Caustic activation of rain showers.
We show quantitatively how the collision rate of droplets of visible moisture in turbulent air increases very abruptly as the intensity of the turbulence passes a threshold, due to the formation of fold caustics in their velocity field. The formation of caustics is an activated process, in which a measure of the intensity of the turbulence, termed the Stokes number St, is analogous to temperature in a chemical reaction: the rate of collision contains a factor exp(-C/St). Our results are relevant to the long-standing problem of explaining the rapid onset of rainfall from convecting clouds. Our theory does not involve spatial clustering of particles.
Wilkinson
Michael
M
Faculty of Mathematics and Computing, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom.
Mehlig
Bernhard
B
Bezuglyy
Vlad
V
D015984:Causality; D003198:Computer Simulation; D044085:Microfluidics / Q000379:methods*; D008956:Models, Chemical; D009038:Motion; D011891:Rain / Q000737:chemistry*