Snowmelt energy balance and its relation to foehn events in Tokachi, Japan

Snow surfaces have unique energy-exchange characteristics, which need to be correctly represented in hydrological and climate models. An intensive field study was conducted in an open farm field in Tokachi, Hokkaido, Japan in which all energy exchange fluxes were monitored during the snowmelt period in 2004. Energy inputs to the snow surface was dominated by net radiation, which provided 75% of total input, while sensible heat contributed significant input on days with strong wind. Snowmelt consumed 80% of energy and evaporation consumed 20%. Sensible and latent heat had similar magnitude but opposite direction, meaning that sensible heat input was nearly cancelled by latent heat loss (i.e., evaporation). Therefore, the snowmelt rate was strongly controlled by net radiation. Compared to previous studies in northern Japan, very high daily evaporation rates, up to 2.2 mm d^-1, were observed during episodic events. High evaporation was caused by the foehn (Tokachi-kaze), characterized by warm, dry, north-westerly wind descending the eastern slope of the mountains. The foehn events were associated with major extratropical cyclones over the sea east, or northeast of Hokkaido. Historical analysis of daily climate data showed that similar high-evaporation events associated with the foehn are common in Tokachi, although the magnitude of the event in 2004 was exceptional. Estimated evaporation rates during the melt periods in 1997-2004 had an average of 0.24 mm d^-1, indicating that evaporation plays a relatively minor role in the overall water balance of the snowpack. However, latent heat flux plays a significant role in energy balance.