The land of Japan is characterized by steep and complex terrain, which may influence the occurrence of extreme weather events such as torrential rain and damaging wind. Since the temporal and spatial variability of winds is very high, the effects of steep/complex terrains are particularly significant in inducing gusty winds. Thus, understanding not only flow structures associated with microscale meteorological phenomena but also terrain effects on wind variability is critically important for the prediction and mitigation of wind disaster. Such understanding should be of use in predicting wind environment as well as in assessing wind energy resources.

In addition, the recent development of mesoscale meteorological models enables one to simulate more accurately the local wind variability under realistically represented meteorological conditions, which lead to evaluating quantitatively the simulated wind fields against observational data in complex terrain. Thus, representing steep/complex terrain in meteorological models with a high spatial resolution becomes more critical for quantitative wind forecasts.

In this study, we develop a high-resolution meteorological simulation system that incorporates 50-m mesh digital elevation map data to a mesoscale meteorological model, the Weather Research and Forecasting (WRF) Model. By resolving small-scale terrain features, the modeling system represents the temporal and spatial variability of strong surface winds induced by mesoscale disturbances. The numerical simulations are performed for high wind cases in a coastal area in order to diagnose the surface wind variability for the safety of railway transportation system. The analysis area chosen is the Shonai Plains, Yamagata Prefecture, Japan, where high wind events frequently occur during the passage of explosively developing extratropical cyclones in winter. The simulations are conducted for the cases in the 2007/2008 winter season.

The high-resolution simulation with the horizontal grid spacing of 300 m can represent short-term and small-scale variability of surface winds that are primarily affected by surface roughness and topography. Even small-scale terrains such as low-topped linear-shaped hill seen in the west of the Shonai Plains have a significant impact on the variability of surface wind speed. Maximum wind speeds at each grid location during the simulation period distribute inhomogeneously over the Plains. In addition, the parameterized TKE values obtained from the turbulence-closure scheme can be used for diagnosing surface strong winds that are resulted from surface roughness and terrain, not from meteorological disturbances. An approach to diagnose gustiness of surface winds from the simulated results is discussed in order to apply the wind simulations to the safety of railway transportation system in a coastal and/or complex terrain.