Abstract: The mountain-valley breeze in Urumqi area, Xinjiang, China has an important impact on the aggregation,diffusion and transport of atmospheric pollutants. The temporal and spatial distribution characteristics of mountain-valley breeze in Urumqi area were analyzed with 10 min mean wind data observed from 33 ground automatic weather stations from 2011 to 2015. The annual average number of days of the mountain-valley breeze was 245 days, more in urban area and north of the city. The sequence of mountain-valley breeze days was as follows:summer(77.5 d) > spring(70.3 d) > autumn(39.3 d) > winter(38.5 d). Although there was an obvious transition period, the mountain breeze and valley breeze moved in opposite directions. The mountain breeze began to appear in the range of 22:00 to 23:00,ended in the range of 08:00 to 11:00 of the next day, the duration was 10-13 h. In contrast,the valley breeze appeared in the range of 11:00 to 14:00 and ended in the range of 19:00 to 21:00, the duration was 6-9 h,which was 4 h more than that of the mountain breeze. The average intensities of the mountain breeze and valley breeze were 1.1 m·s^-1 and 1.6 m·s^-1, respectively. From north to south, the average annual mountain breeze and valley breeze showed a unimodal distribution. For Anningqu, Gaoxinqu, Wulabo and Suigonggou Station, the mountain breeze was 0.7, 0.8, 1.6 and 1.3 m·s^-1, respectively. At the same time, the valley breeze was 1.0, 1.3, 2.5 and 1.7 m·s^-1, respectively. The annual average for mountain breeze was 0.8-1.3 m·s^-1 and for valley breeze was 1.5-1.7 m·s^-1, both were weak in the winter, and strong in spring; in addition, mountain breeze was also strong in summer. The mountain wind was perpendicular to topographic contour, as downslope wind, and its intensity was gradually weakened from the mountains to the valley. The city is a convergence zone of wind speed and wind direction for mountain wind. The pollutants, which are discharged by enterprises in the south of city, are gathered in urban area under the action of mountain wind especially in winter. The flow field distribution of valley breeze is on the contrary with that of mountain breeze. The valley breeze acts as uphill wind and its convergence zone is also in urban area. Part of the pollutants,which are discharged by enterprises in the north of city, are gathered in urban area and the south area of city and finally piped out.

Key words: mountain-valley breeze, spatial and temporal distribution, intensity change