Mesoscale climate models provide indispensable tools to understand land‐atmosphere interactions over urban regions. However, uncertainties in urban canopy parameters (UCPs) and parameterization schemes lead to degraded representation of the drag effect in complex built terrains. In particular, for the widely applied single‐layer urban canopy model (SLUCM) coupled with the Weather Research and Forecasting (WRF) model, near‐surface horizontal wind speed is known to be overestimated systematically. In this study, idealized large eddy simulations (LES) and WRF‐SLUCM simulations are conducted to study the separate effect of UCPs and aerodynamic parameterization on atmospheric boundary layer processes and rainfall variabilities in Phoenix, Arizona. For LES that explicitly resolves surface geometry, significant differences between three‐dimensional (3D) versus two‐dimensional (2D) representation of urban morphology are found in the surface layer and above. When surface drag is parameterized following SLUCM, surface morphologies have little impacts on the mean momentum transfer. WRF‐SLUCM simulation results, incorporated with 3D urban morphology data, indicate that simply refining the frontal area index will reduce the surface drag, which further amplifies the systematic positive bias of SLUCM in predicting horizontal wind speed. Replacing the drag parameterization in SLUCM by LES‐based aerodynamic parameters has evident impacts on near‐surface wind speed. The impact of urban roughness representation becomes the most evident during rainfall periods, due to the important role of surface drag in dictating moisture convergence. Our study underlines that apart from intensive efforts in obtaining detailed UCPs, it is also critical to enhance the urban momentum exchange parameterization schemes.

中文翻译：

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城市粗糙度表征对区域水文气象的影响：理想化研究

中尺度气候模型提供了了解城市地区陆地-大气相互作用的必不可少的工具。但是，城市冠层参数（UCP）和参数化方案的不确定性导致在复杂的构建地形中阻力效应的表示降低。特别是，对于广泛应用的单层城市雨棚模型（SLUCM）和天气研究与预报（WRF）模型，已知系统地高估了近地表水平风速。在这项研究中，进行了理想化的大涡模拟（LES）和WRF-SLUCM模拟，以研究UCP和空气动力学参数化对亚利桑那州凤凰城大气边界层过程和降雨变化的单独影响。对于显式解析曲面几何的LES，在表层及其上层，发现了城市形态的三维（3D）与二维（2D）表示之间的显着差异。当在SLUCM之后对表面阻力进行参数化时，表面形态对平均动量传递几乎没有影响。WRF-SLUCM仿真结果与3D城市形态数据相结合，表明仅改进额叶面积指数将减少表面阻力，从而进一步放大SLUCM在预测水平风速时的系统正偏差。用基于LES的空气动力学参数代替SLUCM中的阻力参数设置对近地表风速有明显影响。在降雨期间，由于表面阻力在指示水分收敛中的重要作用，城市粗糙度表征的影响变得最为明显。