延庆-张家口地区复杂地形冬季山谷风特征分析

基于2016年12月—2017年2月和2017年12月—2018年2月两年冬季的近地面自动气象站逐时观测数据以及张家口探空数据分析延庆-张家口一带（包括张家口崇礼、赤城、海坨、小五台山区，延怀、怀涿、洋河、蔚县盆地以及北京延庆、昌平、怀柔部分平原地区）复杂地形的风场精细化时、空分布特征，揭示不同复杂地形下局地风场的时、空变化规律，加深对复杂地形动力、热力作用对近地面风场影响的认识，为冬季山区风场预报以及复杂地形数值模式改进提供参考。结果表明:晴朗小风天风持续性作为矢量平均风速和标量平均风速的比值，可以作为研究风场变化规律的重要参数。根据风持续性的日变化特征，可以将研究区域内所有站点分为10种类型，分别代表不同局地地形特征的影响，风持续与风向变化的相关也很强。研究区域主要有3种类型的地形风:斜坡风、峡谷风以及较大尺度的山区平原风。不同地形特征下的风场、风持续性存在明显不同的日变化特征，山风和谷风相互转化的时间也不同，山区最早，盆地次之，平原区最晚；山风时段持续时间较谷风时段长，风速小；晴朗小风天实测风反映了实际风场的特征，而排除环境背景风场，弱化地形动力作用后整个冬季的局地风作为理论山谷风，更能反映热力作用下的山谷风特征。 

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.