Cirrus clouds play a crucial role in the Earth’s energy budget. They reflect incoming sunlight and absorb and re-emit terrestrial infrarred radiation. The magnitude of these components depends on the cirrus micro physical properties (e.g., ice crystal number and effective diameter), which can result in either net warming or cooling effects. This thesis investigates the differences in those properties of high- and mid-latitude cirrus, as well as their interactions with atmospheric aerosol and aviation-induced contrail cirrus. To explore these differences, cirrus clouds were measured with the German research aircraft HALO (High Altitude and LOng Range) during the CIRRUS-HL (CIRRUS at High Latitudes) campaign in June and July 2021. A total of 24 flights and 35 hours of in situ measurements of cirrus particle data were collected with the Cloud Droplet Probe (CDP), the Cloud Imaging Probe Grayscale (CIPG) and the Precipitation Imaging Probe (PIP). A comprehensive intercomparison among the instruments, along with a detailed uncertainty analysis, was performed, resulting in an accurate and quality-controlled data set. The results of these measurements show that high-latitude cirrus, compared to mid latitude cirrus, have lower median ice number concentration (0.001 and 0.0086 cm−3), higher median effective diameter (210 and 165 μm), and lower median extinction coefficient (0.042 and 0.072 m−1, respectively). In addition, high relative humidity over ice is observed, particularly at high latitudes, with median values around 125%. The influence of the formation region on cirrus properties was assessed by combining measurements with weather model data and backward trajectories starting at the flight tracks. The results show that a large part of the high-latitude cirrus were formed at mid-latitudes, leading to different properties compared to cirrus formed directly at high latitudes. Simulations from an aerosol-chemistry-climate model were combined with the backward trajectories and a strong contribution of heterogeneous nucleation was identified in the measured cirrus. Thus, the low concentrations of ice nucleating particles at high latitudes (from the model) combined with high ice supersaturation levels might explain the lower ice number concentration and larger effective diameter of cirrus measurements at high latitudes compared to mid-latitudes. Aviation emissions have a large local impact on the cirrus microphysical properties through contrail formation and their evolution to contrail cirrus. The CIRRUS-HL data set shows a higher occurrence of contrail cirrus at mid-latitudes, and a potential impact on natural cloudiness by reducing supersaturation levels at cirrus altitudes. The effect of contrails from future propulsion technologies may depend on background aerosol concentrations. Comparisons of measurements and model data for total aerosol number concentrations show good agreement for the larger particle size mode (> 250nm), but likely an underestimation above 300 hPa in the Aitken mode (> 12nm). By combining observations with model data, this study contributes to enhancing the understanding of the variability in cirrus properties due to different formation mechanisms and aerosol influences, as well as the interaction of natural and contrail cirrus.

Icing conditions that contain supercooled large droplets (SLDs) represent a hazard for aviation. Due to their large inertia, SLDs impact behind the extent of aircraft ice protection systems where the developing ice accretion cannot be removed. At present, aircraft that are vulnerable to SLD icing need to avoid severe icing conditions in general, leading to increased operating costs. New American and European aviation regulations allow the passage through such conditions if the aircraft carries instruments that can detect the presence of SLDs. Upon detection, the pilots can exit the icing cloud before the situation becomes hazardous.
The mass of liquid water contained in an SLD is several orders of magnitude higher than the mass contained in a typical small cloud droplet. However, the number concentration of SLDs is much lower than that of small cloud droplets, consequently, it is challenging to detect SLDs with instruments. As of now, no instruments for the detection of SLD icing conditions are in use on commercial aircraft.
This thesis investigates a combination of two instruments for the detection of SLD icing conditions. The first instrument is the Nevzorov probe for the measurement of liquid and total water content and the second instrument is the Backscatter Cloud Probe with Polarization Detection (BCPD), a non-invasive laser backscatter probe that measures the size and shape of cloud particles between 2 and 42 µm. The Nevzorov probe used in this work carried a new, 12 mm diameter total water content sensor that was added to the instrument specifically for the measurement of SLD icing conditions. Both instruments, the Nevzorov probe and the BCPD, are first analyzed individually in icing wind tunnel tests. The findings from the tests show that the new 12 mm sensor of the Nevzorov probe captures SLDs effectively. No indication was found of SLD being incompletely evaporated due to splashing or of water being swept out of the cone. The collision efficiency of small droplets with the sensor is low and can be compensated with a correction derived in this work. Intensive atmospheric testing ensued the icing wind tunnel measurements. During measurements in Arctic clouds, it could be shown that the fraction of liquid and glaciated particles can be estimated from the BCPD. Measurements in the South of France were able to demonstrate that the detection and discrimination of SLD icing conditions is possible with the Nevzorov probe and the BCPD for SLD icing encounters that are sufficiently long and contain a high number of SLDs.
The results of this work allow future flight campaigns to use the 12 mm sensor of the Nevzorov probe and benefit from its capture efficiency and better sampling statistics. The comparisons of the Nevzorov probe to other instruments can help scientists choose suitable instrumentation for future icing wind tunnel and flight campaigns. Concerning the BCPD, a new method developed in this thesis to estimate the number of ice and water particles could, with small modifications, also be employed for other instruments that use polarization filters. For the detection and discrimination of SLD conditions, future work should focus on extending the sample area of the BCPD further outward from the aircraft skin to measure particle size distributions that are unaffected by the aircraft boundary layer. Furthermore, the false alarm rate of the system could be reduced by incorporating an instrument similar to the BCPD but with a larger size range and larger sample area for the direct detection of SLDs.