Improving the Urban Boundary Layer Wind Speed by Coupling the TKE-ACM2 PBL Scheme with the Building Effect Parameterization Model

Realistically representing the vertical turbulent transport of surface layer fluxes dealt with by the planetary boundary layer (PBL) scheme is of paramount importance in a numerical weather forecasting model. Further complexity arises due to the presence of heterogeneous surface obstacles whose height can be comparable to the model's vertical resolution, which poses a challenge in improving and revisiting the PBL scheme. In this presentation, we derive the numerical method to couple one of the recently validated turbulent kinetic energy (TKE)-based non-local PBL schemes, namely the TKE-ACM2 scheme, with the commonly used multi-layer Building Effect Parameterization (BEP) model in WRF. The behavior of TKE-ACM2+BEP is first examined under idealized convective atmospheric conditions where a simplified staggered urban morphology is prescribed. Its performance is benchmarked against the state-of-the-art large-eddy simulation by PALM and also compared with the operational PBL scheme Boulac+BEP. The idealized simulation results reveal that TKE-ACM2+BEP exhibits superiority in simulating the potential temperature and wind speed profiles compared to Boulac+BEP, corroborating its better non-local treatment of the momentum fluxes near the roughness sublayer. Furthermore, we apply the coupled model to the Pearl River Delta region in South China, where a few extensively urbanized mega-cities exist. The one-month high-resolution wind speed LiDAR observations indicate that TKE-ACM2+BEP provides a more reasonable reproduction of wind speed profiles in the upper surface layer compared to the Bulk methods (i.e., without urban canopy model) by reducing the overestimation at the urban LiDAR site. In addition, the 10-m wind speeds (U10) are compared with surface stations aggregated based on the Local Climate Zone classification. The results suggest that the BEP model can improve the performance of the TKE-ACM2 PBL scheme at low- to moderate-building grids, but may not be consistently better at high-building density grids as it overly reduces U10.