夜間の安定層の破壊に及ぼす地形の影響

(1) 早来と千歳の夜間の気温低下と地形

抄録

To research regional difference of frost damage, meteorological observations were made at Hayakita, located in the south-east of the Ishikari-Yuhutsu plain in Hokkaido. Hayakita has crops that are the most easily damaged by frost in the plain. In particular paddy rice plants are damaged by the first frost when their growth is retarded by a cool summer. The first frost in Hayakita occurs 1 week earlier than in other parts of the plain. To research this problem, Chitose that has different topography from Hayakita, was selected as a control area and some meteorological factors such as air temperature, wind speed, solar radiation, net radiation and downward radiation were compared between Hayakita and Chitose. These areas are at the same altitude and share similar surface features. Although Hayakita is surrounded by hills, Chitose is in the center of the plain and is located 12 kilometers away from Hayakita.
In this paper, the daily minimum air temperatures and 4-hour mean values of the meteorological factors in the springs and falls of 1983 and 1984 were compared between Hayakita and Chitose. Also the variations of these factors on clear nights were compared.
Daily minimum air temperatures in both seasons were not significantly different between Hayakita and Chitose. This means that the advection and accumulation of air mass cooled on surrounding slopes are not significant as causes of the frost damage in Hayakita.
The 4-hour mean value of wind speed at Chitose was always greater than that at Hayakita. However, there were no significant differences for the other factors, such as solar radiation, net radiation and downward radiation between Hayakita and Chitose. The air temperature in Hayakita was often lower than that in Chitose, especially in the lower range of temperatures. This tendency was more remarkable in fall than in spring. Sometimes the temperature difference between the two regions reached approximately 5K in fall, accompanied with a large difference of wind speed.
This large difference of air temperature tended to be observed from 20:00 to 4:00 of clear nights. It was caused by temperatures in Chitose often being rapidly increased by raising wind speed during that time, but both temperature and wind speed in Hayakita remained low and unchanged. When comparing temperature profiles to 80 meters above ground level of both regions, it was noted that the stable layer formed by radiative cooling was destroyed from upper portion to near ground surface at Chitose, while, on the other hand, only the upper portion of the layer was destroyed at Hayakita. However, both stable layers remained during the nights when no temperature difference occurred between the regions. Therefore, the occurrence of temperature differences between the regions is due to the difference in the destruction ratio of the stable layer.
The phenomena mentioned above, often appeared in the center of the plain when the upper wind (geostrophic wind) speed and/or direction changed. The upper wind data were obtained from radiosonde data at the 900mb isobaric surface above Sapporo. The changes of wind were classified by following two patterns: i) upper wind speed very small at first, becoming stronger later, and ii) upper wind direction changing from a direction in which WSR (Wind Speed Ratio: the ratio of surface wind speed to upper wind speed at a direction) is small to other one in which WSR is greater. Greater WSR means greater wind speed at ground level for a given upper wind speed; WSR is influenced by surrounding geographic features. Therefore, the WSR values at the center of the plain, lying between the high mountains to the east and west, were large for the north-south direction and small for the east-west direction. On the other hand, WSR values were relatively small for all directions in Hayakita. Thus wind speed in Hayakita was always smaller than that in the center of the plain.
It can thus be recognized that low temperatures conti