ACPD Atmospheric Chemistry and Physics Discussions ACPD 1680-7375 Copernicus GmbH Göttingen, Germany 10.5194/acpd-11-10557-2011 Initiation of coalescence in a cumulus cloud: a beneficial influence of entrainment and mixing Cooper W. A. 1 Lasher-Trapp S. G. 2 Blyth A. M. 3 National Center for Atmospheric Research, Boulder CO, USA Purdue University, West Lafayette IN, USA National Centre for Atmospheric Science, University of Leeds, Leeds, UK 05 04 2011 11 4 10557 10613 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys-discuss.net/11/10557/2011/acpd-11-10557-2011.html The full text article is available as a PDF file from http://www.atmos-chem-phys-discuss.net/11/10557/2011/acpd-11-10557-2011.pdf

Although rain has been observed to form in warm cumulus clouds within about twenty minutes, calculations that represent condensation and coalescence accurately in such clouds have had difficulty producing rainfall in such a short time except via processes involving giant cloud condensation nuclei (with diameters larger than 2 μm). This model-based study explores a different possible mechanism for accelerating the production of warm rain, one that depends on the variability in droplet trajectories arriving at a given location and time in a cumulus cloud. In the presence of entrainment such droplets experience different growth histories, and the result is broadening of the droplet size distribution. That broadening favours coalescence, leading to embryos that grow to raindrops. These calculations do lead to production of rain that is within the lower range of observations for clouds of Florida, USA, the location on which the input conditions were based. The process emphasized in this study, the formation of drizzle via collisions among droplets in the main peak of the droplet size distribution, complements the growth of precipitation on giant nuclei, which is also an important source of the first rain in the case studied. The results indicate that the mechanism developed here should be considered an important influence on the formation of rain in warm clouds.

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