北京城区近地面比湿和风场时空分布特征

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。

Fine Spatial and Temporal Characteristics of Humidity and Wind in Beijing Urban Area

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.