Articles | Volume 25, issue 10
https://doi.org/10.5194/acp-25-5313-2025
https://doi.org/10.5194/acp-25-5313-2025
Research article
 | 
27 May 2025
Research article |  | 27 May 2025

The critical number and size of precipitation embryos to accelerate warm rain initiation

Jung-Sub Lim, Yign Noh, Hyunho Lee, and Fabian Hoffmann

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This preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).
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Cited articles

Alfonso, L. and Raga, G. B.: The impact of fluctuations and correlations in droplet growth by collision–coalescence revisited – Part 1: Numerical calculation of post-gel droplet size distribution, Atmos. Chem. Phys., 17, 6895–6905, https://doi.org/10.5194/acp-17-6895-2017, 2017. a
Alfonso, L., Raga, G. B., and Baumgardner, D.: The validity of the kinetic collection equation revisited – Part 3: Sol–gel transition under turbulent conditions, Atmos. Chem. Phys., 13, 521–529, https://doi.org/10.5194/acp-13-521-2013, 2013. a, b
Alfonso, L., Raga, G. B., and Baumgardner, D.: The impact of fluctuations and correlations in droplet growth by collision–coalescence revisited – Part 2: Observational evidence of gel formation in warm clouds, Atmos. Chem. Phys., 19, 14917–14932, https://doi.org/10.5194/acp-19-14917-2019, 2019. a, b
Ayala, O., Rosa, B., and Wang, L.-P.: Effects of turbulence on the geometric collision rate of sedimenting droplets. Part 2. Theory and parameterization, New J. Phys., 10, 075016, https://doi.org/10.1088/1367-2630/10/9/099802, 2008. a
Baker, M., Corbin, R., and Latham, J.: The influence of entrainment on the evolution of cloud droplet spectra: I. A model of inhomogeneous mixing, Q. J. Roy. Meteor. Soc., 106, 581–598, https://doi.org/10.1002/qj.49710644914, 1980. a
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Rain formation in warm clouds begins when small droplets collide, but this process can be slow without larger droplets. We used simulations to explore the role of bigger droplets, known as precipitation embryos, in triggering rain. We found that they speed up rain only when their size and number exceed a critical threshold. This threshold becomes larger when collisions are naturally efficient, such as in clouds with broad droplet size distributions or strong turbulence. 
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