Experimental investigation of cloud droplet dynamics at the research station Schneefernerhaus

Abstract

The collision–coalescence of droplets in turbulence is responsible for the fast growth of cloud droplets from 15 to 40 μm in radius, but how exactly it causes this quick growth is not understood. The growth of cloud droplets through collision–coalescence is governed by two quantities: the radial distribution function (RDF), which is a measure for the degree of clustering the droplets exhibit, and the radial relative velocity (RRV), which is a measure for the velocity difference between nearby droplets. In this thesis an in-situ experiment is described, that is designed to simultaneously measure all aspects relevant to turbulent collision–coalescence of cloud droplets: droplet motions, droplet sizes, and properties of the turbulent carrier flow. The experiment is located in the German Alps, on top of the environmental research station Schneefernerhaus, at an altitude of 2650 m where clouds naturally occur. Droplet motions are measured using a particle tracking setup; turbulence statistics are measured with a sonic anemometer that is mounted close by. Droplet sizes are measured using a novel technique that relies only on the droplet intensities as recorded by the particle tracking experiment. A complete derivation of the technique based on Lorentz-Mie scattering theory is given. Droplet sizes measured with this approach are compared to those obtained with a holographic instrument. The experiment is used to measure droplet rms accelerations and the radial distribution function conditioned on (pairs of) Stokes numbers. Both qualitatively agree with literature values, but further research is needed to see why there is no quantitative agreement. This work was supported by the European Union Horizon 2020 program, in particular Marie Skłodowska-Curie actions under Grant Agreement No. 675675."