Mixing and Evaporation at the Cloud Edge and Angular Dynamics of Small Crystals in Viscous Flow

Abstract

How liquid water in atmospheric clouds distributes over droplets of different sizes is important for the initiation of rainfall, and for the optical properties of the atmosphere. Droplet-number densities and droplet-size distributions change rapidly at turbulent cloud edges, where mixing of cloudy and dry air causes cloud dilution and droplet evaporation. The resulting droplet configurations are determined by how individual micrometer-sized droplets are exposed to dry air, by a turbulent flow whose length scales range from millimeters to hundreds of meters.

A first part of this thesis is on mixing and evaporation at turbulent cloud edges. It is explained how several results from numerical simulations can be understood in a simple way by simplifying and non-dimensionalizing a widely used model. The simple understanding makes it possible to interpret empirical data, and to analyze the multiscale nature of mixing and evaporation in clouds. Two statistical models for mixing and evaporation are presented. The first reproduces the broadening of the droplet-size distribution observed in direct numerical simulations (DNS), but has an oversimplified supersaturation dynamics. This problem is partially resolved in the second model, which under some conditions reproduces the supersaturations of droplets in DNS quantitatively.

A second part of the thesis is on the angular dynamics of small particles in flow. How particles spin and tumble is important for many different phenomena, including the intrinsic viscosity of particle suspensions, the settling of ice crystals in clouds, and the motion of plankton. It is predicted that particles that possess a rotation symmetry and a reflection symmetry in a plane that contains the axis of rotation symmetry spin and tumble like spheroids in Stokes flow. This prediction is verified in an experiment where the angular dynamics of triangular particles in a simple shear flow is observed. The angular dynamics of rotation-symmetric particles without the above-mentioned reflection symmetry is analyzed.