cloudy air and clear air. Evaporation is normally expected to occur at this interface. However, continuity of moving air requires cloud-free air above the boundary of rising cloudy air to move upwards in response to the gradient force of perturbation pressure. We used a one and half dimensional non-hydrostatic cloud model and the Weather Research and Forecast model to investigate the impacts of this force on the evolution of cloud spectra. Our study shows that expansion and cooling of ascending moist air above the cloud top causes it to become supersaturated with condensation rather than evaporation occurring at the interface. We also confirm that Eulerian models can describe the cloud droplet activation and prohibit spurious activation at this interface. The continuous feeding of newly activated cloud droplets at the cloud summit may accelerate warm rain formation.

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