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

Lauber, Annika; Henneberger, Jan; Mignani, Claudia; Ramelli, Fabiola; Pasquier, Julie Thérèse; Wieder, Jörg; Hervo, Maxime; Lohmann, Ulrike

doi:10.5194/acp-2020-986

Cited by 2 publications

(2 citation statements)

References 52 publications

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“…Code and data availability. The data used in this study are available at https://doi.org/10.5281/zenodo.4534382 (Lauber et al, 2021a) or referenced herein. The software to reproduce the figures and for calculations done for this study is available at https://doi.org/10.5281/zenodo.4534536 (Lauber et al, 2021b).…”

Section: Discussionmentioning

confidence: 99%

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

et al. 2021

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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.

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Section: Discussionmentioning

confidence: 99%

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

et al. 2021

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“…The analysis utilizes measurements collected during the RACLETS campaign, which took place from 8 February to 28 March 2019 (https://www.envidat.ch/group/about/raclets-fieldcampaign) (Mignani et al, 2020;Ramelli et al, 2020b, c;Lauber et al, 2020). This joint research project offers a unique dataset of orographic clouds, precipitation and snow measurements in an effort to shed light on some fundamental microphysical processes being present in subsequent stages of the lifecycle of clouds (i.e.…”

Section: Observational Datasetsmentioning

confidence: 99%

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

Georgakaki¹,

Bougiatioti²,

Wieder³

et al. 2020

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Abstract. Droplet formation provides a direct microphysical link between aerosols and clouds (liquid or mixed phase), and its adequate description poses a major challenge for any atmospheric model. Observations are critical for evaluating and constraining the process. Towards this, aerosol size distributions, cloud condensation nuclei, hygroscopicity and lidar-derived vertical velocities were observed in Alpine mixed-phase clouds during the Role of Aerosols and Clouds Enhanced by Topography on Snow (RACLETS) field campaign in the Davos, Switzerland region during February and March 2019. Data from the mountain-top site of Weissfluhjoch (WFJ) and the valley site of Davos Wolfgang are studied. These observations are coupled with a state-of-the art droplet activation parameterization to investigate the aerosol-cloud droplet link in mixed-phase clouds. The mean CCN-derived hygroscopicity parameter, κ, at WFJ ranges between 0.2–0.3, consistent with expectations for continental aerosol. κ tends to decrease with size, possibly from an enrichment in organic material associated with the vertical transport of fresh ultrafine particle emissions (likely from biomass burning) from the valley floor in Davos. The parameterization provides droplet number that agrees with observations to within ~25 %. We also find that the susceptibility of droplet formation to aerosol concentration and vertical velocity variations can be appropriately described as a function of the standard deviation of the distribution of updraft velocities, σw, as the droplet number never exceeds a characteristic limit, termed limiting droplet number, of ~150–550 cm−3, which depends solely on σw. We also show that high aerosol levels in the valley, most likely from anthropogenic activities, increase cloud droplet number, reduce cloud supersaturation (<0.1 %) and shift the clouds to a state that is less susceptible to aerosol and become very sensitive to vertical velocity variations. The transition from aerosol to velocity-limited regime depends on the ratio of cloud droplet number to the limiting droplet number, as droplet formation becomes velocity-limited when this ratio exceeds 0.5. Under such conditions, droplet size tends to be minimal, reducing the likelihood that large drops are present that promote glaciation through rime splintering and droplet shattering. Identifying regimes where droplet number variability is dominated by dynamical – rather than aerosol – changes is key for interpreting and constraining when and which types of aerosol effects on clouds are active.

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

Cited by 2 publications

References 52 publications

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

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

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

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