Student Seminar: Sisi Chen

Turbulence enhancement of droplet collisions: how do droplets grow in turbulent clouds?

Abstract

As one of the major features of warm, cumulus clouds, turbulence has long been argued to accelerate droplet collision-coalescence process and thus shorten the warm rain initiation time. Both stochastic models and direct numerical simulation (DNS) have observed a significant turbulent enhancement of geometric collision kernel. And LES studies proposed that turbulence may even increase the amount of rainfall. However, how much contribution the turbulence make from a quantitative perspective is still debatable. In addition, studies on hydrodynamic effects, which determines the collision efficiency, are insufficient to reach a quantitative agreement, and results from DNS studies on the evolution of droplet size distribution(DSD) are rare. A more quantitative evaluation of the effect of small-scale turbulent is needed to unfold the mystery of warm rain initiation process. 
We developed a DNS model to simulate the droplet collision-coalescence process inside cloud adiabatic cores, where the turbulence is assumed to be homogeneous and isotropic. The purpose of our study is to quantify the influence of turbulence (Reynolds number and eddy dissipation rate) on droplet growth to shed light on parameterizations of turbulent collision kernel. Recently, the model includes droplet hydrodynamic effect by explicitly resolving the droplet disturbance flow. In this presentation, I will walk you through the growth history of a droplet by collision and coalescence. Recent attempts on studying the collision efficiency and DSD evolution under turbulence will also be given.