Analysis of characteristics of mountain-valley winds in the complex terrain area over Yanqing-Zhangjiakou in the winter

Based on fine-scale hourly data collected at surface automatic weather stations (AWS) within the study area and sounding data at Zhangjiakou during two winters (from December 2016 to February 2017 and from December 2017 to February 2018), spatial and temporal characteristics of wind field in the complex terrain area over Yanqing-Zhangjiakou (including Chongli, Chicheng, Xiaowutai Mountain, Yuxian, Zhuolu, Huailai Basin, and Beijing Yanqing, Changping, Huairou District) are analyzed. This study reveals the temporal and spatial variations of local wind field over various complex terrain areas, and helps us to better understand the dynamic and thermal effects of complex terrain on the near-surface wind field. Results of the present study provide references for the forecast of wintertime mountain wind field and will be helpful for improvements of numerical models performance over complex terrain areas. It is found that in fair weather days, wind persistence, which is defined as the ratio of vector average of wind speed to scalar average of wind speed, can be used as an important variable to study the variation of wind field. According to the significant diurnal variations in wind persistence during fair weather days, the stations in the study region can be divided into 10 distinctive patterns, representing different influences of local topographic features, and the correlation between diurnal variation of wind persistence and wind direction is quite large. There are three types of topography-induced wind in the study region:slope wind, valley wind and mountain-plain circulation. The spatial scale of the mountain-plain circulation is larger than that of the former two. The diurnal characteristics of the wind field and wind persistence vary significantly with different topographical features, and the mutual transition time between mountain and valley winds is also different. In the mountains, the mutual transition time is the earliest, followed by that in the basins, and the transition time in plain areas is the latest. The duration of mountain wind period is longer than that of valley wind while the mountain wind speed is smaller than the valley wind speed. While the measured winds during fair weather days reflect the characteristics of the actual wind field, the theoretical valley wind that excludes the environmental background wind field and weakens the dynamic effect of topography during the wintertime can better reflect characteristics of thermally driven mountain-valley winds.