Special Nonlinear Physics Seminar: "Settling and collisions of ice crystals in turbulent clouds"

Tue, 30/11/202112:00-13:30
Danciger B Building, Seminar room
Lecturer: Alain Pumir,  Laboratoire de Physique, Ecole Normale Supérieure de Lyon

Small non-spherical ice crystals in clouds settle through a turbulent fluid. As they settle, they can orient themselves, which in turn affects radiation reflection. They can also collide with each other, which leads to aggregation, and to the formation of larger size particles (such as grauples).  I will present the results of a theoretical and numerical study of the problem. The first issue I will discuss concerns the forces and torques acting on small crystals as they move through the fluid, and in particular the importance of fluid inertia to orient the crystals. The dependence of the orientation on the geometric properties of the crystals and on the turbulence intensity can be summarized as an elementary phase diagram. Finally, our results concerning the probability of collisions between ice crystals point to several mechanisms that can bring particles together. Aside from the classical effect induced by the turbulent velocity gradient, the difference in settling velocities between crystals with different orientations can significantly contribute to the collision rate, along with the effect of particle inertia (“sling effect”).