Precipitation formation in low-level mixed-phase clouds: determining relevant processes and drivers based on cloud radar observations from a high Arctic site

Low-level mixed-phase clouds (LLMPCs) shroud large portions of Earth’s surface at high latitudes. They have been shown to dramatically affect the surface energy budget, yet, large uncertainties in their model representation remain, both in climate simulations, and in numerical weather prediction. Both computational limitations and poor understanding of a number of processes taking place in LLMPCs are thought to give rise to such uncertainties. In particular, precipitation formation processes have been relatively understudied in LLMPCs, and reaching a refined understanding is expected to lead to an improvement in model performance, as precipitation determines the cloud’s mass sink, and hence lifetime. In this dissertation, precipitation formation processes are investigated in LLMPCs at the high Arctic site of Ny-Ålesund, based on long-term cloud radar observations. Cloud radars are in fact especially suited for ice microphysical studies, due to the wide spectrum of observational fingerprints of ice microphysical processes that they provide. Doppler radar observations provide information on dynamics, multi-frequency radar observations on ice particle size, and polarimetric radar observations on particle shape and concentration. Radar data are combined with thermodynamic information, which further allows to discriminate between ice microphysical processes, due to their high sensitivity to temperature. In the first part of the dissertation, the relevance of the aggregation process for LLMPCs at Ny-Ålesund is assessed. Aggregation occurs when ice particles collide to form larger ice particles. A long-term dataset of dual-frequency radar observations, as well as thermodynamic information, is used to statistically assess the relevance of aggregation and its sensitivity to varying cloud thermodynamic conditions. The study finds that larger aggregate snowflakes are predominantly produced in LLMPCs whose mixed-phase layer is at temperatures compatible with the growth and subs