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

Abstract

Abstract. 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 ∘C and has been implemented into several models, while all other secondary-ice processes are poorly constrained and lack a well-founded quantification. During 2 h of measurements taken on a mountain slope just above the melting layer at temperatures warmer than −3 ∘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 µm 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.

Microphysical investigation of the seeder and feeder region of an Alpine mixed-phase cloud

“…Drizzle-sized droplets can release small secondary ice particles upon freezing. This process might also be active in cloud top generating cells if the droplets exceed a diameter of about 40 µm, which has been identified as a critical threshold in previous studies (e.g., Lawson et al, 2015;. As highlighted by Lauber et al (2021), the number of secondary ice particles produced by large cloud droplets depends on the droplet freezing rate, the droplet fragmentation probability during freezing and the number of splinters produced per fragmenting droplet.…”

“…This process might also be active in cloud top generating cells if the droplets exceed a diameter of about 40 µm, which has been identified as a critical threshold in previous studies (e.g., Lawson et al, 2015;. As highlighted by Lauber et al (2021), the number of secondary ice particles produced by large cloud droplets depends on the droplet freezing rate, the droplet fragmentation probability during freezing and the number of splinters produced per fragmenting droplet. Since no in situ observations of the cloud properties were available within generating cells to obtain these parameters, the contribution of droplet shattering to the ICNC is not investigated further in this study.…”

“…The data presented in this paper were collected during the RACLETS campaign, which took place in the Swiss Alps in the region around Davos from 8 February 2019 to 28 March 2019. The campaign was designed to observe the pathways of orographic precipitation formation covering the entire aerosol-cloud-precipitation process chain (see also Ramelli et al, 2021;Lauber et al, 2021;Mignani et al, 2021;Georgakaki et al, 2020). In the following, we will describe the relevant instruments and methods which have been used for the analysis of the presented case study.…”

“…This process is known as droplet shattering and has been observed to be strongly dependent on the cloud droplet size and to be potentially effective over a large temperature range (Keinert et al, 2020;Korolev and Leisner, 2020). Previous field studies have observed the presence of drizzlesized droplets in the size range of 100 to 300 µm in regions of strong vertical updrafts (e.g., Hauf and Schröder, 2006;Ikeda et al, 2007;Lauber et al, 2021). Thus, droplet shattering could increase the ICNC in generating cells by several factors if supercooled drizzle drops are present in the updraft regions.…”

On the drivers of droplet variability in alpine mixed-phase clouds

“…Orographic clouds and the precipitation they generate play a major role in alpine weather and climate (e.g., Roe, 2005;Grubisic and Billings, 2008;Saleeby et al, 2013;Vosper et al, 2013;Lloyd et al, 2015). The formation and evolution of orographic clouds involves a rich set of interactions at different spatial and temporal scales encompassing fluid dynamics, cloud microphysics, and orography (Roe, 2005;Rotunno and Houze, 2007).…”