Large Amplitude Fluctuations of Mountain Waves Observed Downwind of Ou Mountain Range

On the early morning of February 1, 2020, a helicopter accident occurred near Koriyama City, Fukushima Prefecture, Japan (for details, see Aircraft Accident Investigation Report AA2024-1, Japan Transportation Safety Board). A high-resolution weather simulation was conducted for this case using the Japan Meteorological Agency’s non-hydrostatic model. On that day, due to a winter monsoon, westerly winds were predominant in the Tohoku region, and a prominent hydraulic jump was observed downwind of Ou Mountain Range. According to a simulation with a horizontal resolution of 1km, a situation where the vertical flow was particularly large near the accident site was reproduced. Furthermore, when the horizontal resolution was increased to 100m, fluctuations were observed in which the amplitude of the hydraulic jump increased on a one-minute scale, and the updraft and downdraft at the helicopter’s flight altitude exceeded 10 m/s. These large amplitude fluctuations were accompanied by a significant negative perturbation in surface pressure, suggesting the possibility of capturing the large amplitude fluctuations (pulsating downslope wind) in downwind of the mountain.