Four-year monitoring of rock temperature and snow conditions relevant to frost weathering in the northern Japanese Alps

Frost weathering is a fundamental process controlling bedrock fracturing and rockfall activity in alpine environments, and its understanding requires evaluating the influence of snow cover on the thermal distribution at the bedrock surface (Matsuoka and Murton, 2008; Eckerstorfer et al., 2012; Haberkorn et al., 2017). In the Japanese Alps, characterized by heavy rainfall and snowfall, rock temperature data remain limited compared to the European Alps, which lie in the same mid-latitude zone as Japan. Moreover, long-term monitoring in the Japanese Alps has been confined to areas with relatively low snow accumulation (e.g., Matsuoka, 2019). In this study, we aim to clarify the conditions favorable for frost cracking in the heavy snow region by monitoring rock temperature, maximum snow depth, and snow-cover duration under various snow conditions on Mt. Hakuba (2,932 m a.s.l.) and Mt. Shakushi (2,812 m a.s.l.), located in the northern Japanese Alps.

From October 2021 to September 2025, we continuously monitored rock temperatures at depths mainly of 2 cm, 20 cm, and 40 cm, and locally down to a maximum depth of 120 cm. In addition, aerial surveys were conducted using a Cessna aircraft and UAVs. 3D point cloud models of rock slopes for different seasons were generated from aerial images using SfM/MVS analysis. Maximum snow depth at each sensor location was obtained from point cloud distance calculations between models representing the maximum snow accumulation and the snow-free period when bedrock is exposed. From the rock temperature data, several indices related to frost cracking were calculated, including annual freeze–thaw cycles (Matsuoka, 2002), freezing degree days, the duration of temperatures within the frost cracking window, and thermal gradient conditions (Kellerer-Pirklbauer, 2017). These indices were compared with interannual variations in snow depth and snow-cover duration.

In the results, snow-cover duration varied substantially between years, resulting in pronounced differences in frost cracking indices at each site. In addition, the height of snow cornices formed along ridges was greater than in heavy snow years, even when the overall snow cover duration was short, highlighting strong spatial heterogeneity in snow conditions near ridges. Although longer snow covers tended to raise winter rock temperatures, fractured bedrock sites showed lower temperatures and high freezing degree days. Future changes in frost cracking indices were calculated for each site based on various air-temperature warming scenarios, revealing markedly different trends depending on the scenario. These findings provide valuable insights for assessing present and future frost cracking potential in snow-rich alpine regions of Japan.