Transient downslopewinds under the influence of stationary leewaves from the Zao mountain range

Severe downslope winds accompanying turbulence, which are of great concern to aviation safety, often occur on the lee side of Zao mountain range in winter. Special Doppler lidar observations and numerical simulations with a grid spacing of 100 m were carried out focusing on the downslope winds around the Sendai airport on 14 February 2008. The model reproduced stationary lee waves with an approximate 20-km wavelength, consistent with the observed horizontal scale in water vapor images from the Japanese geostationary multifunctional transport satellite (MTSAT-1R). The simulated lee waves are accompanied by strong vertical shear, which leads to the separation of a vortex sheet from the surface, where small-scale vortices are generated successively. The vortex sheet is under favorable conditions for Kelvin-Helmholtz instability and may account for the generation of small-scale vortices. The simulated vortices, which are advected downstream, are accompanied by weak low-level wind patches. The Doppler lidar observation also captured the weak low-level wind patches with a horizontal scale of 1000-m moving downstream near the surface. An observed temporal variability in line-of-site wind speed decreased with increasing altitude from the surface. This feature was reproduced by the model in a qualitative sense, although the model tended to overestimate (underestimate) the variability below (above) a height of approximately 800m.