Inversion and Fluctuations of Nocturnal Air Temperature in the Kakuda Basin

Abstract

Sudden warming during nocturnal air temperature changes has been explained in terms of the mixture of the inversion layer. This paper intends to report on the fluctuations attributed to another reason.Many mulberries have been grown for sericulture in the Kakuda Basin lying in the north end of the Abukuma Mountains. These mulberries have frequently suffered from frost damage in the late spring. Investigating the nocturaal temperature in this basin, Sasaki and Makita (1972) suggested that the degree of frost damage is related to the fluctuations of nocturanal air temperatures, and Kikuchi (1972) reported that the fluctuations were accompanied by increase in wind speed. Therefore, it is necessary that the fluctuations are recognized under inverse nocturnal temperature conditions.(1) MeasurementThe vertical distribution of air temperatures from 0.3m. to 10m. was measured with recording thermister-thermometers. The observation points were F, G, H, I and MB (Fig. 1). The wind speed was observed at point K. Except for obviously bad weather nights, the air temperatures for 11 nights were analyzed mainly at point G.(2) Some examples of inversion and the fluctuations of nocturnal temperaturesMarch 19-20 (Fig. 2-A) The wind was weak (0-2m/sec.) and the intensity of inverstion from 0.3m. to 2m. (Δ2) showed little fluctuation (0.6-1.3°C), but there were wide changes from 0.3m. to 10m. (Δ10) (1.3-4.3°C). The fluctuations began to appear after 2000 hours and large fluctuations corresponded to increases in wind speed. Warming appeared at every height at 0200 hours and warming at 10m. was especially great. At 0300 hours, warming was remarkable at 10m. but did not take place at 0.3m. Therefore, inversion was intensified at the peaks of the fluctuations.April 1-2 (Fig. 2-B) The wind was strong (4-11/sec m) and the intensity of inversion Δ2 was about 1°C, and Δ10 did not fluctuate widely (0.3-1.5°C). Under these conditions, fluctuations began at 2130 hours and were associated with the times of wind strengthening. The lower the height, the larger the warmings were (for example, at 2230 hours 0.1°C at 10m., 0.4°C at 2m., and 0.6°C at 0.3m.).For these two nights, on the night with strong winds, the inversion of the upper layer was weak and the temperature fluctuations were greater in the lower layer. On the night with weak wind, however, the greater the height, the larger were the temperature fluctuations. Inversion at the peaks of the fluctuations were, therefore, strengthened on the night with weak winds, and were weakened on the night with strong winds.(3) The amplitude and time intervals of air temperature fluctuations in 11 nightsFig. 3 shows that there are many small fluctuations and that the numbers decrease as the fluctuations enlarge. Fig. 4 shows the differences of the fluctuations with height. In this figure, the longitudinal axis is the difference between the mean value of the fluctuations at 10m. on one night and that at 0.3m., and the lateral axis is the mean value of the wind speed at Kakuda City on one night. The wind records are lacking for three night. Except for the night of March 29-30 when the inversion ceased to exist, it is shown in this picture that the fluctuations are larger in the upper layer during the night with weak wind but are equal or somewhat larger in the lower layer on the night with winds over about 3m/sec.Fig. 6 shows the frequency of time intervals of the fluctuations and the mean maximum value at 50 minute intervals.(4) Areal extent of air temperature fluctuations at nightThe nocturnal warmings at point G were associated with increases in wind speed at K, and Fig. 7-A shows the good synchronous fluctuations at points F and G. It is inferred from these phenomena that the fluctuations in nocturnal air temperature change may occur areally, but further investigation is re