Impact of urban morphology representation on regional hydrometeorology: an idealized study

Abstract:

Numerical simulations based on mesoscale weather and climate models provide indispensable tools to understand physical processes of land-atmosphere interactions in complex urban environments. However, uncertainties in urban canopy parameters (UCPs) and parameterization schemes in current urban land surface models lead to degraded representation of key features in the complex built terrain, e.g. the drag effect of urban canopy on wind fields. 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 simulations are conducted to study the separate effect of UCPs and aerodynamic parameterization scheme on regional urban hydrometeorology. For LES that explicitly resolve the surface geometry by means of the immersed boundary method, significant differences between three-dimensional (3D) versus two-dimensional (2D) representation of urban surface morphology are found in the surface layer and above. When surface drag is parameterized following SLUCM, distinct surface morphologies have little impact on the mean momentum transfer. The sensitivity of SLUCM-WRF to UCPs is then investigated by implementing 3D urban morphology data in the default drag parameterization scheme. Results indicate that simply refining the frontal area index based on 3D urban morphology will reduce the surface drag, which further amplifies the systematic positive bias of SLUCM in predicting horizontal wind speed. On the other hand, replacing the default drag parameterization in SLCUM by LES-based aerodynamic parameters have evident impact on near-surface wind speed. The different impacts by morphology data and aerodynamic parameterization becomes more evident when large-scale forcing involving rainfall occurs. Our study underlines that apart from intensive efforts in obtaining detailed UCPs, it is important to enhance the urban parameterization schemes in current WRF model.