Hayakita, which is surrounded by hills like a basin, has strong stable air layers due to radiative cooling and is compared with Chitose which lies in the center of Yuhutsu plain. The Hayakita area also suffers from frost damage since low temperatures remain until early morning (Matsuoka et al., 1987). It was considered that the upper wind plays an important role in maintaining stable layers in Hayakita. In this study, relations between upper wind speed and surface wind speed were investigated.
Wind speeds were measured at Hilltop as well as at Hayakita and at Chitose. The wind speed at Hilltop was compared with the upper wind speed which is at the 900mb isobaric surface, above Sapporo (V₉₀₀). The analysis revealed;
i) Wind speed at Hilltop (U_T) is greater than 2m/sec.
ii) Wind speeds at 900mb and 800mb isobaric surface above Sapporo (V₉₀₀ and V₈₀₀ respectively) are greater than 2m/sec.
iii) Vertical shear (|V₉₀₀-V₈₀₀|) is less than 6m/sec.
The regression equation between wind speed at Hilltop and at 900mb isobaric surface is given in eq. (1). The correlation coefficient (r) is significant to 1% level. Thus it may be stated that the wind speed at Hilltop represents upper wind speed at the 900mb isobaric surface.
The relations between wind speed at Hilltop (U_T) and, those at Hayakita and Chitose (U_H and U_C respectively) were investigated on cloudy nights. It was considered that this type of situation, which was judged on the basis of 4-hour mean cloud amount being over 9.0 at the Chitose airport, has nearly neutral conditions aerodynamically. Eq. (2) shows relations between U_T and U_H, and, U_T and U_C. The gradient of the regression line for Chitose is greater than that for Hayakita. From the relation between U_H and U_C, eq. (3) was obtained using a hypothesis of logarithmic law of wind speed profile. The roughness parameters at Hayakita and Chitose were calculated to be 0.43m and 0.09m respectively, using the methods employed by Kondo and Yamazawa (1983). If this value at Chitose is substituted for Z_0C in eq. (3), the roughness parameter at Hayakita (Z_0H) is calculated to be 0.42m which is nearly equal to 0.43m as mentioned above. Thus roughness parameter (Z₀) shown by eq. (3) can express topographical feature.
On the clear nights which have below 2.0 of mean cloud amount, the relations between wind speed at Hilltop, and wind speed at Hayakita or Chitose resulted by eq. (4). These nights have stable conditions aerodynamically. The gradients of the regression equations are smaller than those under the neutral conditions, but the tendency that gradient at Chitose is greater than that at Hayakita, remains in eq. (4).
The relationship between the time taken to destroy stable layers by entrainment and topographical feature, was investigated on the nights when there was wind speed increases at Hilltop. The crosscorrelation between the sequence of wind speed at Hayakita (or Chitose), and that at Hilltop was calculated for this analysis. Under steady state conditions, time taken to destroy stable layers (L) is represented as eq. (8). By inputting observed values into eq. (8), the regression equations are given in eq. (9). The regression constant at Hayakita (B=1.13) is larger than that at Chitose (B=0.96). The hypothesis that B is a unique constant for each location is justified. Thus, if regression equations such as eq. (9) are determined, we can predict the duration of low temperature periods using potential temperature gradient in the stable layer and upper wind speed.

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