Agro-meteorological studies on the down slope wind I

The phenomenon of the slope wind caused by cool air near the ground surface of a slope at clear nights furnishes an interesting problem in micro-meteorology, and at the same time is one of the principal subjects in agro-meteorology, as it apt to generate local differences of low temperature and to cause frost damage on crops especially in cold season. Many investigations on slope wind have been carried out, but it seems that most of them have been founded on only surface observations. Hence the author tried to make observations not only in the surface layer but also in the layer up to the height of about 250 m, and to deduce some conclusions about the slope wind and allied phenomena. The observations were carried out chiefly at a small island (Hakatajima Island in the Inland Sea of Seto), whose area is about 22 km^2 and maximum height is about 304 m from the mean sea level, and partly at a large slope of a mountain (Ogi-yama, Beppu City, Oita Pref.) in order to obtain some comparative data. Though these observations were made from 1949 to 1964, only the results of 1961-1963 are given as the objects of investigation in this paper. The paper is made up of two parts, the first part deals chiefly with the phenomenon of the slope wind and may be said to be micro-meteorological, and the second part concerns with the temperature distribution associated with the slope wind and the frost damage on crops and hence has a character of agro-meteorology. The results of t h e observations are summarized as follows: /A. Concerning the slope wind itself/ (1) The down slope wind on clear and calm nights arises immediately after sunset and lasts till the sun rises next morning, so long as there is no remarkable change in weather condition. The wind velocity, in many cases, is one meter per second and the wind goes on blowing at an almost constant speed all night long. (2) The depth of the down wind layer in a small valley is 25-30 meters thick, and the air temperature there falls till about 4℃, the maximum wind velocity being generally found in the five meters height. (3) The down slope wind found on ridges of either side of a valley and also in the upper part of the valley itself is weaker in strength and more irregular than that in the valley, but has the characteristics of down slope wind blowing down along the slope, the thickness being small and not constant. (4) The observed down slope wind coincides with Prandtl\u27s theory either in regard to the vertical distribution or to the magnitude of its maximum value, but the observed air temperature distribution does not agree with the theory. The reason for the discrepancy is not easy to explain the fact that, though the theory treats the two-dimensional phenomenon, it is three-dimensional in the actual case, seems at present to be the only possibility of explanation. /B. Concerning the influence of the general wind upon the d own slope wind/ (1) When the general wind rises in the inverse direction, the inversion layer grows weak and the velocity of the down slope wind falls. The relation between velocities of the winds is generally expressed by a negative linear function, and when the general wind attains to the velocity of about 1.5 m/s, the down slope wind disappears. (2) Similar relation holds also when the general wind rises in the same direction with the down slope wind, i.e., the latter grows weaker as the former becomes more remarkable. The wind velocity in the valley reaches a minimum and after then increases, showing that the general wind predominates also in the valley. /C. Concerning the circulation of the down slope wind/ Synthesizing the results of the wind in and above a valley of the north-west peninsula at Hakatajima, a return current against the down slope wind was evidently found to exist. As the wind velocity was very weak (less than 1.0 meters per second) and easily disappeared when the general wind intervenes, the domain of the return current was difficult to decide exactly, however, the height of its lower and upper limits seemed to exist between 50-110 m and 100-180 m respectively above the coastal region.晴夜の斜面上の地表付近で発生する冷却空気によつて生ずる斜面下降風は,物理的には対流の一相であつて興味深いが,それらに関連して生ずる夜間接地気温の局地的な気温差は,冬季における農作物の低温障害並びに凍霜害等に関連して,農業経営の合理化の上から極めて重要な問題である.我々の観測並びに調査は,主に瀬戸内海にある伯方島(Lat.34°14\u27N,Long.133°05\u27E)で行なわれたが,比較資料を得るために別府市扇山斜面等においても実施した.なお,これらの観測・研究は1949年より1964年に及んでいるが,主体となつている観測は1961年より1963年の間に行なわれたものである.次にこれらの観測の概要を説明する. (1) 静穏な晴夜における斜面下降風は日没後直ちに発生し,特別な変化のない限り日の出まで続き,地表付近の風速は多くの場合1m/s内外で,夜中大した変化もなくほとんど定速で吹き続いている. (2) 斜面下降風の厚さは20m-30mで,気温低下は地表付近までに約4℃であり,風速の最大は大体5m層に現われている. (3) 側壁の稜線上及び谷に入るまでの上流部における斜面下降風は,谷間のものよりも弱勢でかつ不規則であるが,斜面下降風の特性をそなえ,斜面にそうて流下しており,その厚さは一定でないが相当うすく,10m以下であると思われる. (4) 斜面下降風の観測結果はPrandtlの理論とよく一致する.しかし温度分布の方は一致しない.これは理論は二次元的現象を取扱つているのに対し,実際には三次元的な要素が影響しているためではないかと考えられる.1961年に行なつた観測結果の中で,斜面下降風に対する一般風の影響を要約して説明すると次のごとくなる. (i) 晴夜であつて一般風の介入がなければ斜面下降風は定常的に全夜吹きつづき,系統的な流出時刻は見当らない. (ii) 斜面下降風が発達しているときに一般風が発生すると,逆転屠が弱まつて下降風の風速が低下し,両者の風速の関係はほぼ負の一次関係で表わされるとみてよく,一般風が1.5m/sぐらいに達すると斜面下降風は停止する. (iii) 斜面下降風の背面から一般風が吹き込む場合も上記の関係が成立することは興味深いが,これら事実はさらに多くの地形について観測した後で議論されなければならない.伯方島にある谷で,1962年に行なつた観測結果の中で,斜面下降風によつておこる循環系についての問題を要約するとつぎのようになる. (i) 伯方島の北西部半島上の谷状地における,地上250m高度までの風の観測結果を総合して,かなり明瞭な下降風に対する帰還流の存在することがわかつた.風速は微弱(1.0m/s以下)であつて弱い一般風の介入によつて容易に消失し,厚さ等を正確に決定することは困難であるが,海岸における帰還流の下限の高さは50m-110m,上限は100-180mの間で変化しているようである