Continuous secondary-ice production initiated by updrafts through the melting layer in mountainous regions

Lauber, Annika and Henneberger, Jan and Mignani, Claudia and Ramelli, Fabiola and Pasquier, Julie T. and Wieder, Jörg and Hervo, Maxime and Lohmann, Ulrike. (2021) Continuous secondary-ice production initiated by updrafts through the melting layer in mountainous regions. Atmospheric Chemistry and Physics, 21 (5). pp. 3855-3870.

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Official URL: https://edoc.unibas.ch/83019/

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An accurate prediction of the ice crystal number concentration in clouds is important to determine the radiation budget, the lifetime, and the precipitation formation of clouds. Secondary-ice production is thought to be responsible for the observed discrepancies between the ice crystal number concentration and the ice-nucleating particle concentration in clouds. The Hallett-Mossop process is active between -3 and -8 degrees C and has been implemented into several models, while all other secondary-ice processes are poorly constrained and lack a well-founded quantification. During 2h of measurements taken on a mountain slope just above the melting layer at temperatures warmer than -3 degrees C, a continuously high concentration of small plates identified as secondary ice was observed. The presence of drizzle drops suggests droplet fragmentation upon freezing as the responsible secondary-ice mechanism. The constant supply of drizzle drops can be explained by a recirculation theory, suggesting that melted snowflakes, which sedimented through the melting layer, were reintroduced into the cloud as drizzle drops by orographically forced updrafts. Here we introduce a parametrization of droplet fragmentation at slightly sub-zero temperatures, where primary-ice nucleation is basically absent, and the first ice is initiated by the collision of drizzle drops with aged ice crystals sedimenting from higher altitudes. Based on previous measurements, we estimate that a droplet of 200 mu m in diameter produces 18 secondary-ice crystals when it fragments upon freezing. The application of the parametrization to our measurements suggests that the actual number of splinters produced by a fragmenting droplet may be up to an order of magnitude higher.
Faculties and Departments:05 Faculty of Science > Departement Umweltwissenschaften > Geowissenschaften > Umweltgeowissenschaften (Alewell)
UniBasel Contributors:Mignani, Claudia
Item Type:Article, refereed
Article Subtype:Research Article
Publisher:European Geosciences Union
Note:Publication type according to Uni Basel Research Database: Journal article
Identification Number:
Last Modified:06 Sep 2021 16:15
Deposited On:06 Sep 2021 16:15

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