Effect of terrain on leeward mesoscale field

  • Masato YABE, 2016: Effect of terrain on leeward mesoscale field .

Abstract


In this study, we examined the effects of Sado island in Niigata Prefecture on leeward field in winter monsoon season, through statistical analysis and numerical experiment using WRF model.  In statistical analysis, we class field selected winter monsoon cases based on wind direction and Frude number, and averaged radar reflctivity distribution in each class ed cases. As a result, an area of fewer precipitation is seen in the lee side of Sado island near the Niigata city in each wind direction. However, the relationship between Frude number and the area of fewer precipitation is not recognized.  Numerical experiments were also performed in order to reveal the charactarastic of the atmospheric environment around Sado island in the two cases whose Frude number is different. In the experiments, we reproduced the typical features of the winter monsoon day, including the arae of fewew precipitation near Niigata city. In addition, we performed the tracer experiments to cliarfy the wind field around Sado island. In the tracer experiments, the tracers tended to go over Sado island rather than go around the island in both case. This suggested that the air ahead of Sado island tends to go over the island regardless of the Frude number values.  Moreover, in order to investigate the impact of Sado island on the lee side, sensitivity experiments were performed. The sensitibity experiments were performed in each cases under the condition, in which altitude of Sado island were changed to 0m; PLANE experiments, and the landuse of Sado were changed to sea; SEA experiments. The results showed that in CTRL experiments and PLANE experiments, the area of fewer precipitation was recognized, on the other hand in SEA experiments was not recognized. Further research revealed that ground roughness of Sado island affect the lee side. And it was suggested that the dynamic effect which reduce the wind speed and form down wind area in the lee side of Sado was seen. And furthermore, thermodynamic effect which reduce the sensitibity heat flux and latent heat flux over the Sado island and the lee area was seen.

Keywords: orographic effect、precipitation、Sado island、winter monsoon、Frude number