The importance of alpine blowing snow for cloud processes

Viaro, Samuele; Sigmund, Armin; Thomas, Evan; Lehning, Michael

doi:10.5194/egusphere-2026-2132

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https://doi.org/10.5194/egusphere-2026-2132

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24 Apr 2026

| 24 Apr 2026

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

The importance of alpine blowing snow for cloud processes

Samuele Viaro, Armin Sigmund, Evan Thomas, and Michael Lehning

Abstract. Numerical models are known to fail in reproducing the large gap that exists between measured ice nucleating particle and ice crystal number concentrations in alpine regions. Improvements have been made by adding different sources of secondary ice production mechanisms into the models. Blowing snow has been identified as an additional possible source of ice particles. Driven by this assumption, we investigate the effect of blowing snow particles using the numerical model CRYOWRF, in which a new saltation scheme has been implemented to better represent the boundary conditions necessary for the blowing snow equations. First, ice crystal number concentrations are compared with measured data from Jungfraujoch, in the Swiss Alps, showing the importance of secondary ice production, blowing snow and microphysics scheme. Then, erosion and deposition patterns are also analyzed, as well as the influence of blowing snow on precipitation. It is shown that our implementation of blowing snow dynamics improves significantly the match between observed and simulated cloud particles.

Received: 15 Apr 2026 – Discussion started: 24 Apr 2026

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Samuele Viaro, Armin Sigmund, Evan Thomas, and Michael Lehning

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RC1:
'Comment on egusphere-2026-2132', Anonymous Referee #1, 12 May 2026 reply
Review of “The importance of alpine blowing snow for cloud processes” by S. Viaro, A. Sigmund, E. Thomas, and M. Lehning submitted to ACP for publication
Many studies have shown that the number of ice crystals observed in boundary layer clouds is much larger (by orders of magnitude) than the available ice nucleating particles. This paper investigates whether blowing snow can be an effective secondary ice production mechanism that increases the number of ice crystals in cloud systems in alpine regions. Nested WRF simulations with an innermost 1km nest are run using 2 different land surface models, 2 different microphysics schemes, and with and without blowing snow. The ISHMAEL microphysical scheme predicts the evolution of ice habits and density while the Morrison scheme uses 3 categories of ice with fixed habits and densities. There are many other differences between the two schemes as well, specific to this study, the secondary ice production mechanism by rime splintering following the parameterization of Hallett and Mossop (1974) is included in both schemes but implemented differently. An additional secondary ice production mechanism of ice-ice collisions is included in a sensitivity study with the Morrison microphysics. Blowing snow is modeled with a two-moment scheme but importantly, only interacts with the microphysics through the water vapor source-sink term.
Summary: This study shows that blowing snow can reach heights that impact cloud processes, but the studies of the different microphysics and SIP mechanisms are very limited, and I think additional studies need to be done to identify what processes cause the different results. Also, to understand how blowing snow impacts cloud processes the blowing snow ice crystals need to be included in the microphysics. The conclusions are overstated or not supported by the model studies. For example, in the conclusions it says, “When blowing snow particles are included in these schemes, the overall effect on the results is comparable to a simulation with a standard microphysics scheme that includes secondary ice production.” Is this meant to describe the MOR_noBS and MOR_BSorig runs? I don’t see this result in the model studies presented and discussed. It also says in the conclusions, “We also confirm the importance of adding secondary ice production mechanisms to improve the estimation of ice particles”, which is not supported by the model studies.
Main Comments:
This is a very interesting study and it is shown that blowing snow can reach heights that will impact cloud processes, but I find the sensitivity studies to be incomplete since the additional ice crystals due to blowing snow are not included in the microphysics. Including blowing snow ice crystals in the microphysics would impact accretion of cloud droplets on ice/snow, rime splintering, among other processes. It would not be that difficult to include ice crystals due to blowing snow in the microphysics and I think this is necessary to identify the “importance of alpine blowing snow for cloud processes”. I also think these additional studies are more important, given the title of the paper, than studies of erosion and deposition patterns.

I find the results shown in Figure 8 difficult to interpret. The green-yellow regions in MOR_noBS and MOR_bs are exactly the same, even in regions of high wind speeds. Blowing snow is only playing a role when near-surface RH is low? I don’t think this is a good demonstration to show how blowing snow can be a source of the missing ice crystals. Even in the regions where blowing snow fills in the gaps without ice, the values are less than 0.01/L.

Figure 7: The impact of ice-ice collisions in MOR_bs is very marginal, I would not say there is a consistent improvement in the simulation of ICNC.

In the first 12 hours, blowing snow does increase ICNC and IWC closer to observed levels in both microphysical schemes but has marginal impacts for the rest of the simulation. What is the explanation for this?

Minor comments:
The paper is well-written and the figures clearly illustrate the main results of the study.

Panels in figures need to be lettered not referred to as top/bottom, left/right, etc.

Page 11: Is MOR_noBS the same as MOR_noBSeq?

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Citation: https://doi.org/10.5194/egusphere-2026-2132-RC1

Samuele Viaro, Armin Sigmund, Evan Thomas, and Michael Lehning

Supplement

https://doi.org/10.5194/egusphere-2026-2132-supplement

Model code and software

CRYOWRF-WRF WSL-CRYOS https://gitlabext.wsl.ch/atmospheric-models/CRYOWRF

Samuele Viaro, Armin Sigmund, Evan Thomas, and Michael Lehning

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Short summary

Our research focuses in understanding snow processes at the surface-atmosphere interface, and their influence at larger scales. The use of our advanced numerical model, which includes blowing snow equations, proved to be valuable in predicting the observed ice crystal concentration number at an alpine site. We further demonstrate the effect of blowing snow particles in precipitation and erosion-deposition patterns. Finally, secondary ice production mechanisms are also treated.


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