【作者】 王腾蛟；

【导师】 张镭；

【作者基本信息】 兰州大学 ， 大气物理学与大气环境， 2013， 硕士

【摘要】 随着城市化水平的不断提高,城市热岛效应及其对区域气候、环境和人体健康的影响己获得广泛关注。近50年来,兰州市城市化进程加快,城市热岛效应显著。但是,兰州市独特的河谷地形分布与复杂的下垫面性质,使得兰州市边界层结构及地表辐射能量平衡等特征较其他下垫面有很大不同。因此，开展对兰州市热岛效应的时空特征及形成机制的研究具有非常重要的现实意义。通过对比兰州市区站和榆中郊区站55年(1956～2010年)的逐日平均气温、最低气温和最高气温观测数据,总结了兰州市热岛效应的长时间尺度的演变规律,并定量分析了热岛效应对城市站温度序列的影响。在成功利用WRF中尺度数值模式模拟复杂地形自然下垫面边界层的基础上,运用WRF模式耦合城市冠层模块(UCM)对兰州市冬季个例(2005年1月25-27日)和夏季个例(2009年7月27-29日)的城市热岛效应进行了高分辨率数值模拟。通过与去城市下垫面敏感性试验的对比,系统地分析了城市下垫面对兰州市边界层、山谷风环流、地表辐射平衡及地表能量平衡的影响程度。主要结果如下(1)兰州城市热岛效应明显,并且热岛效应在最低气温上表现最显著。55年来,兰州市热岛强度的年代际变化具有明显的阶段性特征：1986年是显著的跃变点,跃变前后年平均气温热岛强度升高了1.04℃。夏季的热岛强度最大春秋季次之,冬季最小；但冬季热岛强度的增强趋势最为显著,增幅可以达到3℃。兰州市热岛强度同样具有明显的日变化特征：夜间强,白天弱,冬季正午还会出现城区温度低于郊区的冷岛效应。1956～2010年,年平均热岛增温率和增温贡献率分别为0.34℃/10a和69.4%,城市热岛效应对兰州站气温序列的影响不容忽视。1986年增温跃变后,年平均热岛增温率和增温贡献率分别减小到0.13℃/10a和22.4%,自然因素引起的增温贡献比重增加。冬季平均最低气温的热岛增温率最高,可以达到0.84℃/10a。(2)WRF中尺度数值模式能很好地模拟出复杂地形条件下的边界层结构及地表辐射和能量平衡场特征。通过与兰州大学半干旱气候与环境观测站(SACOL)观测资料的对比发现,加入了非局地混合作用以及边界层顶夹卷作用的YSU边界层参数化方案能够更好地模拟出对流边界层的的结构特征,而MYJ局地闭合方案则对稳定边界层的模拟表现更优。(3)兰州市冬季的热岛强度日变化曲线呈现双峰结构,峰值出现在19：00和06：00,谷值出现在13：00；夏季则变为单峰结构,峰值出现在20：00,谷值出现在10：00。冬季热岛效应维持时间长于夏季。通过与去城市下垫面模拟结果对比可知,城市下垫面上的近地层气温高于自然下垫面,并且在夜间表现最明显,冬季温差约为2.4℃,夏季则可以达到4.0℃。城市由于建筑物对风的阻挡作用导致近地层风速小于自然下垫面,风速差值约为1.0-1.5m.s-1。城市下垫面对兰州市热岛强度的贡献率约为40%,而其他因素(如人为热排放等)对热岛强度的贡献也相当可观。夜间,兰州站200m以下的近地层大气保持了白天的混合层特征；热岛环流的上升运动促进了山风环流,使得Urban试验上升气流达到的高度高于Nature试验近400m。冬季白天,由于山峰加热效应,两试验模拟的位温廓线在400m-600m高度间均存在一个脱地逆温层；11：00～15：00兰州站上空300m以下出现弱上升气流,抑制了谷风下沉支气流的发展。夏季白天,太阳辐射强烈,谷底的大气迅速升温,剧烈的湍流运动使得河谷中的大气混合均匀,1000m以下大气呈现近中性结构；16：00～20：00河谷中的气流转为上升运动,上升高度超过1000m,此时段内热岛环流与谷风环流相互促进。兰州市城乡地表辐射平衡存在差异的重要原因是城乡下垫面属性的不同。城市地表长波辐射的加热作用是造成城市热岛强度夜间强于白天的主要原因。城市下垫面对太阳辐射的“截陷”效应导致净辐射通量大于自然下垫面。由于建筑材料的不透水性,城市下垫面潜热通量远小于感热通量,且城市的储热能力明显增强。更多还原

【Abstract】 With the improvement of urbanization, urban heat island (UHI) effect and its associated impact on regional climate, environment and human health have received wide attention. In the recent fifty years, the urbanization of Lanzhou has been development rapidly. Accordingly, Lanzhou has experienced a significant UHI effect. However, the unique river valley topography and complexed surface properties, making the boundary layer structure and the surface radiation energy balance characteristics has great different with the other citys. Therefore, the study which focus on the temporal and spatial characteristics as well as the formation mechanism of UHI effect over Lanzhou, has very important practical significance.Based on the daily average temperature, maximum temperature and minimum temperature data from Lanzhou urban station and Yuzhong rural station, this paper investigate the long-term variation of UHI effect for the time period1956to2010in Lanzhou, and quantitative analysis the impact of UHI effect on temperature series in Lanzhou urban station. On the other hand, Weather Research Forecasting (WRF) numerical model and its coupled single-layer Urban Canopy Model (UCM) are used to simulate the UHI effect over Lanzhou during25-27January2005(winter case) and27-29June2009(summer case). In order to evaluate the impacts of urbanization over Lanzhou, the land use data that inversed from MODIS lkm resolution data is applied. A sensitivity experiment that without urban underlying is designed to investigate the effects of urban underlying on urban boundary layer structures, valley wind circulation, land surface radiation balance and land surface energy budget. The main results are as follows: (1) The UHI effect of Lanzhou is obvious, especially for minimum temperature. For the past55years, the interdecadal variability of lanzhou UHI intensity has obvious stage features, the largest rise in the1980s and1990s. The UHI intensity is strongest in summer, and weakest in winter. But the increasing rate of UHI in winter, that is much higher than in any other seasons, can reach3.0℃. The UHI intensity demonstrates distinctive diurnal variation, with UHI intensity being stronger and more stable in nighttime than it is in daytime. The urban cool island effect, in which surface temperature in urban areas is lower than that in rural areas, also occurs at the winter noon.During1956-2010, the impact of UHI effect on temperature series recorded in Lanzhou is significant. The warming rate of annual average UHI is0.34℃/10a, and its contribution to surface temperature warming trends is69.4%. Around the year1986, an apparent temperature jump occured in northwest China. Therefore, temperature changes during1986-2010in lanzhou is mainly caused by natural factors, while contribution rate of UHI effects caused by human factors is relatively reduced.(2) Mesoscale numerical model WRF can well simulate the boundary layer structrue, characteristics of land surface radiation balance and energy in the complex terrain conditions. The simulated results are compared with the observations provided by Semi-Arid Climate Observatory Laboratory (SACOL) of Lanzhou University. The comparison results reveal that the non-local scheme (YSU) with the entrainment flux proportional to the surface flux is favourable under unstable conditions. Under stable conditions, the local TKE closure schemes (MYJ) perform better than the first-order approaches.(3) Diurnal variation of UHI intensity in winter presents the double-peak structure, the peak value appeared at19:00and06:00, while the valley value appeared at13:00. But diurnal variation of UHI intensity in summer presents the single-peak structure, the peak value appeared at20:00, while the valley value appeared at10:00. The UHI effect in winter maintain longer than in summer. Surface air temperature on urban underlying is higher than on natural underlying, and showed the most obvious during the night. The temperature difference is about2.4℃in winter, while can reach4.0℃in summer. Surface wind speed of urban is less than natural underlying surface, the wind speed difference is about1.0-1.5m-s-1, which is caused by the resistance of building to wind. The contribution rate of urban underlying to Lanzhou UHI intensity is about40%. Accordingly, Other factors (e.g., anthropogenic heat emission) also play important role for UHI intensity.The urban surface air keep the characteristic of mixing layer at nighttime. Due to the heating effects of mountain peak, there is a inversion layer exist above the city400m-600m at winter daytime. During the summer daytime, that the vertical turbulent motion is severe, atmospheric boundary layer presents nearly neutral structures. Furthermore, the UHI effect also has a significant impact on valley breeze circulation. At afternoon, the weak upflow caused by UHI circulation could restrain the formation and development of the valley breeze.Different of the underlying attributes can lead to a greater difference in surface radiation balance between urban and rural areas. The main reason of UHI intensity stronger in nighttime than in daytime is heating effect of urban surface long-wave radiation. The trapping effect of urban underlying on solar radiation could lead to the net radiation in urban areas are more than that in non-urban areas. In summer, the magnitude of surface latent heat flux on natural underlaying is about10times as much as that in winter. Because of materials waterproofness of building, the latent heat flux is far less than sensible heat flux, while the heat storage capacity is obvious enhanced.更多还原