| dc.description.abstract | Puli basin is located in the central mountainous area in Taiwan, the surrounding terrain height is about 800 to 2000 m. Under a weak synoptic weather system, the thermally driven circulations such as the upslope/downslope wind and mountain/valley wind generally develop and dominate the local airflow in Puli basin.
The observed sounding data indicated some characteristics of atmospheric structure. During the daytime, there is a strong westerly wind preduced by the thermal heating in the near surface layer and upper level over Puli basin. During the nighttime, the radiation cooling causes the nighttime inversion layer in the lower level and forms the nighttime stable layer (SBL). Strong easterly wind is also observed above SBL.
However, it is difficult to accurately simulate the development of atmospheric motions over the mountainous area. In order to understand the characteristics of the planetary boundary layer (PBL) structure and the local circulations over the complex terrain in Taiwan, the Weather Research and Forecasting (WRF) Model at a fine resolution (600-m) was applied with the high resolution terrain data. Simulation results were evaluated with the observed sounding data and surface stations to characterize the PBL structure and local airflows over the central mountainous area of Taiwan.
The model performance is better represented at fine scale resolution than the coarse resolution. The ambient wind speed decreases significantly, and it reduces the overestimation of the surface wind speed in the whole Taiwan area. The performance is not only good at the wind field but also good with a correlation coefficient of temperature at 0.9 (highly correlated). The use of the high resolution topographical data from Shuttle Radar Topography Mission which was published by National Aeronautics and Space Administration enhances the WRF simulation performance, particularly over the complex terrain. And it also helps characterize the PBL structures and local air flow in Puli basin. In the morning, the nocturnal stable boundary layer (SBL) disappears and the wind speed remains low due to the divergence induced by developing upslope wind. In the afternoon, atmosphere become well-mixed and the westerly flow which is composed of the sea breeze and up-valley wind prevailing over the basin. During the nighttime, there is strong easterly wind above the nocturnal SBL by the downslope wind.
In addition, two different PBL scheme (Yonsei University, YSU and Shin-Hong scale-aware, SH) were applied to study the influence of the PBL physical processes on the simulated vertical structures over the complex terrain in Taiwan. The main difference between the two PBL schemes is the algorithm for nonlocal PBL parameterization and the scale dependency of the subgrid-scale transport. The SH PBL scheme considers a more accurate nonlocal heat flux profile and multiplies the grid-size dependency function with the vertical transport term which is found in large-eddy simulation. By using SH PBL scheme, the resolved motion can be improved and the simulated convection structure can be maintained at the gray zone resolution. The results from SH PBL scheme show the better simulation of the theta and wind speed profile compare to the YSU scheme, and highlight the characteristics of observation such as a strong westerly wind in the near surface in the afternoon.
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