A Foehn Event Enhanced by the Synoptic Scale Dry Intrusion

摘要

The purpose of this article is to report the result of our effort to uncover the nature, and possible cause(s), of the high wind that was responsible for some human and property damage in the morning of 2 March 2003 in Yaita city, Totigi prefecture. Our approach is through the analysis of observed data and all data used are conventional. It is found that the damage-causing high wind was a foehn-like downslope wind, which occurred when a bomb-class midlatitude cyclone passed over the Kanto area, and Yaita city was situated on the lee side of the mountain range in the strong northwesterly flow in the back side of the cyclone. The unique feature of the present high wind event is that it was followed by another foehn-like episode, in that surface temperature was higher and dewpoint temperature was lower than in the preceding foehn, while the wind was less violent. It is concluded, through the analysis of the precipitation field, that the first foehn was wet (thermodynamic), whereas the second foehn was dry (dynamic). Further, in our quest to understand why the surface air was warmer and dryer in the second foehn, it is found that, by the time of the commencement of the second foehn, the region of the unusually strong mid-level descending motion, representing the dry intrusion, had approached the Kanto area and this helped bring down the air possessing high potential temperature and low mixing ratio into the downslope wind in the second foehn. Finally, it is pointed out that a well-defined low-level jet was present in the northwesterly flow behind the cyclone center and the strong wind associated with the low level jet was instrumental in the present foehn event. Thus, presented here are documentations showing the sequence of events associated with successive foehn formations, that occurred in a quite limited local area but was intimately linked with the evolution and structure of a synoptic-scale midlatitude cyclone.