Parameterizing the impact of heterogeneous urban canopies on local climate using nudging

A multi-layer urban canopy parameterization using a nudging approach has been developed and integrated into the mesoscale atmospheric model METRAS to improve the urban surface representation. Through modifying the existing nudging equation in METRAS, urban canopy effects on local climate such as reduction of mean wind speeds, enhanced aerodynamic turbulent fluxes as well as urban heat island phenomenon are represented. The nudging technique has the advantages of simple implementation and reasonable model results, moreover, it is widely used as a data assimilation approach in global-scale models. One of the objectives of such a canopy parameterization using nudging is thus to prepare for its direct usage in the global-scale models with a typical resolution of 1 km so as to improve meteorological variables calculation for heterogeneous urban canopies.

In this work, the city Hamburg with heterogeneous surfaces was selected as the study area. Urban canopy information for Hamburg such as building height and building surface fraction (i.e., the ratio of the surface area occupied by buildings to the total plan area) were obtained from the 3D city model of Hamburg LoD1 (Level of Detail 1). We also used the Local Climate Zone map, which defines a range of values of the urban canopy parameters for each urban class. Sensitivity studies using these two types of canopy data are performed to gain a better understanding of how detailed should urban canopy data be to serve as input for an urban canopy parameterization. Model simulations are made for summer 2020 with particularly focusing on the maximum urban heat island phenomena. Simulation results for different meteorological situations are presented and the impacts of model resolution will be discussed.