城市化影响下的京津冀地区暴雨中尺度系统发展机理研究

The rapid development of urbans could effect the weather and climate change over urban regions, which always intense the meteorological disasteres. In current, The urban floods is a major problem over urban regions in China because of the rapidly urbanization. Beijing-Tianjin-Hebei region (BTHR) is one of three urban groups in China, and the frequent urban floods occurred over BTHR, which always caused by meso-scale torrential rain systems, has became a focus attention by society. Based on the introducing of higher resolution terrain data, newly land use data and coupling with the urban canopy model with WRF and ARPS, the torrential rain processes will be simulated and the fine structures of meso-scale torrential rain systems will be diagnosed. The horizontal wind partition method of harmonic-cosine in a limited region and ageostrophic splitting method (the atmospheric motion is decomposed into advection process and adjustment process. The advection process causes ageostrophic motion, and the adaptation process makes it adapt to geostrophic state. The interaction of the two processes can be analyzed by splitting method) will be adopted to study the dynamical processes of the meso-scale torrential rain systems, such as the meso-scale convergence and divergence, dynamical and heat advection processes under effects of complex terrain, urban heat island and urban urban canopy will be analyzed by using the simulation results from WRF nad ARPS models over BTHR. Especially, the effect of terrain on the convection triggering mechanism under benefit circulation, the action of local dynamical lift and vertical motion caused by convergence, the strong convergence(divergence) and rotational motions, the instantaneous concentration mechanism of moisture and energy, etc., will be diagnosed. The mechanism study in this project does favor to understanding the dynamical process of meso-scale torrential rain systems over BTHR, which is helpful for improving the forecasting accuracy of torrential rain over BTHR, and is important in disaster prevention and mitigation of urban floods in China.

城市快速发展会影响到城市局地天气气候，具有"放大"气象灾害的作用，内涝暴雨就是城市面临的主要气象灾害之一。京津冀地区是中国三大城市群之一，暴雨内涝频繁发生，已成为各界关注的热点。本项目以提高模式地形分辨率、引入较新的下垫面类型资料并耦合城市冠层模式为基础，基于调和余弦风场分解和非地转框架下的分解分析理论（把大气热动力过程造成天气系统出现非平衡以及非平衡调整到新平衡态的两个过程分解并分析其作用），研究复杂地形、城市热岛和城市冠层、中尺度辐合辐散、大气动热力平流对京津冀地区城市内涝暴雨系统发展的影响，尤其是复杂下垫面在有利大气层结下如何触发对流，中尺度辐合造成的局地动力抬升和垂直运动、强辐合辐散和涡旋运动对京津冀暴雨过程中水汽和能量的瞬时集中机制等。基于本项目机理研究，增进城市化影响下京津冀暴雨中尺度系统发展动力过程的认识，有助其预报准确率提高，对满足国家对城市内涝暴雨防灾减灾需求有重要意义。

城市快速发展会影响到城市局地天气，具有"放大"气象灾害的作用，内涝暴雨就是城市面临的主要气象灾害之一。京津冀地区是中国三大城市群之一，暴雨内涝频繁发生，已成为各界关注的热点。项目通过反演和拼接MODIS观测的最新的地表覆盖数据，得到新的能表示京津冀城市扩张的下垫面特征的土地利用类型（Landuse），并替换中尺度数值模式WRF中原有Landuse。以NASA提供的90m高分辩地形数据替换了WRF目前最高分辨率为30s的地形数据。通过提高模式地形分辨率、引入新的Landuse资料并耦合城市冠层，模拟分析京津冀城市下垫面扩张对本地区暴雨的影响及机理。结果显示新的Landuse能更好反映出京津冀地区近地面的城市“热岛”和“干岛”效应。对京津冀地区持续性暴雨过程的敏感性数值模拟对比分析显示，扩大城市下垫面的城市区域地面温度及其与2m气温的差异较原有类型的模拟结果有明显升高，模拟暴雨强度增强，部分地区雨量增强大于15mm。城市下垫面的增加导致了地表潜热通量增加，直接影响了边界层通量与大气自由对流层的能量交换，利于持续性降水的发生发展。对京津冀地区的两次极端暴雨过程的新旧Landuse的模拟对比分析显示，新的Landuse模拟得到的地面暖心和地气温差中心正好位于北京和天津城区。城市扩张造成的热岛效应对近地面大气的加热引起了城市上空的上升运动加强和水汽垂直输送增强；在近地面低层风场上，气流经过地面高粗糙度的北京和天津城市区域时，城市区域尺度的气流发生了绕流，造成了这俩个城市下风方的水平风水平切变增强，进而引起中尺度气旋性环流增强，利于该区域低层的中尺度辐合增强和水汽局地集中，降水增强。对造成京津冀地区短时强降水的强对流飑线过程的数值模拟分析揭示了太行山地形激发重力波后，重力波在山顶附近的不稳定层结（形成与太行山地形的热力差异有关）大气中发展导致山顶附近有单体对流触发，对流下山过程中发展组织成飑线并造成北京和河北地区短时强降水。多过程模拟分析还显示出，切变线上生成的中尺度涡旋的发生发展的动力旋转和辐合及其引起的水汽局地辐合抬升对京津冀地区暴雨和强对流的触发和发展有重要影响，而切变线上中尺度涡旋的形成位置及其发展与局地水平和垂直风切不稳定有关。