Shallow mixed-phase clouds (MPCs) occur extensively in the Arctic, and are known to play a key role for the energy budget. While their characteristic structure is nowadays well understood, the significance of different precipitation-formation processes, such as aggregation and riming, is still unclear. Using a 3-year dataset of vertically-pointing W-band cloud radar and K-band Micro Rain Radar (MRR) observations from Ny-Ålesund, Svalbard, we statistically assess the relevance of aggregation in Arctic low-level MPCs. Combining radar observations with thermodynamic profiling, we find that larger snowflakes (mass median diameter above 1 mm) are predominantly produced in shallow MPCs whose mixed-phase layer is at temperatures between -15 and -10°C. This coincides with the temperature regime known for favoring aggregation due to growth and subsequent mechanical entanglement of dendritic crystals. Doppler velocity information confirms that these signatures are likely due to enhanced ice particle growth by aggregation. Signatures indicative of enhanced aggregation are however not distributed uniformly across the cloud deck, and only observed in limited regions, suggesting a link with dynamical effects. Low Doppler velocity values further indicate that significant riming of large particles is unlikely at temperatures below -5°C. Surprisingly, we find no evidence of enhanced aggregation at temperatures above -5°C, as is typically observed in deeper cloud systems. Possible reasons are discussed, likely connected to the ice habits that form above -10°C, increased riming, and lack of already aggregated particles precipitating from higher altitudes.

The sizes and shapes of ice crystals influence the radiative properties of clouds, as well as precipitation initiation and aerosol scavenging. However, ice crystal growth mechanisms remain only partially characterized. We present the growth processes of two complex ice crystal habits observed in Arctic mixed-phase clouds during the NASCENT campaign. First, are capped-columns with multiple columns growing out of the plates’ corners that we define as columns on capped-columns (CCC). These ice crystals originated from cycling through the columnar and plate temperature growth regimes, during their vertical transport by in-cloud circulation. Second, is aged rime on the surface of ice crystals having grown into faceted columns or plates depending on the environmental conditions. Despite their complexity, the shapes of these ice crystals allow to infer their growth history and provide information about the in-cloud conditions. Additionally, these ice crystals exhibit complex shapes and could enhance aggregation and secondary ice production.