Climatological characteristics of the Ibuki-oroshi and its blowing mechanism

  • Shunsuke YAMADA, 2016: Climatological characteristics of the Ibuki-oroshi and its blowing mechanism .

Abstract


In this study, the mechanism of strong wind called “Ibuki Oroshi” which occurs in the Nobi Plain spreading mainly in Aichi and Gifu were investigated. First, based on the previous research, we defined Ibuki oroshi in this research. As a result, 136 cases occurred between 2012 and 2014, and it turned out that it mainly occurs from the end of winter to the spring. Next, we classified Ibuki oroshi as five patterns from the distribution of ground wind when it occurred. Type I is when strong Northwest wind is blowing in all of Aisai, Ogaki and Ibigawa, type II is when strong winds is blowing in Ogaki, type Ⅲ(northwest wind in Aisai)、Ⅳ(west-northwest wind in Aisai) is when weak wind is blowing in Ogaki, type V is when westerly wind is blowing in all of Aisai, Ogaki and Ibigawa. In this study, we analyzed case I and type II in detail. In three years, there are 23 cases of type I, mainly occurred in the winter, and it tends to be a strong wind in any area of the Nobi Plain. In addition, from the observation data of the radio sonde in Wajima, it was found that the inversion layer existed at a position higher than the summit of Ibuki Mountains (altitude around 1000 m). This is seen in 21 cases out of 23 cases, and it can be said that it is a feature of Type I as a whole. In addition, from the results of backward trajectory analysis based on the results of numerical simulation, it was revealed that the air existing near the ground of the Nobi Plain when Type I occurred had passed over the Ibuki mountains from the northwest direction and rapidly descends on the leeward slope, and it reached near the ground. From these results, it is thought that Ibuki oroshi type I is “oroshi kaze”. There are 25 cases of type II, mainly caused by the approach of high pressure and the passage of low pressure in the spring. Strong winds in Ogaki spread to the east. In addition, from radio sonde observation data on the day of occurrence, it was revealed that there were 21 cases which there was an inversion layer at a position lower than the summit of Ibuki Mountains. Numerical simulation results also showed that the existence of this inversion layer existed above the Sekigahara. And its altitude was lowered downwind. Furthermore, from the results of the numerical simulation, it is known that the wind direction changed and the wind speed was also increasing only around the surface of the Sekigahara. Furthermore, from the results of backward trajectory analysis, it was found that a wide range of air flowed into the Sekigahara during strong winds, spreading as a strong wind in the wide area of the Nobi Plain after passing through Sekigahara. And it was revealed that the increase of the wind speed was mainly inside the Sekigahara, and its downwind. From these results, it is considered that Ibuki oroshi type II is gap wind.

Keywords:local wind, gap wind, oroshi kaze, inversion layer, Ibuki Oroshi