Ice aggregation in Arctic shallow mixed-phase clouds: enhanced by
dendritic growth and absent close to the melting level
Abstract
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.