Datensatz

Zusammenfassung

A new cloud particle spectrometer has been developed in this work with the purpose of obtaining in-situ measurements on different research aircraft. It is based on the principle of digital in-line holography and delivers six images per second of the recorded interference pattern from the laser background and particles suspended in the approx. 35 cubic centimeter large sample volume. Particle position and two-dimensional in-focus images of particles are determined via numerical reconstruction based on Fraunhofer diffraction pattern analysis. Particle properties such as diameter, area and mass are calculated from the reconstructed images. This new instrument, called "HALOHolo", has been calibrated in the laboratory w.r.t. particle detectability and sizing accuracy. An instrument intercomparison with other cloud particle spectrometers confirmed the determined instrument characteristics in general. Measurements in liquid-phase, mixed-phase and ice clouds have been obtained from three airborne field campaigns where HALOHolo was operated on two different aircraft. The liquid-phase clouds measured were Arctic stratocumulus clouds, where the connection between variability of particle number concentration and mean volume diameter was investigated. The measured data were indicative of inhomogeneous mixing between the cloud and the surrounding air. A developing tropical rain shower was sampled in the process of glaciation and its microphysical evolution has been characterized. The local environment of ice crystals surrounded by supercooled liquid water droplets was studied and the spatial distribution of ice crystals and droplets was found to be a uniform random distribution for the majority of measurements in the rapid glaciation zone of the cloud. In addition, the simultaneous occurrence of small columnar ice crystals among larger rimed particles (graupels) has been documented in a variety of cases, which were most likely to be found in strong updrafts. Finally, the spatial structure of cirrus clouds has been investigated on scales between 30 m and 300 km. These clouds did sometimes exhibit bimodal distributions of particle maximum dimension, which were supposedly induced by mixing processes. In some of the cases, a connection between dynamics and microphysics could be established from the holographic measurements and the meteorological data recorded by the aircraft. It could also be shown that the size distribution of ice crystals can be described as a superposition of log-normal distributions from each individual particle habit. In this thesis, it could be shown that the new instrument is capable of delivering scientifically useful data, which extend beyond conventional instruments like the Forward Scattering Spectrometer Probe, the Cloud Imaging Probe or the Cloud Droplet Probe, and it could even provide new insights in some aspects of cloud physics in all three microphysical classes of clouds. While the majority of the data analysis is able to run automatically, some critical steps like particle classification are still semi-automatic.