Snowpack characteristics on steep frozen rock slopes

Data from 27 snow profiles taken in frozen rock walls at two sites in the Swiss Alps reveal that steep slopes have distinctive snowpack characteristics. Snow pits were dug in 50-65 degrees slopes at elevations between 2900 and 3600 m asl on north-and south-facing slopes at Gemsstock and Jungfraujoch Sphinx in the winters 2012-2013 and 2013-2014. There were marked contrasts in snow characteristics between the two aspects, yet strong intersite similarities. Under the influence of intense solar radiation, basal ice layers and multiple hard melt-freeze crusts formed on the south-facing slopes. Soft layers of facets and depth hoar developed between the crusts. On the shady north-facing slopes, thick basal melt-freeze crusts formed when snow persisted during stable weather periods in autumn. The dominant snow grain types in winter were facets and depth hoar. When solar elevation exceeded slope angle from mid-April onwards, gravity-driven percolation of melt water flowing parallel to the frozen rock surface from areas with warm protruding rocks led to the formation of thick basal ice layers in the north-facing slopes. Windward slopes were covered with rime and glaze during storms, regardless of aspect and season. Despite widespread snowpack instability, the formation of large slab avalanches was hindered by the pronounced roughness of the rock surfaces. The main drivers contributing to the distinctive character of snow covers in frozen rock walls are the negative rock surface temperatures, enhanced/minimized solar radiation and multidirectional fluxes of water, vapour and heat induced by the steepness of the rock slopes.