A Numerical Study of the Local Downslope Wind "Yamaji-kaze" in Japan

Abstract

In order to study the Yamaji-kaze-a typical downslope wind found in Japan, the two-dimensional flow over an asymmetric mountain is simulated by use of a non-hydrostatic model. The Yamajikaze front and the reversed wind behind the front-characteristic features of the Yamaji-kaze-are explained in terms of the internal hydraulic jump and its associated circulation.
Numerical experiments for a homogeneous atmosphere show that the behavior of the internal hydraulic jump is significantly affected by the inverse Froude number and the shape of the mountain. When the inverse Froude number is large, a quasi-steady state solution such that the hydraulic jump remains on the lee side of mountain is obtained, with the associated reversed flow being generated just behind the hydraulic jump. In the case of the Yamaji-kaze, the asymmetry of the Shikoku Mountains and the blocking effect of the Chugoku Mountains impede the propagation of the Yamaji-kaze front and allow the reversed wind to occur more readily.
For the case of the Yamaji-kaze observed on 21 April 1987, a notable inversion layer was found at a level near the mountaintop. It is confirmed, by numerical experiments of a heterogeneous atmosphere with the observed thermal stratification, that the surface wind strengthens in the presence of the inversion when compared to that without the inversion. Development and propagation of an internal hydraulic jump are qualitatively simulated under the observed thermal stratification and timechanging wind profile. On the basis of the experimental results a conceptual model of the Yamaji-kaze is proposed.