The impact of fluctuations and correlations in droplet growth by collision-coalescence revisited. Part II: Observational evidence of gel formation in warm clouds

Abstract. In recent papers (Alfonso et al., 2013; Alfonso and Raga, 2017) the sol-gel transition was proposed as a mechanism for the formation of large droplets required to trigger warm rain development in cumulus clouds. In the context of cloud physics, gelation can be interpreted as the formation of the lucky droplet that grows by accretion of smaller droplets at a much faster rate than the rest of the population and becomes the embryo for raindrops. However, all the results in this area have been theoretical or simulation studies. The aim of this paper is to find some observational evidence of gel formation in clouds by analyzing the distribution of the largest droplet at an early stage of cloud formation, and to show that the mass of the gel (largest drop) is a mixture of a Gaussian and a Gumbel distributions, in accordance with the pseudo-critical clustering scenario described in Gruyer et al. (2013) for nuclear multi-fragmentation.