Spatial and temporal variations in glacier aerodynamic surface roughness during the melting season, as estimated at the August-one ice cap, Qilian mountains, China

Abstract

The aerodynamic roughness of glacier surfaces is an important factor governing turbulent heat transfer. Previous studies rarely estimated spatial and temporal variation in aerodynamic surface roughness (z0) over a whole glacier and whole melting season. Such observations can do much to help us understand variation in z0 and thus variations in turbulent heat transfer. This study, at the August-one ice cap in the Qilian mountains, collected three-dimensional ice surface data at plot scale, using both automatic and manual close-range digital photogrammetry. Data were collected from sampling sites spanning the whole ice cap for the whole of the melting season. The automatic site collected daily photogrammetric measurements from July to September of 2018 for a plot near the center of the ice cap. During this time, snow cover gave way to ice and then returned to snow. z0 was estimated based on micro-topographic methods from automatic and manual photogrammetric data. Manual measurements were taken at sites from the terminals to the top of the ice cap; they showed that z0 was larger at the snow and ice transition zone than in areas that are fully snow or ice covered. This zone moved up the ice cap during the melting season. It is clear that persistent snowfall and rainfall both reduce z0. Using data from a meteorological station near the automatic photogrammetry site, we were able to calculate surface energy balances over the course of the melting season. We found that high or rising turbulent heat, as a component of surface energy balance, tended to produce a smooth ice surface and a smaller z0 and that low or decreasing turbulent heat tended to produce a rougher surface and larger z0.