doi:  10.3878/j.issn.1006-9895.1805.18140

Fine-scale structures and formation of strong winds over mega city during a cold surge
摘要点击 595  全文点击 74  投稿时间:2018-03-21  修订日期:2018-05-01
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基金:  国家自然科学基金 41775094
中文关键词:  冷空气  局地强风  风场扰动  大涡模拟  城市建筑物
英文关键词:  Cold surge, Local strong winds, Wind disturbances, Large-Eddy Simulation, Buildings
向杰勋Xiang Jiexun中山大学大气科学学院
陈桂兴Chen Guixing中山大学大气科学学院
姜平Jiang Ping重庆市气候中心
吴乃庚Wu Naigeng中国气象局广州热带海洋气象研究所
温之平Wen Zhiping复旦大学大气科学研究院
Citation:Xiang Jiexun,Chen Guixing,Jiang Ping,Wu Naigeng,Wen Zhiping.2018.Fine-scale structures and formation of strong winds over mega city during a cold surge[J].Chinese Journal of Atmospheric Sciences (in Chinese)
      地面强风可直接造成人员伤亡和经济损失,严重影响出行安全、工农业生产等社会秩序。强风的发生与天气系统和复杂下垫面的共同作用密切相关,在城市区域尤为明显。受数值模拟技术和计算资源的限制,对实际天气条件下大范围城区的强风现象进行建筑物分辨率的大规模数值模拟研究仍是一个挑战。本研究利用中尺度气象模式嵌套流体计算动力模式的超高分辨率局地气象预报系统,对强冷空气过程造成广州市区的一次强风事件进行数值模拟,深入探讨强风的精细结构和形成机制。结果表明,伴随着强冷空气入侵,广州市区的平均风速和风场高频扰动均明显增强,且在城市冠层顶尤为明显,呈现区域不均匀的三维结构,数值模拟与地面观测相一致。较大范围的强风速和阵风主要出现在建筑物较为低矮的老城区上空,并持续影响下游河道等开阔区域。在高层建筑密集的新城区,虽然整体风速明显减弱,但能在平行风向的街道狭管和下游区域形成局地强风。特别是,超高层建筑群引起显著的垂直环流,导致强风扰动向下传播,造成最大风速达8 m/s的地面局地强风,阵风指数接近2。上游建筑群引起的风场扰动呈现大尺度湍流结构,能沿着平均气流传播影响数公里之远的下游地区。当风场扰动经过广州塔等单体超高层建筑时,可在其两侧绕流区再次加强,形成局地强风。局地强风和阵风还出现在垂直于风向排列的沿江高层建筑群的侧边,与建筑屏风的阻挡效应和缺口溢出有关。研究结果促进认识城市强风的时空特征和物理机制,有助于提升城市气象的精细化预报水平。
      Local strong winds could be hazardous to the street structures and pedestrians, which affects the outdoor activities and traffic safety. The formation of local strong winds is associated with both mesoscale weather and underlying surfaces, particularly over the urban areas where a large number of tall buildings exist. The building-resolving simulation of strong winds over a large city domain at given atmospheric conditions has been challenging, because of the limitations in numerical models and computational resource. Here we investigate a typical event of strong winds induced by cold surge in Guangzhou city using an advanced local weather prediction system, in which a mesoscale model is downscaled to a Computational Fluid Dynamics (CFD) model with Large-Eddy Simulation. Both CFD simulations and observations suggest that, during cold surge, mean wind speed and high-frequency disturbances are enhanced significantly in the city particularly at the top of urban canopy. There are also evident differences between the old and new districts with different buildings characteristics. A large area of strong winds and disturbances are seen above the old district with densely low buildings and its downstream open areas. Over the new district with tall buildings, although the regional mean wind speed is reduced, local strong winds occur to the main streets parallel to wind direct. In particular, high-rise buildings can induce obvious veridical circulation that can lead to a downward movement of wind disturbances and cause near-surface strong winds. Such eddies of the building-induced strong winds can propagate downstream with a considerable distance of up to several kilometers. They can be reinforced when they are passing other high-rise buildings such as Canton Tower. Local strong winds also occur to the flanks of tall buildings aligned along the Pearl River which is perpendicular to ambient wind direction. The blocking effect of those buildings results in an along-river air flow which then outflows the gap without buildings and leads to local strong winds. These findings have important implications for understanding the fine-scale structures and formation of local winds in the city, which helps to improve the prediction of urban winds.
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