[A-CC32] Glaciology

[ACC32-03] Major factors for distribution of red snow on a glacier in Alaska

*Yukihiko Onuma¹, Nozomu Takeuchi² (1.Japan Aerospace Exploration Agency, 2.Chiba University)

Keywords:Snow algae, Bio-albedo effect, Red snow, Numerical modeling, Satellite observation

Red snow phenomenon, snow surface turns red, occurs on glaciers worldwide during summer, causing an albedo reduction by microbial activities known as the bio-albedo effect, which affects the hydrological cycle through accelerated snowmelt. The red snow phenomenon is caused by the blooms of cryotolerant phototrophs called snow algae. Because snowpacks have limited water required for their growth, the existence of liquid water is one of the limiting factors for the blooms. However, red snow does not necessarily appear on the melted snow surfaces in glaciers, and it is still unclear where red snow appears on snow surfaces. We previously developed the global snow algae model Bio-MATSIRO, which reproduces the spatiotemporal distribution of red snow with a few tens kilometers of scale globally based on meteorological conditions, but it is questionable that the model can simulate detailed red snow distribution on the finer glacier scale. In this study, Bio-MATSIRO was downscaled to 60m horizontal resolution to simulate red snow on the Harding Icefield in Alaska, where prominent red snow occurs broadly, we investigated the process of red snow appearance. To downscale the spatial resolution of the model, topographic conditions (elevation, etc.), glacier area and meteorological conditions in the Harding Icefield were prepared from ArcticDEM Mosaic, Sentinel-2 level-2 image and ERA5-Land reanalysis data, respectively. The data with 60 m horizontal resolution were used to calculate the biomass of a snow alga, Sanguina nivaloides, on the Harding Icefield in the summer of 2019. The model results showed that red snow appeared at the low-elevation areas of the ice field in early July and spread over the entire ice field by mid-August. On the other hand, the red snow index estimated from the reflectance ratio of the red and green bands observed by Sentinel-2 indicated that red snow appeared at the low elevations in the north-western area of the ice field in early July. The satellite observations also showed that red snow was widespread, but the area was limited to the north-eastern area throughout the summer season. Although the model was able to reproduce the timing of the red snow appearance in early July, it was unable to reproduce the difference between red snow distributions in the east and west or north and south areas, suggesting that processes not considered in the model are related to the red snow distribution. In the Kenai Peninsula where the Harding Icefield exists, the soil area spreads across the north-western peninsula, and the wind blows from the northwest during summer. The prominent red snow in the north-western may be due to the supply of algal cells to the snow surface via the atmosphere. Hence, we established a simple bioaerosol diffusion model to discuss aerosol deposition processes. The simulation indicated that the deposition was higher in the north-western, north-eastern and southern, in that order. The result suggests that differences in the red snow cells accumulated from the atmosphere are the major factor affecting the red snow distribution.