【作者】 杨龙；

【导师】 胡和平；

【作者基本信息】 清华大学 ， 水利工程， 2014， 博士

【摘要】 城市化作为人类活动对水循环影响的重要表现形式，一方面通过改变下垫面属性，对地表产汇流特征产生直接影响，另一方面通过地表能量分配及其他城市环境要素（如气溶胶，人为热源等）改变区域降雨特性，从而对地表的水文过程产生间接影响。论文围绕城市地区夏季暴雨及洪水问题，基于多源观测数据和数值模拟手段，开展城市下垫面对降雨以及洪水过程影响规律的研究，对理解城市地区陆气耦合反馈机制以及建立防洪减灾体系有重要的科学价值与实践意义。论文从我国夏季降雨整体时空格局及其与东亚夏季风的关系出发，分析了北京夏季降雨年际变化特征，发现北京夏季降雨在年际上呈波动减少的趋势，与东亚夏季风变化一致；但城区夏季降雨在2000年之后显著增加，与北京建成区面积急剧增加的变化趋势一致。同时，揭示了2008~2012年北京夏季累积降雨量和强降雨频次在空间上有两个聚集中心，分别位于城区以及城市下风向的怀柔、密云和平谷地区。基于耦合城市冠层模型的中尺度气象模式WRF对北京地区2008~2012年夏季降雨的空间格局进行了分析和模拟，从气候角度揭示了城市下垫面对北京地区夏季降雨的影响机制。开发了基于雷达反射回波强度的暴雨云团识别算法SCI-2D，并将其用于分析夏季暴雨云团通过城区时的演变规律，研究表明城市下垫面对暴雨云团有分离和激发的作用。基于气块浮升理论发现城区地表的热量传输过程改变了大气边界层厚度、抬升凝结高度以及自由对流高度，为对流活动提供了良好的动力和水汽条件，从而使得城区降雨强度和频次得到显著增强。对于典型湖滨城市而言，城市地表热量传输过程对降雨的影响还与湖陆风环流有关。以美国密尔沃基市-密歇根湖地区为例，通过对该地区夏季典型降雨事件的数值模拟和分析，揭示了城市热岛环流和湖陆风环流的耦合效应及其对湖滨城市夏季降雨的影响机制。基于暴雨云团追踪算法TITAN，识别了暴雨云团通过城区时形态和结构的演变特征，进一步验证了北京地区研究得到的结论。以城市化流域为研究对象，定量分析了城市下垫面以及城市化引起的降雨空间分布的改变对流域洪水响应的影响。通过设置虚拟流域及数值实验，发现忽略降雨的空间分布会低估城市化引起的流域洪水量级的增幅，最高可低估50%。此外，降雨空间分布特征对流域年径流量的年际变异性也有显著影响。论文综合了下垫面改变和陆气耦合反馈机制，完善了城市化流域水文响应规律的分析框架。更多还原

【Abstract】 As one of the important anthropogenic influences on water cycle, urbanizationexhibits a direct impact on surface hydrology by modifying the characteristics of landsurface conditions; on the other hand, urbanization also alters the partition of surfaceenergy balance and urban environment (e.g., aerosols, anthropogenic heat), whichchanges the characteristics of regional rainfall regime and ultimately feed back to thesurface hydrology. This study focuses on summer heavy rainfall and flooding eventsover urban environment. Urban modification of heavy rainfall and flooding wasexplored based on multi-source observations and numerical atmospheric/hydrologicmodels. The results are of remarkable scientific as well as practical meanings to furtherthe understanding on interactions between land surface and atmosphere, and improvethe design of flood-control systems in urban areas.The connection between the spatio-temporal patterns of summer rainfall and EastAsian summer monsoon (EASM) was examined over China. Under the control ofEASM, North China is experiencing a significantly decreasing trend of summer rainfallduring1961~2012, with Beijing metropolitan region included. However, summerrainfall over the urban core region of Beijing witnessed an increasing trend after2000,in accordance with the abrupt changes of built-up areas. Two ‘hot spots’ of higherfrequency of heavy rainfall events and summer total rainfall were observed over theBeijing metropolitan region within the period of2008~2012. One is located over theurban core region, and the other resides in the climatological downwind region. WeatherResearch and Forecasting (WRF) model coupled with single layer urban canopy model(SLUCM) was used to examine the urban signature in the spatial pattern of2008~2012summer rainfall over Beijing. A new storm cell identification algorithm SCI-2D wasdesigned to analyze the dynamics of storm cells over urban region. The urban coreregion of Beijing tends to split the storm cells into smaller ones and trigger new smallcells as well. Urban thermodynamic properties play a key role in enhancing theconvective potentials by increasing the depth of planetary boundary later (PBL), liftedcondensation level (LCL) and level of free convection (LFC), which ultimately elevatedrainfall intensity and frequency over the urban region. The impact of thermodynamic properties on summer rainfall is affected by regionalcirculation, e.g., Lake Breeze for lake-shore cities. This study further examined a typicalsummer heavy rainfall event over Milwaukee-Lake Michigan region, US. The couplingmechanisms of urban circulation and Lake Breeze and their effects on the rainfallprocesses were identified. The dynamics of storm cells approaching the city, asdisclosed with the storm tracking algorithm TITAN, were in accordance with theconclusions of the study in Beijing metropolitan region.An integrated assessment of urban impact on hydrological responses was carriedout by setuping up a virtual urbanized basin and customized sensitive numericalscenarios. The numerical scenarios were the combinatins of different imperviouscoverages (as represented by urban ratios) and rainfall spatial patterns, assuming thaturbanization processes could noticeably alter the rainfall spatial patterms for urbanizedbasins. Numerical experiments show that the amplification of flood magnitude inducedby urban sprawl could be underestimated as large as50%with the ignorance of rainfallspatial pattern. In addition, rainfall spatial pattern also plays an indispensable role in theinterannual variability of annual runoff. This study synthesized the direct impact ofurbanization and the indirect impact through the feedback of land surface/atmosphereon hydrological responses, which improved the analytical framework of hydrologicalresponses over urbanized basins.更多还原