Understanding rainfall: the role of turbulence and large deviation theory

Understanding the mechanism of rainfall from ice-free cumulus clouds is a challenging problem, because of the low rate of collisions between microscopic water droplets settling under gravity. It has been proposed that turbulence may facilitate collisions between droplets. I shall describe the mechanisms for enhancement of collision rates due to turbulence, and the latest simulation results quantifying this mechanism. It appears to be insufficient to explain rainfall. An alternative approach is therefore required. Recently, I showed that large deviation theory can explain how raindrops result from a succession of unusually rapid collisions. The onset of rain showers can be surprisingly rapid, much faster than the mean time required for a single collision. The work on turbulent enhancement of collision rates involved various collaborations, primarily with Alain Pumir, Bernhard Mehlig, Vlad Bezuglyy, and Michel Vosskuhle. The talk is based upon a review article, Collisional Aggregation due to Turbulence, A. Pumir and Michael Wilkinson, Ann. Rev. Cond. Matter Phys., 7, 141, (2016) and a paper Large Deviation Analysis of Rapid Onset of Rain Showers, M. Wilkinson, Phys. Rev. Lett. 116, 018501, (2016).