Influence of snow cover and grain size on the ground thermal regime in the discontinuous permafrost zone, Swiss Alps

摘要

The presence or absence of frozen ground within the discontinuous mountain permafrost zone is on a small scale largely determined by local site characteristics. Snow cover changes spatially at short distances caused by relief and temporally from year to year. Substrate grain size influences the heat exchange processes between air and surface and causes differences in the ground thermal properties. Based on data of four shal low boreholes and 25 temperature loggers, the effect of these factors on the ground thermal regime in two recently deglaciated forefields in the Engadin, Swiss Alps is investigated. The annual depth of the cold wave is dependent on the characteristics of the snow cover and can be twice as deep in winters with a snow height below 0.8 m. In contrast, the active layer thickness is constant even though the duration of the snow-free period changes substantially from year to year. A difference in mean annual ground surface temperatures of 4.3 ℃ is registered over distances of 50 m. The postulated negative thermal anomaly in blocky surfaces is confirmed by our data. However, permafrost conditions are also recorded at one site with fine-grained debris (sand/gravel) that is attributed to its geomorphological position and a strong coupling between air and ground surface temperatures in early winter. Thus, the ground thermal regime shows a large spatial and temporal variability as a function of snow cover evolution, snow height, surface substrate, and relief. The short data record does not yet enable us to detect any climatic signal within the discontinuous permafrost.