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Fine Spatial and Temporal Characteristics of Humidity and Wind in Beijing Urban Area
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摘要: 利用2008—2012年北京城区平均5 km的高密度自动气象站逐时观测数据，分析了北京城区近地面比湿、风向和风速的时空精细分布特征，初步探究了城市下垫面对局地气象要素的影响机制。研究表明：夏季白天北京城区为干岛，冬季城区表现为弱湿岛特征。受城市效应的影响，北京城区与郊区比湿日变化有明显差异。近地面10 m风受到地形、城市和季节性盛行风的共同影响。当气流经过城区时有明显的绕流现象。夏季05：00—10：00 (北京时，下同)，受山风、弱的夏季偏南风和城市热岛共同作用，气流向城市中心辐合。冬季15：00—19:00，受季节盛行风偏北气流和谷风偏南气流的共同作用，在城区形成一条西北—东南走向的辐合线。对风速研究发现：城市粗糙下垫面使北京城区风速减小，二环路和三环路之间存在一条“n”状的风速小值带。由此可见，除已开展较多研究的城市热岛效应外，北京城市效应对近地面湿度和风场亦有显著影响。Abstract: The temporal and spatial characteristics of specific humidity, wind speed and direction in Beijing urban area and urban effects are investigated in terms of hourly automatic weather station data during 2008-2012.Results show that values of specific humidity (q) in urban areas are lower than those in rural area during summer daytime and early night in Beijing, which is known as "urban dry island"(UDI). Values show a multi-center distribution, due to the non-uniform distribution of non-evaporating urban impervious-surfaces, which decrease evapotrans-piration, increase run-off, and thus lower urban specific humidity levels. In winter, urban values of q are bigger than rural ones at most hours which are affected by anthropogenic emissions. Studies on 10-m wind directions show that they are affected by seasonal prevailing winds, topography and urban effects. During summer valley-breeze time, southerlies bypass Beijing urban area because of buildings, while air flows converge into city during summer mountain-breeze time due to combined effects of topography, urban effects and seasonal prevailing wind. During winter breeze-time, a convergence line is formed in northwest-southeast direction in urban area. Wind speeds are reduced by the large Beijing surface-roughness. A low-speed region is observed in more urbanized areas between Second Ring Road and Third Ring Road due to its high surface-roughess. It shows that humidity and air flow are strongly affected by urban effects besides the much studied "urban heat island" in Beijing. The local urban effects have to be taken into account in fine weather forecasting. In addition, the result will contibute to the discussion of urban atmospheric environmental governance and city planning and construction.
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图 1 研究区域情况 (a) 北京地区地形分布 (黑色圆框内为本研究范围，黑色圆点为本文选取的郊区代表站), (b) 研究区域遥感图像和自动气象站 (黑色圆点) 分布 (白色三角为城区代表站；红色环路分别代表二环路、四环路和六环路)
Fig. 1 Study area (a) Beijing area with topographic-heights and study area (black square), rural stations (black dots), (b) remote sensing images of study area with AWS sites (black dots) (white triangles denote urban sites; red circles denote the Second, Forth, Sixth Ring Roads)
图 2 2008—2012年平均2 m比湿水平分布
(单位：g·kg-1；方框分别为北京二环路和四环路；阴影为地形高度)
Fig. 2 Seasonal averaged 2-m specific humidity during 2008-2012 in Beijing urban area
(unit:g·kg-1; circles denote the Second and Fourth Ring Roads, the shaded denotes terrain height)
图 3 夏季和冬季城区、郊区比湿日变化
Fig. 3 Diurnal variation of q of urban and rural areas in summer and winter
图 6 不同季节山风、谷风风向 (矢量) 分布 (阴影为地形高度)
Fig. 6 Wind direction (vectors) of mountain-breeze and valley-breeze in four seasons (the shaded denotes terrain heights)
图 7 2008—2012年四季平均10 m风速分布
(单位：m·s-1; 方框分别代表为北京二环路和四环路，阴影为地形高度)
Fig. 7 Seasonal averaged 10-m wind speed during 2008-2012
(unit: m·s-1; circles denote the Second and Fourth Ring Roads, the shaded denotes terrain heights)
图 8 研究范围内平均10 m风速日变化分布
Fig. 8 Diurnal variation of regional averaged 10-m wind speed in different seasons
图 10 不同环路区域内平均风速与郊区平均风速差日变化
Fig. 10 Diurnal variation of averaged speed difference between different loop areas and rural sites
表 1 各季节山风、谷风开始和结束时间
Table 1 The beginning and ending time of valley and mountain breeze in four seasons
季节 山风 谷风 起始时刻 结束时刻 起始时刻 结束时刻 春季 02:00 10:00 11:00 01:00 夏季 05:00 10:00 11:00 04:00 秋季 22:00 12:00 13:00 21:00 冬季 20:00 14:00 15:00 19:00 
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