Ice Aggregation in Low‐Level Mixed‐Phase Clouds at a High Arctic Site: Enhanced by Dendritic Growth and Absent Close to the Melting Level

Low‐level 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 data set 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 larger than 1 mm) are predominantly produced in low‐level 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 colder than −5°C. Surprisingly, we find no evidence of enhanced aggregation at temperatures warmer than −5°C, as is typically observed in deeper cloud systems. Possible reasons are discussed, likely connected to the ice habits that form at temperatures warmer than −10°C, increased riming, and lack of particle populations characterized by broader size distributions precipitating from higher altitudes.Plain Language Summary: Low‐level mixed‐phase clouds (MPCs), that is, shallow clouds containing both liquid droplets and ice crystals, form frequently in the Arctic region. Their characteristic structure—consisting of one or multiple liquid layers at sub‐zero temperatures, from which ice crystals form and precipitate—is nowadays well understood. However, the processes that lead to the growth of ice crystals into snow have been overlooked. Using a 3‐year data set of radar observations from Ny‐Ålesund, in Svalbard, Norway, we are able to identify situations when the ice particle growth is dominated by aggregation of several individual crystals. Combining radar observations with temperature information, we find that larger snowflakes are only produced in MPCs if their liquid portion is at temperatures between −15 and −10°C. This coincides with the temperature regime known for favoring aggregation due to growth and subsequent entanglement of branched crystals. Surprisingly, we find no evidence of enhanced ice aggregation at temperatures warmer than −5°C, as is typically observed in deeper cloud systems. Possible reasons are discussed, likely connected to the ice crystal shapes that develop at temperatures warmer than −10°C, increased liquid droplet production, and lack of particles precipitating from higher altitudes.Key Points: Low‐level mixed‐phase clouds (MPCs) at Ny‐Ålesund produce large aggregates predominantly at dendritic‐growth temperatures. Enhanced aggregation due to dendritic growth occurs intermittently in limited regions of the MPC. The typically observed enhanced aggregation zone close to 0°C is absent in low‐level MPCs at the site.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/50110000165