microdroplets in turbulent flows

Here is a movie, obtained by DNS, representing the evolution of the turbulent vapour field (in blue -- light blue represents moist regions and dark blue represents dry regions). The white points represent cloud droplets, evolving in this strongly fluctuating environment, which provides a huge variety of conditions broadening the size spectrum.

Warm clouds are basically composed by small solid particles (sand, salt, combustion products...), water droplets of very different sizes (radii range from around .001 mm to 2 mm) suspended in a moist air. Embryonic droplets, formed onto the surface of the suspended solid particles, must grow of six orders of magnitude in radius, in order to produce rain. Two growing mechanisms are basically active in warm clouds: condensation and collection. Collection is based on collisions of two droplets which remain toghether - with a high probability - to form a bigger drop (coalescence). It is very efficient, but it requires some large (R > .02 mm) droplets in order to initiate. Therefore immediately after their formation, droplets are not yet large enough to collide. Here condensation is fundamental because it is the only mechanism which can lead droplets to grow.
We have focused on these condensing droplets which at the end of the condensation stage compose the population of colliding droplets. We performed direct numerical simulations of real turbulent flows, in appropriate conditions, advecting droplets and vapour. In this way we were able to follow the evolution of the size distribution of millions of droplets. We aimed to find evidences for basic mechanisms justifying in situ observations which claim the presence of very different droplets (observation known as spreading of the size spectra). This observation represents a problem, since the classical equation of evolution of the droplet radius by condensation tells us that small droplets grow faster than large droplets, what clearly narrows the size distribution. A lot of different spreading mechanisms have been proposed, but no general agreement has been achieved in this respect. The presence of a population of very different droplets is a fundamental requirement for assuring the efficiency of successive collisions.
The main result we proved is the presence of strong correlations between the vapour field and the droplet trajectories which eventually segregate in the sole moist regions of the cloud. The same correlation produces lucky particles, which experience for a very long time a highly moist environment, and other particles experiencing less moist regions. This naturally produces a very large variety of size evolution, and therefore a very broad spectrum. Of course, the variety of evolutions is due to the presence of a strongly fluctuating vapour field, provided by turbulence.