Inclusion of Building-Resolving Capabilities Into the FastEddy? GPU-LES Model Using an Immersed Body Force Method

As a first step toward achieving full physics urban weather simulation capabilities within the resident-GPU large-eddy simulation (LES) FastEddy (R) model, we have implemented and verified/validated a method for explicit representation of building effects. Herein, we extend the immersed body force method (IBFM) from Chan and Leach (2007, ) to (i) be scale independent and (ii) control building surface temperatures. Through a specific drag-like term in the momentum equations, the IBFM is able to enforce essentially zero velocities within the buildings, in turn resulting in a no-slip boundary condition at the building walls. In addition, we propose similar forcing terms in the energy and mass conservation equations that allow an accurate prescription of the building temperature. The extended IBFM is computationally efficient and has the potential to be coupled to building energy models. The IBFM exhibits excellent agreement with laboratory experiments of an array of staggered cubes at a grid spacing of Delta=1 mm, demonstrating the applicability of the method beyond the atmospheric scale. In addition, the IBFM is validated at atmospheric scale through simulations of downtown Oklahoma City ( Delta=2 m) using data collected during the Joint Urban 2003 (JU03) field campaign. Our LES IBFM results for mean wind speed, turbulence kinetic energy, and SF6 transport and dispersion compare well to observations and produce turbulence spectra that are in good agreement with sonic anemometer data. Statistical performance metrics for the JU03 simulations are within the range of other LES models in the literature. Plain Language Summary A significant majority of social and economic activities are logically concentrated around densely populated urban areas. Consequently, accurate modeling and prediction of urban- scale weather entails a tremendous benefit to society in many ways. Herein, we extend the immersed body force method (IBFM), which allows explicit representation of building effects in microscale numerical models, to be applicable to disparate scales and to effectively control building surface temperatures. This computationally efficient method is implemented into the GPU-accelerated large- eddy simulation (LES) FastEddy (R) model, with the purpose of facilitating a path toward realistic street-scale operational weather forecasting in the near future. We validate the extended IBFM with observations at laboratory scale and urban-scale field measurements over downtown Oklahoma City during the Joint Urban 2003 field campaign. Our LES IBFM results for mean wind speed, turbulence kinetic energy, and SF6 transport and dispersion compare well to observations, and the corresponding statistical performance metrics are within the range of other LES models in the literature employing body fitted and immersed boundary approaches.