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

Part 2: Non-Linear Aspect of the 3-D Flow over a Mountain Range with a Col

Abstract

Following the previous work (Saito and Ikawa, 1991a), the three dimensional effect of the orography of the Shikoku Mountains on the Yamaji-kaze is studied numerically, focusing on the effect of a col to the flow over a mountain range. The geographical characteristics of the Yamaji-kaze are explained in terms of the non-linear aspect of the three-dimensional flow over a mountain range with a col.
Flow régimes of the three-dimensional flow over a mountain range with a periodic col are examined by use of a linear analytic solution for a homogeneous environmental atmosphere and numerical experiments using a non-hydrostatic model. Two parameters, a non-dimensional mountaintop height lh_m, and the ratio of the amplitude of the col to the mean mountaintop height h_c/h_m, are used to prescribe the mountain shape. The critical values of lh_m which divide the flow regimes are about 20% smaller in the numerical experiments compared to the linear theory. When the mountain range has a col, wave breaking occurs readily in the lee of the peak of the ridge, and the internal hydraulic jump and reversed flow behind the jump occurs for smaller lh_m compared to the two-dimensional cases. On the other hand, in the lee of the col, a strong wind area easily extends leeward, and the appearance of the jump is unclear in many cases. The behavior of the hydraulic jump is quite sensitive to the existence of the col, while the upstream blocking is rather insensitive to the existence of the col. The response of the hydraulic jump to the existence of a col has qualitative similarities to the behavior of the hydraulic jump in a shallow water flow over a mountain range in a channel of variable width, which has been presented by Saito (1992).
Numerical experiments using the real orography of Shikoku Island and the thermal stratification observed on 21 April 1987 are performed as an example of the simulation of a typical Yamaji-kaze, and the development and movement of the internal hydraulic jump is simulated under a time-changing wind profile. It is shown that the timing of the onset of the Yamaji-kaze, which is too early in the two-dimensional simulation, is ameliorated in the three-dimensional simulation. The geographical characteristics of the Yamaji-kaze can be explained by the reversed flow behind the hydraulic jump which stays in the lee of the Shikoku Mountains and a low-level strong wind area which extends in the lee of the col of the Shikoku Mountains. A further improved surface wind pattern is obtained by inclusion of the surface friction. An improved version of the conceptual model of the Yamaji-kaze is proposed on the basis of the results of the three-dimensional experiments.