Case study of the downslope wind of Japan “Rokko-oroshi”

In Japan, several local strong winds that are unique to a particular region are known by names such as “oroshi,” “dashi,” and “kaze,” including Akagi-oroshi and Kiyokawa-dashi. In the area around Kobe City, northerly downslope winds from Mt. Rokko, or “Rokko-oroshi,” blow. However, there have not been many studies on Rokko-oroshi, and its nature is not yet fully known. In particular, the nature of Rokko-oroshi in the middle of Mt. Rokko (34.7321°N, 135.2539°E) and temperature changes induced by the blow of Rokko-oroshi have not been studied so far. For example, during the blowing of Rokko-oroshi, wind speeds up to 12 m/s can be recorded at the downwind while experiencing a rapid temperature decrease of between 5 and 8°C. In this study, winds and temperatures in the middle of Mt. Rokko were observed to investigate the characteristics and temperature reduction effect of Rokko-oroshi. First, characteristics of wind speed and direction in the middle of Mt. Rokko were clarified to determine the intensity and duration in which Rokko-oroshi blew. Second, hourly changes in temperature, wind speed, and wind direction were analyzed to relate variations in wind speed and direction with temperature reductions upwind and downwind of Mt. Rokko and within Kobe City. Lastly, radiosonde measurements upwind and downwind of Mt. Rokko were inspected to elucidate the physical mechanisms necessary for the generation of Rokko-oroshi.