A LOCAL CLIMATOLOGICAL STUDY OF WIND IN THE MIYAKO ISLAND

Abstract

In order to study the local distribution of winds in the Miyako Island, one of the Ryukyu Islands, Southwest Japan, an attempt was made to observe the wind-shaped trees of Casuarina equisetifolia as an indcator of prevailing wind conditions. According to the climatological values observed at the Miyakojima meteorological observatory located at a point of 24°47'N, 125°I7'E, 39m a. s.l. , NE-NNE winds prevail from October to March. These winds are relatively stronger than the winds from May to August. Namely, monthly mean wind velocity is 6.5m/s in the cold season, and 5.3m/s in August. S-SSW winds are striking from May to July. The wind-shaped trees were observed at the 83 points in the island with an area of 159km^2 and an average of 3-4 trees was obtained at each point. Fig. 1 illustrates the scale of wind-shaped trees, which is determined in reference to the previous studies in the other regions. The result is shown in Fig. 2. Important facts revealed in this figure are: 1) The coastal region with a width of 500m-1,000m facing NE has the prevailing NE-ENE winds. On the other hand, there is also the ENE wind region at the south-western part of the island. Between these two regions, SSE-SE winds develop in a region with a width of 8km. They become S wind near the Hirara City. 2) Comparing the observed result of wind-shaped trees with those by the instruments at the meteorological observatory, the wind-shaped trees of Casuarina equisetifolia indicate the wind conditions during the warmer season, especially from August to September. The summer conditions of monthly mean wind velocity 5m/s with monthly mean temperatures of 27-28℃ overcome the winter conditions of monthly mean wind velocity 6.5m/s with monthly mean temperatures of 17-19℃ in the tree growing forms. 3) It is supposed that the winds are weaker in summer and stronger in winter in the ENE-NE wind regions than in the region near the meteorological observatory. 4) The coastal region facing NE has strong ENE-NE winds, because the north-easterly winter monsoon hits directly this coast. On the other hand, the other ENE-NE wind region at the south-western part of the island is considered to de appeared as strong falling lee winds crossing over the low hills with a height of 80-100m a. s. l. 5) An interesting phenomena of "shrub dunes" composed of Robinia pseudo-acacia \vas observed at the southeastern part of the islands, as shown in Fig. 3. Above the "shrub dune", the wind-shaped trees of Casuarina equisetifolia were seen. The upper part of the dune and the Casuarina trees do show smooth stream lines. The cross-section of the "shrub cune" resembles to that of barkhan-type sand dune.