Comparative Study on Fine Characteristics of Surface Wind Field in Winter and Early Spring over Yanqing Complex Terrain during Beijing Winter Olympics

Wind is one of the most important meteorological conditions in previous Winter Olympics， and it is the primary factor that affects the mountain events for Beijing Winter Olympics.Understanding the fine distribution law of wind can provide important theoretical basis for track construction， wind forecast and prevention measures.Using hourly observation data from surface automatic weather stations at different altitudes in Yanqing mountain area of Beijing Winter Olympics from December 2017 to March 2022， this study investigated the characteristics of local wind field during winter and early spring （Mar， Paralympics period） under complex terrain， focusing on comparing the frequency of wind speeds and directions， as well as the diurnal and seasonal variations.Firstly， all stations were grouped into four categories using the K-Means clustering algorithm， and Groups 1 to 4 represent the low-elevation Yangqing suburb area， the northeastern foothills transition area， the southwestern transition area and the high-elevation mountain top area， respectively.Subsequently， fine-grained characteristic analysis was conducted on each group separately.Results show that： （1） The frequency of strong winds is closely related to the altitude， with higher altitudes generally having a higher frequency of strong winds.In Groups 1~2 （altitude below 1000 m）， the frequency of light winds （≤3.3 m·s-1） exceeds 80%， while the proportion of strong winds （≥10.7 m·s-1） is 0%.In Group 3 （above 1000 m）， the frequency of light winds decreases to below 75%， and strong winds occasionally occur for less than 1%.In Group 4 （above 1800 m）， there is a significant shift in the wind speed frequency distribution， with the frequency of strong winds increasing to above 10%， which is much higher during winter compared to early spring.（2） There are significant local variations in the distribution characteristics of wind directions.Group 4 is primarily dominated by large-scale winter monsoonal circulation， resulting in a prevailing northwesterly wind， with rare concurrence of other wind directions.Groups 1~3 are influenced by a combination of large-scale circulation， valley wind circulation and underlying surface conditions， leading to different frequencies for each wind direction.（3） The diurnal variation exhibits contrasting characteristics between high and low elevations.Groups 1~3 show lower wind speeds at night and higher wind speeds during the day， while Group 4 shows a reserved pattern and an obvious small wind “window period” in midday.Groups 1~3 exhibit distinct daily transitions in wind direction， occurring after sunrise and sunset， whereas Group 4 does not show any diurnal change.（4） From a seasonal perspective， there are significant local differences between early spring and winter.Compared to winter， Group 2 exhibits a daytime wind speed increase in early spring， and Group 3 exhibits a nighttime decrease， while Group 4 exhibits a significant decrease in wind speeds throughout the day.Wind directions in early spring are relatively more variable， with an evident increase in northeasterly winds in Group 1， a delay of about 3 hours in the transition of valley wind circulation in Group 2， and an increase in southwesterly winds in Groups 3~4.This study contributes to a deeper comprehension of the fine-scale spatiotemporal patterns of near-surface local wind fields within complex terrains， and can offer crucial background clues for Winter Olympics and small-scale mountainous meteorological monitoring and forecasting.