An Urban Surface Energy Balance Model

Abstract

In this research, a model to simulate the urban surface energy balance (SEB) model is developed. Urban geometry plays an important role in the SEB and the resulting surface temperatures. A well-known effect of how building geometry affects radiation is shadowing and trapping of LW radiation. The goal of the energy balance model is to quantify the difference in SEB for different urban geometries and thereby guide the choice of building-resolving methods for turbulence models. In order to capture the surface geometry, two algorithms are developed to calculate the sky view factor (SVF) and shadow factor (SF). The SVF represents how much of the sky is visible for a surface and influences diffuse radiation. The SF determines whether a surface receives direct solar radiation, based on the solar trajectory. The surface energy balance is simulated for both a 0.5m resolution AHN data grid and LES-averaged grids, which usually have larger grid cells. The LES method considers 3D grid cells, where a grid cell has volume ΔxLES ⋅ ΔyLES ⋅ ΔzLES. An LES grid cell is considered part of a building if the volume is more than half filled, and empty if it is less than half filled. The two different grids result in large differences in SVF and SF values, which result in temperature differences up to around 6 K for the simulations run in this study. The simulations are affected by all simulation parameters, of which the aerodynamic resistance, outside temperature and building temperature, are discussed. From the results, we conclude that the choice of grid has a significant impact on surface temperature and energy fluxes.