Characteristics of mountain glaciers in the northern Japanese Alps

Since 2012, seven perennial snow patches in the northern Japanese Alps have been determined to be very small glaciers (VSGs: <0.5 km(2)). However, it had not been determined how such glaciers could be maintained in such a warm climate. In this study, we calculate the annual mass balance, accumulation depth, and ablation depth of five of these VSGs, covering 2015-2019 for four of them (2017-2019 for the fifth) using multi-period digital surface models (DSMs) based on structure-from-motion-multi-view-stereo (SfM-MVS) technology and images taken from a small airplane. The results indicate that, due to snow acquired from avalanches and snowdrifts, these VSGs are maintained by an accumulation in winter that is more than double that from the snowfall, thereby exceeding the ablation in summer. Therefore, we classify them as topographically controlled VSGs. We find very small yearly fluctuations in their ablation depth; however, their annual mass balance and accumulation depth have large yearly fluctuations. The annual mass balance, which mainly depends on the accumulation depth, showed accumulation throughout each glacier during heavy snow years and ablation throughout each glacier during light snow years. This characteristic differs from the upper accumulation area and lower ablation area that exists on most glaciers. These VSGs lack a positive annual mass balance gradient, which suggests that they are not divided by a distinct glacier equilibrium line altitude (ELA) into an upstream accumulation area and a downstream ablation area. Moreover, compared to other glaciers worldwide, we find the mass balance amplitude of VSGs in the northern Japanese Alps to be the highest measured to date.