Microphysical properties and interplay of natural cirrus, contrail cirrus and aerosol at different latitudes

Abstract

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.