Temporal Evolution and Spatial Structure of the Local Easterly Wind "Kiyokawa-Dashi" in Japan PART I: Coherent Doppler Lidar Observations

Abstract

The Kiyokawa-dashi (strong local wind) was observed with a coherent Doppler lidar at the exit of the Mogami Valley, in Kiyokawa (38.80°N, 140.01°E, 20 m MSL) during the summers of 2003 and 2004. Used to define days on which the Kiyokawa-dashi wind blew were meteorological data obtained from four automatic weather stations installed at four observation sites: Kiyokawa, Karikawa (38.79 °N, 139.99°E, 15 m MSL), Mawadate (38.81°N, 139.92°E, 10 m MSL) and Hirono (38.83°N, 139.85°E, 5 m MSL). When the Kiyokawa-dashi blew, the synoptic patterns tended to be high-pressure in the east and low-pressure in the west. After the Kiyokawa-dashi was extracted, the data measured by the coherent Doppler lidar with a high spatial and temporal resolution were then utilized to investigate it. On the basis of the range height indicator and Plane position indicator scans, the coherent Doppler lidar was able to detect the three-dimensional dynamical structure of the Kiyokawa-dashi up to 8-10 km downstream from the Kiyokawa observation site. The coherent Doppler lidar clearly showed that a low-level critical layer (horizontal wind speed Approx 0 m s -1 ) existed at altitudes of 0.5-1.2 km, and that a strong easterly/southeasterly wind confined below the critical layer blows away about 10-km downstream of the Mogami Valley. Intensive upward/downward vertical motion under the critical layer was indicated by the velocity-azimuth display scan. The line-of-sight wind speeds obtained by high-frequency plane position indicator scans were high, not only near the exit of the Mogami Valley but also over the entire Shonai plain, and the distributions of the line-of-sight wind speeds were almost steady, indicating the existence of transient disturbances.