In the upper reaches of the Azusa River, which flows between the Yari-Hotaka mountain range and the Jonen mountain range, air temperatures below -20°C are frequently observed during the severe winter. Monthly runoff in the upper reaches of the Azusa River reaches its minimum in February and increases from May to July. This is because the amount of water resulting from snowmelt within the basin is added to rainfall. Episodic snowmelt occurred in February 2024, which should have been the severe winter, and runoff in the upper reaches of the Azusa River also episodically increased. At the Taisho-ike observation site, snow depth decreased by 58 cm between February 12th and 22nd. During this period, temperatures rose due to the inflow of warm air from the south. A heat balance analysis was conducted to clarify the factors behind episodic snowmelt during the severe winter. Heat balance analysis of the snow surface at the Taisho-ike observation site revealed that during this period, in addition to downward shortwave radiation, downward longwave radiation also increased, and downward sensible heat flux was also observed at times. The large downward longwave radiation and sensible heat flux were unique event in February 2024, and it is believed that the abnormal temperature rise was the cause of the episodic snowmelt during the severe winter.

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