Airborne in-situ Measurements of Microphysical Properties of Supercooled and Mixed-Phase Clouds

Kurzfassung

Clouds in the mixed-phase temperature regime between -38 °C and 0 °C (235 K and 273 K) are often misrepresented in climate prediction models. The largest uncertainty is the accurate estimation of the degree of glaciation, i.e., the distribution of ice and supercooled droplets within the cloud, which determines its radiative properties. In order to better represent these clouds, it is crucial to improve the understanding of microphysical processes and properties determining the cloud phase. With the purpose of extending the data-base of mixed-phase clouds, airborne insitu measurements were taken in less frequently probed convective clouds above Southern Germany, in July 2021. This data, recorded by both the Cloud and Aerosol Spectrometer (CAS-DPOL) and the Cloud Imaging Probe (CIP), was combined and evaluated. Prior to evaluation, the CAS-DPOL was calibrated and instrument related measurement biases, such as Shattering, Coincidence, and Ram Rise Effects, were discussed and eliminated to ensure the best quality of results. A phase identification (liquid/ice) of particles with sizes > 50 µm was conducted, using the particle images of the CIP-Grayscale with a resolution of 15 µm. Clouds enclosed in five isothermal flight segments, at temperatures of -35 °C, -23 °C, -13 °C, -7 °C, and -1 °C, with particle sizes > 3 µm and a liquid water content > 0.01 gm−3 were evaluated regarding number concentration, medium volume diameter, liquid and ice water content, particle size distribution, as well as particle shape and phase. Additionally a closer look was taken into values determining ambient conditions, such as temperature, relative humidities with respect to water and ice and vertical velocities within a segment. Finally, clouds were categorized into four groups, ’Wegener-Bergeron-Findeisen/Large Ice’, ’Secondary Ice’, ’Coexistence’ and ’Mostly Liquid’, based on a classification method introduced by Costa et al. (2017), taking into account all derived parameters, particle images and ambient conditions. The results confirm the importance of the Wegener-Bergeron-Findeisen process for the glaciation stage of clouds, but also display differences between microphysical processes in the mixed-phase layer of convective clouds and low-level clouds, due to diverging updraft velocities. Furthermore, I find that phase-determining mechanisms cannot be assumed to be homogeneously distributed within a cloud, strongly depend on vertical velocities and are often times interwoven rather than separate from each other. In order to improve climate models, it is therefore crucial to further evaluate and compare microphysical processes and properties in the mixed-phase layer of different cloud species and to obtain more statistical data of in-situ measurements.