An examination of boundary layer structure under the influence of the gap winds in Urumqi, China, during air pollution episode in winter

Tethered-sonde measurements of atmospheric profiles were performed at Urumuqi, capital of the Xinjiang Uyghur Autonomous Region of China, from 29 December 2008 to 14 January 2009. The data were used to examine the boundary layer structure during this severe air pollution period. Diurnal evolution of local wind flow near Urumqi was simulated using the fifth-generation Pennsylvania State University-National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5). Measurements from operational radiosonde data showed that a southeasterly elevated low-level jet often intruded upon Urumqi through the middle Tianshan Mountain pass to the south of the city. The tethered-sonde measurements showed that calm and northwesterly winds prevailed near the surface in Urumqi, whereas the southeasterly winds of relatively higher speed were dominant above similar to 400 m. Both temperature inversion and humidity inversion frequently occured during day and nighttime. Temperature inversion intensity could sharply rise as the stronger elevated southeasterly gale (ESEG) happened. Model simulations showed that the winds near the surface around Urumqi remained calm during nighttime and developed toward the mountains during daytime. As cool air flow in the basin confronted the southeasterly winds from the pass in the lower layer, they formed a convergence line around Urumqi city, which was not favor for dilution of air pollutants. Implications: There is little knowledge about the relationship between air pollution and the local gap wind (well known as the southeasterly gale [SEG]) in Urumqi during winter. On the basis of tethered-sonde measurements and model simulations, the authors showed that the SEG, if only occurring above similar to 400 m (called the elevated SEG [ESEG] in this study), did alter the urban boundary-layer structure and favor formation of a convergence zone near the surface, which was beneficial for occurrence of severe air pollution episode. Therefore, it needs to further study how the ESEG maintains and impacts on atmospheric environment.