Abstract
In my previous papers I have statistically studied the individual cases of cyclonic rainfalls in Japan. As the result of my investigation, I have found that the cyclonic rainfalls are usually much affected by the mountain ranges. Dr. T. Okada has proposed in his Text book on Meteorology that the actual regional distribution of rainfall in our island Empire during the visitation of cyclone results from a combined effect of cyclonic and orographic rainfalls. We have affirmed this fact by means of many typical examples which were classified according to the tracks of cyclones.
We have found that the orographic effects are of two kinds; the first one is due to the forced ascending motion of air along the mountain side, and the second one is that which results from the horizontally deflecting motion of lower winds caused by the mountain ranges. In the case of the orographic effect of the first kind, heavy rainfalls usually occur in the mountainous region in general while in the case of the second kind heavy falls occur in the plane or the valley near the mountain. The colder surface wind is usually mild and blows in an opposite direction to or at about right angles to the upper wind. Now we may propose that the second kind is a result of the formation of the surface of discontinuity in the atmospheric strata due to the mountain ranges.
In the present paper we will consider the heavy rainfalls which lasted over one or several days associated with tropical cyclones, which travelled over Formosa and its neighbourhood. The typhoons, which pass over this district may be generally divided into four classes according to their tracks, that is, (i) typhoon which moves west or north-westward passing along the northern coast of Formosa; (ii) typhoon which moves in the same direction passing through the southern part of Formosa; (iii) typhoon which passes through the middle part of the island from southeast to northwest; (iv) typhoon which travels along the eastern coast of Formosa from south to north. During the visitation of the typhoon of the first category, heavy rainfall often occurs on the western side of the mountain range. A fall of 1034mm, the record of the daily maximum amount of rainfall in Japan occurred at Hunkiko on Augut 31, 1911, associated with a large typhoon of the same class. On the eastern side of the mountain range the precipitation is very slight, owing to a descending motion of ærial current over the mountain barrier. In the case of the typhoon of the second class, the heavy rainfall occurs in the eastern and southern parts, and is rare in the northwestern region. In the case of the typhoon of the third class, the heavy fall occurs in the mountainous region of the southern, northern and northeast parts. In the case of the typhoon of the fourth class heavy fall occurs in the eastern parts of the island.
On the occasion of the cyclonic centre situating far from Formosa, the heavy local rainfall associated with southwest wind in summer occurs sometimes in the southern part but in winter the heavy rainfalls at the north extremity of this island.
