Study of optimal protection of crops on the cliff from wind damage Absract The purpose of this study is to examine the methods for protecting crops against typhoon wind on islands area formed by the cliff. In the first examination, the authors measure the wind speed distribution near the cliff using the multipoint observation system composed of sonic anemometers. Then, the wind shielding effects of windbreaks net were evaluated using observation data. The observation of typhoon was done at Miyagi island in Okinawa prefecture in Japan. Okinawa is an area frequently attacked by heavy typhoons. For example, maximum instantaneous wind speed of 74.1m/s was recorded on the occasion of typhoon 0314 (MAEMY) in 2003. Since then, maximum instantaneous wind speed of 60m/s and more was frequently recorded. Thus, Okinawa's agriculture has become difficult to maintain stable cultivation. The observation field at Miyagi island has 90m above sea level, the top on the island is almost flat and 1,130,000 square meters of cultivated land. Cliffs surround the island, and there were about 5m high woodland on the cliff. Characteristics of windbreaks net are divided into top and bottom, which are arranged in two rows. It is called the uneven dual-screen windbreak (UDSW) .The height of UDSW is 400cm in top and 200cm in bottom, and the mesh size of net is 1mm. The windbreaks net are placed in a rectangular, 120m for long side, 100m for short side. The results of wind tunnel experiment for the UDSW show the same wind shielding effects as the general windbreak nets. Further more, the optimal arrangement of UDSW is expected to secure the adequate amount of airflow and sunlight. Wind speed distribution is measured during Typhoon 0310, 0315, 0404, 0418. The maximum instantaneous wind speeds measured at Miyagi island were respectively 54.4m/s, 30.1m/s, 18.4m/s, 36.4m/s by present observations. The mean wind direction used in analysis was respectively NE, ESE, NE, SW. In addition, the Verification of wind shielding effect of windbreaks net was carried out for Typhoon 0310. The result shows that the mean wind speed increases with distance from the cliff in all of wind directions. Especially, within the range of about 300m from the cliff, the wind increased rapidly. On the other hand, turbulence intensity tended to decrease with distance from the cliff. The relations between the distance from the cliff edge and the mean wind speed were indicated a quadratic function of the distance from the edge. This seems to mean that the internal boundary layer is developed above the cliff ground. The mean wind speed ratio measured at a height of 3m to 6m from the ground was about 0.9. However, the influence of topographic relief and sparse forest zone caused to increase the turbulence intensity and decreased the mean wind speed. For example, in Typhoon 0315, the turbulent intensity and the mean wind speed in such irregular ground surface were 1.6 times larger and 30% lower than in the flat surface, respectively. The authors have estimated the wind shielding effect of the UDSW by a quadratic approximation formula based on the measured data. The linear with the high correlation showed between the mean wind speed and the distance from the cliff. The correlation coefficient is higher than the relation between the net. Wind characteristics have a deep connection with not only the distance from the net but also the distance from the cliff. Specifically, the wind speed at 28m leeward from the UDSW was reduced by 25% at a height of 6m, and that at 90m leeward was reduced by 17%. The wind speed ratio at a height of 3m to 6m was increased with distance from the UDSW about 0.8 to 0.9. The maximum instantaneous wind speeds of 50m/s or more were measured at every observation point, and regardless of inside or outside of the net. It shows that the maximum instantaneous winds have no concern with the distance from cliff or UDSW. Key words: typhoon, windbreak net, wind speed reduction ratio, cliff,sonic anemometer