On the Discrimination and Interaction of Droplets and Ice in Mixed-Phase Clouds (Phasendiskriminierung und Interaktion von Wassertropfen und Eispartikeln in Mischphasenwolken)

Mixed-phase clouds, consisting of both supercooled liquid droplets and ice particles, play a major role in the life cycle of clouds and the radiative balance of the Earth. However, mixed-phase cloud processes are still rather poorly understood and represent a great source of uncertainty for climate predictions. The main reason for this is the insufficient understanding of the microphysical properties of mixed-phase cloud particles. The biggest challenge is the correct discrimination of droplets and ice particles. In this work, the Particle Habit Imaging and Polar Scattering (PHIPS) probe, an airborne in situ cloud instrument, is used to investigate the composition and microphysical properties of mixed-phase clouds. It combines optical microscopy with polar nephelometry to simultaneously measure the angular scattering behaviour while acquiring stereo-microscopic images of single cloud particles. Based on PHIPS data, a novel method to determine the phase of individual cloud particles based on their angular light scattering behaviour is presented. Comparisons with manually classified in situ data show that the algorithm is able to confidently discriminate spherical droplets and aspherical ice particles with a 98% accuracy. Furthermore, a sizing method based on single particle scattering data is presented. Combined, this allows the determination of phase discriminated particle size distributions in a size range of $50 \leq D \leq 700 \, \text{µm}$ and $20 \leq D \leq 700 \, \text{µm}$ for droplets and ice, respectively. This fills the gap between the commonly used forward scattering instruments and optical array probes. The PHIPS probe was deployed during three in situ aircraft field campaigns in the Southern Ocean, the Arctic and the US east coast. In over 250 flight hours, an extensive data set of single particle microphysical data over a wide range of ambient cloud conditions was acquired. Using the aforementioned newly developed methods, the phase composition of the sampled clouds is analysed and the difference between clouds in high latitudes of the northern and southern hemisphere is discussed. Furthermore, riming, the accretion of droplets by ice particles, is investigated based on manual classification of PHIPS\u27 stereo-micrographs. Riming is observed on over 30% of the investigated ice particles in a size range from $20 \leq D \leq 700 \, \text{µm}$ in clouds between -10$^\circ$C$\,\leq\,$T$\,\leq\,$0$^\circ$C. The meteorological conditions of riming are investigated and the correlation of ambient parameters with riming state and riming degree are discussed. It is shown that riming increases the light scattering in the angular range from $\theta = 42^\circ\,\text{and}\,170^\circ$ by up to 135% compared to unrimed particles. Further, particles with faceted, crystalline build-up which is aligned to the lattice structure of the underlying particle are described. For these particles, which are believed to be the result of vapor deposition during the ageing process of rimed particles, the term "epitaxial riming" is proposed