Spatial snowpack properties in a snow-avalanche release area: An extreme dry-slab avalanche case on Mt. Nodanishoji, Japan, in 2021
Abstract. An extreme dry-slab snow-avalanche occurred on 10 Jan. 2021 at Mt. Nodanishoji, Gifu, Japan, during a heavy snowfall. The avalanche ran down approximately 2,800 m and caused damage to trees and infrastructures. Although this avalanche was estimated to be the second largest in Japan, physical snowpack properties and their vertical structure and spatial distribution, that caused the avalanche, were not addressed in the release area just after the avalanche fall, mainly due to unsafe and lousy weather. Based on a snow depth distribution observed by an unmanned aerial vehicle and a numerical snowpack simulation in the avalanche release area, the spatial distributions of the mechanical snowpack stability and slab mass and their temporal evolutions were estimated in this study. The procedure was validated by comparing the calculation results with the observed snowpit and spatial snow depth data. The results indicated that two heavy snowfall events, ~3 and 10 days before the avalanche onset, generated two different weak layers made of precipitation particles and associated slabs above other weak layers. The older weak layer was only generated on the northward slope due to its low temperature, whereas the newer layer was predominant over the avalanche release area. The fraction of contributions of the slabs associated with the two weak layers to the total slab mass over the calculation domain was found to be 1 : 2.
Yuta Katsuyama et al.
Status: final response (author comments only)
RC1: 'Comment on nhess-2023-5', Anonymous Referee #1, 28 Feb 2023
- AC1: 'Reply on RC1', Yuta Katsuyama, 14 Mar 2023
RC2: 'Comment on nhess-2023-5', Anonymous Referee #2, 03 Mar 2023
- AC2: 'Reply on RC2', Yuta Katsuyama, 14 Mar 2023
Yuta Katsuyama et al.
Yuta Katsuyama et al.
Viewed (geographical distribution)
Thank you for the opportunity to review this work investigating snowpack conditions relating to a large slab avalanche event on Mt. Nodanishoji, Japan in January 2021.
In this work, the authors attempt to characterize conditions snowpack conditions in the release area of the avalanche using UAV-derived snow depth data from a field campaign in March 2022 and snowpack information derived from SNOWPACK model results forced with an operational atmospheric model with 5 km spatial resolution. The authors interpret their results as indicating two heavy snowfall events in the two weeks preceding the avalanche resulted in two weak layers of precipitation particles which contributed to the avalanche’s release. I found the paper generally well-structured and understandable, but it may benefit from some language editing.
Case-study documentation of large snow avalanches plays an important role in better understanding the processes behind these events. In particular, work such as the present study which combines a variety of field and model-based methods can help address scientific questions related to the spatial variability of snowpack conditions leading to large avalanches and falls well within the scope of NHESS. The authors’ premise to combine UAV-supported snow depth mapping and snowpack modeling to better characterize a high-impact avalanche event can be of interest to a broad audience of scientists and practitioners reading NHESS.
My main concern with this work stems from the lack of field data presented from the event, or even the season of the event, itself. Although the authors have made an effort to justify their methods by, for example, showing the similarity in wind conditions (Figure 11) between the snow season in which the avalanche occurred and the snow season in which they conducted field work, the assumption that fully modeled snow distribution and snowpack conditions (e.g. Section 4.4) can robustly represent a specific avalanche event in the absence of any field validation data from the event itself seems rather difficult to justify in this case. Without the inclusion of additional data which can help validate the authors’ model outputs, I am unsure if this work is publishable as an event case-study or as a methodological work. I do not have an adequate mental image of this avalanche after reading the paper and cannot even identify on the provided figures (e.g. Figures 4, 8, and 9) where in the study domain the avalanche occurred.
The authors have clearly put a great deal of effort into this paper and present a compelling methodological premise for event documentation. I wonder if photos of the event / event’s aftermath (i.e. such as those in the referenced Japan Avalanche Network’s event report) could serve as first-order, qualitative validation data? Do any snow profiles from the 2020/2021 season exist in the area in which the avalanche occurred? Such data could, along with a much more comprehensive discussion of model and result uncertainties help bring this work to a publishable level.
Line 23 – please specify if you are referring to destructive size or relative size here; the JAN citation has this as a size 4 avalanche.
Line 25 – second largest avalanche in Japan ever recorded? This line needs clarification.
Line 48-50 – this citation and information does not seem to fit with the other specific high-magnitude events included as background.
Figures – please add some sort of scale to the maps in the Figures 3, 4, 6, 8, 9
Section 4.4 – these results represent the core of the paper, based on the title, but also, in my opinion, represent the shakiest results with uncertainty and error transmitted throughout the model chain (e.g modeled weather -> modeled snowpack conditions -> modeled stability index). I think a more comprehensive treatment of uncertainty needs to be included in any future iterations of this work.
With regards to the weak layers identified via SNOWPACK modeling (as an example of where more discussion or validation data are needed) – as I understand the results, two layers of preserved precipitation particles under up to 2 m of storm snow resulted in the avalanche release. However, the conclusion states “the temperature difference between snow layers depending on aspect would cause these two different WLs through the metamorphosis process.” I would think that metamorphosis of any kind in the snowpack would result in the destruction of the precipitation particles, either through rounding or facet development. In any case, although I have never worked in a similar snow climate, it’s a bit difficult for me to imagine preserved precipitation particles persisting as a weak layer under a 2 m slab, especially over a ten-day period in the case of WL-B. Perhaps a figure showing the modeled snowpack stratigraphy prior to avalanche release (ideally supplemented with some observed snowpack data from the 2020/2021 season) could help clear this up?