From cloud condensation nuclei to cloud droplets: a turbulent model

Abstract

The formation of a warm vertical cloud is classically described by the evolution of microscopic droplets [1]. Droplets start to nucleate around solid particles in a favourable environment (i.e. for positive supersaturation in the absence of solute effect). Then, if the available vapour is sufficient, they grow by condensation. Once their size is large enough, their terminal velocity is no more negligible and they start to collide with both slower and faster droplets. The resulting growth by coalescence is explosive and eventually a precipitation can occur. This simple description is able to capture the basic mechanisms behind the first development of the cloud.

Nevertheless, the classical description leaves some problems open and also leads to a serious inconsistency. Indeed, the prediction for the condensational growth states that smaller droplets grow faster than larger droplets. As a result, during the condensation stage the droplet population becomes more and more homogeneous, while the growth slows down. On the one hand, this slowing process cannot lead to a precipitation in reasonable times; on the other hand, the explosive process ensured by collisions cannot occur, if all the droplets share the same size (i.e. the same terminal velocity). Therefore, the size distribution (known as size spectrum) during the condensation stage must broaden in some way. Such a contradiction with the classical prediction is confirmed by experimental observations in clouds, where a broader size spectrum is detected [2, 3].