An Improved BEP-BEM Based Urban Canopy Scheme Derived from High-Fidelity Simulations.

Understanding the physical processes of urban climate and their interactions with larger-scale regional and global climate is crucial for providing insights into equitable climate change adaptation and mitigation solutions in cities, especially under accelerating climate change. The Building Effect Parameterization (BEP) and Building Energy Model (BEM), developed by Martilli et al. (2002, BLM) and modified by Santiago & Martilli (2010, BLM), predict how urban structures and characteristics influence local weather and climate. We modified the BEP model to better fit different areas of Chicago by incorporating results from Large Eddy Simulations (LES), focusing on improving its accuracy at the street level (2 m from the surface) for both densely packed and open urban areas.

Specifically, LES were performed using the spectral-element code nekRS/nek5000 to examine the flow structure across the urban canopy. A parametric analysis of simplified/theoretical cases with various packing densities (λp = 0.16, 0.25, 0.44) and under different stratification conditions (Ri = 0.25, 0.5, 1) was conducted. Our LES results were validated using DNS data from the work by Coceal et al. (2007, JFM). After validation, the turbulence structure and turbulence length scale profiles across the urban canopy height were revised using our LES simulations. The modified length scale were then implemented in the BEP model within WRF.

We conducted a comparative study of three models: i) the original BEP-BEM from Martilli et al. (2002), ii) the updated version by Santiago & Martilli (2010), and iii) the modified BEP-BEM from the current study. Our analysis focused on a real-world heat event that took place in Chicago on August 24, 2023, with a peak temperature of 37.8°C (100°F). The numerical results were validated against observations of 2-m temperature and 10-m wind speed. For comparison reasons we used 70 m surface data downscaled 2 km sensing data within the metropolitan area of Chicago. The differences between models were examined, and the sensitivity of the various BEP versions and implementations was assessed with regard to different urban parameters (wall conductivity, wall/roof/ground albedo, etc.) and BEM parameters (ventilation rates, wall emissivity, etc.), and their effects on near-surface temperatures and building energy consumption.

The improved BEP-BEM parameterization and parametric study enhance our understanding of urban climate and can be used to evaluate the efficacy and effects of different adaptation and mitigation strategies (such as various roofing options, artificial/green roofing, green spaces, etc.) at the building, neighborhood, and city levels.