Characteristics and a Mechanism of the Wind in the Ishikari Lowland, Hokkaido

  • Shuhei ENDO, 2016: Characteristics and a Mechanism of the Wind in the Ishikari Lowland, Hokkaido .

Abstract


In the Ishikari lowland, strong winds sometimes blow from northwest or southeast. The season and the resions of these strong winds, however, are not revealed sufficiently. Also, the structure of generating these winds is not clear. The purpose of this paper is to clarify characteristics and to purpose the mechanism of the strong wind in Ishikari lowland. In analyzing, the strong wind was defined as the wind that blew faster than 8 m s-1 at the AMeDAS observation point. Also, when the wind direction of AMeDAS Sapporo was from the west to the north, it was defined as the northwesterly wind type, and the time that wind direction was from the east to the south was defined as the southeasterly wind type. First, AMeDAS data was analyzed. As a result, it was found that the strong wind of the northwesterly wind type in the Ishikari lowland could be divided into two types, “high-pressure in west and low-pressure in east” pattern (HWLE pattern), and DEPRESSION pattern. HWLE pattern, which is famous pressure pattern in winter around Japan, could be seen mainly in from January to March, and strong winds blew only on the inflow side of the lowland. On the other hand, DEPRESSION pattern were frequently seen in from March to May, and strong winds blew only on the outflow side of lowland. Strong winds of southeasterly wind type in the Ishikari lowland could also be divided into two types, “high-pressure in east and low-pressure in west” pattern (HELW pattern) and WESTERN DEPRESSION pattern. HELW pattern could be seen many times from April to May, and there were many strong winds inside the lowland and entrance of the inflow side. WESTERN DEPRESSION was frequently seen in from September to December, and strong winds blew only in the area from the left bank of the inflow side of the lowland to the entrance. Next, we conducted a numerical reproduction experiment using WRF for the case of strong winds corresponding to the DEPRESSION pattern of the northwesterly wind type. As a result, when a strong wind was seen on the outflow side, it was found that a clear inversion layer existed over the middle part of the lowland. It was also found that the bottom height of the inversion layer was more likely to descend toward the south. In the region where the inversion layer existed, the horizontal high-speed wind was restricted under the inversion layer, and the wind speed above the inversion layer was low. Since a inversion layer was seen clearly, the two-layer fluid internal Froude number Fi was calculated. As a result, it was found that Fi continued to increase from the middle of the lowland to outflow. This means that the regime of flow passing through the Ishikari lowland is changing from the subcritical flow to the supercritical flow. To explain the mechanism of generating northwesterly strong winds corresponding to the DEPRESSION pattern, the theory of uncompressive two-layer fluid flowing on the topography where the bottom height and valley width change gradually is applied. As a result, the mechanism of strong northwesterly wind is suggested as followed. The regime of flow changes from the subcritical flow to the supercritical flow when passing through the lowland, accordingly the thickness of the lower layer continue to decrease with the change in the valley width, and the flow continues to accelarate. Even in the numerical simulation experiment, the bottom height of the inversion layer descend toward the south side of the lowland, and the thickness of the lower layer decrease. Therefore, it is shown that the suggested mechanism obtained by using the theory of two-layer fluid can explain the reason of generating strong wind. This strong wind mechanism has been shown as one of mechanisms of gap wind in conventional studies. In addition, the feature that the wind becomes strongest on the downstream side of the narrowest part of the valley is also consistent with the previous study of gap wind. Ttherefore, it is concluded that the strong wind of DEPRESSION pattern of the northwestern wind type seems to be a typical gap wind.

Key words: Ishikari lowland,inversion layer,gap wind,internal Froude number,subcritical flow,supercritical flow