宮古島の風の小気候的調査

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.