2019年冬季北京海陀山局地环流特征及机理分析

Local circulation characteristics and mechanism analysis of Haituo mountain in Beijing during winter 2019

海陀山作为北京冬(残)奥会的主要室外赛场之一,其复杂的地形对风场的精细化预报提出了严峻的挑战,亟需开展加密的风场观测提高对复杂地形下局地环流特征及其影响机理的认识,并为提升赛区精细化预报与服务提供数据支撑。基于2019年度海陀山观测试验,利用加密自动气象站、激光测风雷达、涡动相关仪、云高仪等多源数据,对海陀山风场的水平分布、垂直结构、山谷风转换时间和不同背景风场影响下的局地环流特征进行了统计分析;从地形动力、热力和背景风场三个方面探讨了海陀山局地环流的影响机理。结果表明:弱背景风场下,海陀山局地环流特征显著,以热力驱动的山谷风环流为主导,并叠加了背景气流过山的影响。低海拔(1650 m以下)站点,山风和谷风表现为近乎相反的方向;而在中(1650-1800 m)、高(1800 m以上)海拔的山脊或山峰附近的站点,山风和谷风均以偏西为主。中低海拔站点的谷风明显强于山风,高海拔站点则相反;而在强西北背景风影响下,海陀山局地环流特征消失,整个山谷风向趋于与背景风一致,昼夜风向的差异较小,山谷中风速随海拔高度降低而减小。

The Haituo mountain,one of the main outdoor venues of the Winter Olympic(Paralympic)Games,presents a severe challenge for refined prediction of wind field due to its complex terrain.Therefore,it is imperative to carry out intensive wind field observations to improve our understanding of local circulation characteristics and influence mechanism of complex terrain,and provide data support for the improvement of refined prediction and service in the competition area.Based on data obtained in the observational experiment at Haituo mountain in 2019,the horizontal distribution,vertical structure,mountain-valley wind conversion time and local circulation characteristics under the influences of different background wind fields are statistically analyzed using multi-source data such as data collected by automatic weather stations,Doppler wind lidars,eddy covariance systems,ceilometer and cloud radars,etc.The influence mechanism for local circulation in the Haituo mountain is discussed from three perspectives,i.e.,topographic dynamic and thermodynamic impacts and background wind field influences.The results show that the local circulation characteristics in the Haituo mountain are significant under calm weather conditions when the background winds are weak.The local circulation is dominated by thermally-driven valley wind circulation superimposed by the influence of background airflow passing over the mountain.At low-altitude(<1650 m)stations,mountain and valley winds show opposite directions;at mid-(1650-1800 m)and high-(>1800 m)altitude stations located near the mountain ridges and peaks,mountain and valley winds both are dominated by westerly winds.Valley wind is significantly stronger than mountain wind at the low-and mid-altitude stations,while the opposite is true at high-altitude stations.In contrast,under the influence of strong background winds,local circulation characteristics disappear and the wind direction in the whole valley tends to be consistent with the background wind,while the difference in wind direction between the daytime and nighttime is small and wind speed decreases with increasing height.