Clear-Air Turbulence Upstream of a Hydraulic Jump Leading to a Helicopter Accident—A Large Eddy Simulation

Abstract

This study investigates clear-air turbulence that triggered a helicopter accident in the lee of a mountain range near Koriyama, Japan, on 1 February 2020. Using numerical weather prediction models with horizontal resolutions of 1 km and 100 m, we examined the environmental conditions and mechanisms responsible for the turbulence. The models successfully reproduced a downslope windstorm with a hydraulic jump. The finer-resolution model (100 m), which is regarded as a large eddy simulation, revealed intense turbulence induced by Kelvin–Helmholtz instability at the boundary between the downslope wind and the overlying stagnant layer upstream of the hydraulic jump. Vertical wind speeds exceeding 10 m $s^{-1}$ are associated with the turbulence at the flight level. These results highlight the importance of high-resolution simulations for capturing hazardous clear-air turbulence in mountainous regions.