Using a simple parameterization for representing the effect of heterogeneous urban canopies in atmospheric models

Increasing urbanization requires a better understanding and representation of urban effects and urban canopy processes in weather and climate modelling at various scales. In this work, an urban canopy parameterization (UCP) based on an extended nudging approach is presented. The UCP is implemented in the mesoscale model METRAS with a horizontal resolution of 500 m. The city of Hamburg with heterogeneous surfaces was chosen as the study area. Urban canopy information for Hamburg, including building height and building surface fraction (i.e., the ratio of the surface area occupied by buildings to the total plan area), obtained from the 3D city model of Hamburg LoD1 (Level of Detail 1). were used as input for the UCP.

Model results show that the urban canopy parameterization based on an extended nudging approach can reproduce urban effects on the wind and temperature fields. For example, the UCP can simulate the wind reduction effects due to canopy obstacles in different model levels, with the largest wind reduction simulated in the urban areas with highest values for building surface fraction (densely built areas). In addition, the urban heat island effect as simulated with the single layer flux aggregation scheme in METRAS, was enhanced using the urban canopy parameterization, with the highest intensities in the densely built areas. The results suggest that nudging is a useful tool for modeling aerodynamic and thermodynamic urban effects, which are both important components in understanding the urban environment. Additionally, nudging is an approach that is relatively easy to implement or already implemented in many models. It can be applied to a variety of urban scenarios and resolutions and thus can be a simple approach to better represent urban effects in global-scale weather and climate models.