Boundary Layer Characteristics Over Complex Terrain in Central Taiwan: Observations and Numerical Modeling

The development of the atmospheric boundary layer and flow circulation becomes complicated over complex terrain. A field campaign involving the acquisition of tethersonde, ozonesonde and 10-m meteorological tower observations was conducted during September 2–8, 2013, over the Puli Basin located in the mountainous area of central Taiwan to investigate the evolution of the planetary boundary layer (PBL) and flow circulation. In addition, numerical simulations using the Weather Research and Forecasting (WRF) model were performed to examine the three-dimensional air flow variations. Analysis of the tethersonde data indicated a well-mixed boundary layer structure and a steady westerly flow over the Puli Basin during the daytime that became further enhanced in the afternoon. The 600-m resolution WRF model simulation showed that the enhanced wind flow in the afternoon was due to the inland penetration of the onshore sea breeze, which also transported high concentrations of O₃ from the western metropolitan area into the basin. During the nighttime, a stable boundary layer formed, above which a nocturnal low-level jet formed throughout the campaign period in the Puli Basin. The WRF model is able to reproduce the general variations in the wind flows and thermal structures. Furthermore, the diagnostic PBL height estimated based on the critical Richardson number provides an enhanced understanding of the evolutionary characteristics of the PBL over complex terrain.