Outbreak of Hazardous Local Gusts by Interactions of Gradient of Potential Temperature near the Ground with Surrounding Terrain Effect

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Our study showed how the interaction of the gradient of potential temperature near the ground and the configuration of surrounding terrain affects the development of a stronger wind gust using some observation examples and numerical simulation and paying attention to the phenomenon in which the strong wind running over a ridge during a typhoon passage becomes stronger near the foot of a mountain.(1)A cooling box mounted in a floor of a wind tunnel could make a stronger temperature gradient of an approaching flow in a small wind tunnel by combining a device which heats an upper layer of an approaching air by a hot wire.(2)Some wind tunnel tests showed that a temperature gradient, i.e., stability of atmospheric air, affected strongly the growth of a strong wind region near the leeward foot of a two-dimensional mountain model.(3)In the case of the two-dimensional mountain model which has a V-shaped valley, some wind tunnel tests showed the generation of the strong wind region different fr … More om the two-dimensional mountain model.(4)A parametric study on the wind flow over the mountain ridge line with a valley by several commercially available software shows that the locations of the simulated strong wind regions were in good agreement with the wind-tunnel-test result.(5)Although Ogi City in Saga Prefecture was no less than 300km away from the course of Typhoon No.23 in 2004, the big wind damage concentrated in the area. Some analyses by our numerical simulation indicated that a shallow valley in a 1000-meter high mountain which is located in a windward of Ogi City had affected strongly the acceleration of the strong winds in the area. Moreover, the simulation results became more similar to the observed wind speed at the site by taking in the data of actual aerologic observation data for the initial condition (wind-speed inflow conditions) of the calculation.(6)I think, however, the present numerical-simulation technique is difficult to realize precise reproduction of the instantaneous wind speed containing high-frequency components of gust. For forecasting of actual peak gusts, the further creative ingenuity of wind flow simulation is required.(7)Similarly as a result of parametric study of outbreak of a strong local wind seen at the western Fukuoka City area in March, 1998, it turned out that the development of the strong gust in this region can be better reproduced by operating the gradient of potential temperature with Froude number of about 1.6 in addition to the surrounding terrain effect. Less