Ground surface temperature and seasonal snow cover interactions in a Himalayan glacierized catchment (upper Langtang Valley, Nepal)

Abstract:

It is acknowledged that snow cover influences the thermal regime of the underlying ground and plays an important role in permafrost dynamics on a seasonal timeframe. In addition, both permafrost and snow are important components of natural freshwater storage systems in high mountain regions. It is therefore essential to study both cryosphere components as part of an integrated system, particularly in monsoon dominated regions such as in the Himalayas, where the concomitance of snow accumulation and snow melt seasons complicates ground thermal processes.

Where present, snow exerts a primary control on underlying surface ground temperature due to its high albedo and low thermal conductivity. Whereas various studies have emphasized the role of snow in preserving the active layer from air temperature changes during the snow accumulation season, snow has also been observed to contribute to ground surface warming via heat advection during the melting season. Under projected climate warming, where both permafrost and snow environments are likely to undergo significant changes, it is crucial to gain better insights into the thermal interactions between snow, permafrost and its active layer. This is especially true in the context of high-mountain hydrological, ecological and geohazards studies, where the dynamics and timing of (sub-)surface water outflow resulting from ground surface thawing are important drivers to be considered.

In this study, we report on the use of near-surface ground temperature as a mean to infer snow cover dynamics in cryosphere environments from upper Langtang Valley, Nepal. 35 ground temperature sensors were installed between 2013 and 2016 and at altitudes ranging from 4550 to 5542 masl in the vicinity of Yala Glacier. The devices were buried a few centimeters either below ground surface, within rock outcrops or within voids between rocks. Surface ground temperature data was acquired throughout the Oct. 2013-Oct. 2019 period, during which individual sensors recorded from one to four annual cycles. Based on climate data analysis from nearby automatic weather stations and snow cover reconstruction from MODIS satellites, we investigate how air surface temperature and seasonal snow cover in upper Langtang Valley control underlying ground surface temperature, and highlight the role of snow ripening and snow melt-out processes in this interaction. It appears from this study that, in high mountain environments with seasonal snowpack such as encountered in Northern Nepal, near-surface ground temperature is a useful proxy to assess snowpack occurrence and evolution, on the condition that the ground below the snow blanket remains frozen at least partially during the winter season. Determining in this way the timing of snowpack degradation has important environmental implications, notably for a better understanding of (sub-)surface water runoff dynamics at the end of the winter season.

Keywords: Ground surface temperature; Soil active layer; Permafrost; Snow cover; Snow ripening; Snow melt-out; Langtang; Nepal; Hindu Kush Himalaya.