[A-CC45] Glaciology

[ACC45-P04] Time-lapse photography of red snow algal blooming on Gulkana Glacier in Alaska

*Keisuke Shibuya¹, Nozomu Takeuchi¹, Yukihiko Onuma¹, Ryohei Abe¹, Suzunosuke Usuba ¹, Takumi Suzuki¹, MASATO ONO¹, Hiroyuki Oguma (1.Graduate School of Science and Engineering, Chiba University)

Keywords:

Red Snow,Time-lapse photography,Gulkana Glacier,Alaska

Red snow phenomena caused by snow algal blooming can be observed worldwide during the melt season. Their blooms can reduce the surface albedo of snowpacks and accelerate snowmelt. This study aimed to determine the onset of red algal blooms in the upper reaches of the Gulkana Glacier, Alaska, using time-lapse photography taken at 1-hour intervals from July 3 to August 28, 2023. For the analysis, a total of 185 images were used, selected from clear sky conditions between 9:00 and 14:00, when solar radiation and weather conditions were relatively stable. Eleven regions of interest (ROIs) were selected on the glacier snow surface within the images, and the ratio of Red to Green pixel values (R/G ratio) was calculated for each region as a proxy for red snow biomass. The image analysis visually confirmed that red snow appeared on parts of the glacier surface starting in mid-July and gradually expanded through August. While the R/G ratio for each ROI fluctuated significantly with time of day and weather conditions, stable values were obtained by limiting the analysis to clear-sky conditions. By setting a threshold for the R/G ratio and determining the onset date when the ratio exceeded this threshold for each ROI, it was found that red snow appeared earliest on the south-facing slope (July 9), followed by the flat area of the main glacier flow (July 21), and then the upper reaches on the eastern side (July 23-24). On high-elevation snow surfaces near the summit, the R/G ratio did not exceed the threshold, indicating no red snow occurred. The chronology of red snow appearance cannot be explained solely by elevation; instead, surface slope, aspect, and the associated flow of meltwater are suggested to be contributing factors. These results suggest that red snow on glaciers does not necessarily develop from downstream to upstream with increasing elevation, but is instead determined by local topographical conditions on the glacier surface.