The Mechanism of Local Wind “Matsubori-kaze” and Relationships with Cold Air Pool Formed in Aso Caldera

  • Shingo NAKAMURA, 2015: The Mechanism of Local Wind “Matsubori-kaze” and Relationships with Cold Air Pool Formed in Aso Caldera .

Abstract


A gorge called “Tateno gorge” is located at the exit of Aso caldera in Kumamoto Prefecture, Japan, in which the Matsubori-kaze, a gap wind, is often observed. Matsukubori-kaze is outflow of cold air pool formed in Aso Caldera at clam night. For two cases on 28 - 29, October 2015 and 29 - 30 October 2015, we examined the relationships between the formation process of cold air pool in Aso caldera and Matsubori-kaze using by observation data and numerical simulations with the Weather Research and Forecasting (WRF) model. In this study, we observed the ‘Matsubori-Kaze’ two consecutive nights of October 28–29 and 29–30, 2015. The ground wind speed of the former was: moderate east wind from the sunset (17:23) to 21:00, and later from 21:00 for about 2 hours, it rapidly increased from 1m/s (approx. 2.24 miles/h) to 3.8m/s (8.5 miles/h). On the other hand, the ground wind speed of the latter was: after the sunset from 17:25, it increased gradually for 15 hours up to 4.2m/s (9.4 miles/h). And immediately after its peak, the wind speed decreased steeply. As mentioned, on the days the ‘Matsubori-Kaze’ occurs, their time series variations can present different patterns. We considered the causes of such two different patterns of the time series variations from the observation of the surface pressure. Calculating the east-west element of the pressure gradient of the synoptic scale by MSM analytical value indicated the pressure gradients were opposite from the ‘Matsubori-Kaze’ in either of the nights. However, as we compared the two nights, the first night was observed with greater pressure gradient. From this result, it is presumed that, in the night of 28-29, the ‘Matsubori-Kaze’ did not occur until the cold layer developed strong enough to overcome the pressure gradient of the synoptic scale, but once it passed a certain threshold, the cold air started flowing out and the wind speed in the valley rapidly picked up. It is also presumed that, in the night of 29-30, the ‘Matsubori-Kaze’ was not prevented by the synoptic scale pattern, and it started flowing out simultaneously as the cold layer developed. Such patterns of the shifts of wind speed closely resemble those of sea breeze front. It is known that the sea breeze front tends to occur when the power of the pressure gradient works to prevent the sea breeze from in-coming. It may be possible to explain the difference of the time series variations of the ‘Matsubori-Kaze’ type II by the analogy of the sea breeze, which is gravity current.

Keywords: local wind, cold air pool, WRF, gap wind, gravity current