Monitoring of thermal conditions and snow dynamics at periglacial block accumulations in a low mountain range in central Germany

The Rhoen Mountains, a relict periglacial landscape in central Germany, feature a wide range of openwork block accumulations. Although located in a temperate climate, those have characteristics comparable with cold regions of higher altitude or latitude such as arctic-alpine species, longer lasting snow patches or the discussed existence of summer or even year-round ice lenses in one of the largest of these landforms in the Central German Uplands. This study aims for a characterization of the microclimatic conditions of two neighbouring block accumulations. Therefore, temperatures were registered by data loggers along profiles, and snow dynamics were monitored using time-lapse cameras and terrestrial laser scans. These observations are finally compared with geophysical measurements to address the question of potential isolated low-altitude permafrost occurrences. Mean ground surface temperatures show an inverse thermal gradient along the Schafstein block accumulation. Furrows were identified as the cold spots in winter, whereas snow melt holes are signs of a chimney effect. In summer, cold air flows out at ventilation holes along the front causing temperatures of up to 25°C below air temperatures, although no clear signs of permafrost were detected. Temperature correlations reveal periods indicative of a recurring internal summer air circulation. Coarse blocky substrate also favours ground cooling of the smaller Mathesberg block accumulation compared with its surroundings. Winter temperatures are influenced by a persistent snowbank forming due to drifting and blowing snow at the leeward edge of a plateau as little amounts of snow are sufficient to be redistributed by westerlies. The prolonged melt of the snowbank might have had or still has a local hydrological and geomorphological impact. Uncertainties remain regarding the behaviour of the microclimate of block accumulations in a warming climate. Being ‘cold spots’ of high ecological value further investigations are suggested.