Breakdown of the night time urban heat island energy budget

This conceptual study aims at identifying the dominant factors involved in the night time urban heat island energy budget at building level for an idealized 2D urban geometry. For this purpose a simulation model has been developed which combines radiative transfer, conductive heat transfer and convective heat transport by Computational Fluid Dynamics (CFD) modelling at 1 m spatial resolution. A wide range of building height (H) to street width (W) ratios are considered. Starting from radiative equilibrium, complexity is added with each next test case, adding the long wave trapping effect, heat transfer by conduction and finally by convective transport of sensible heat. It is found that the long wave trapping effect is the main mechanism controlling the surface temperature for lower H/W ratios. With increasing H/W ratio the long wave trapping effect equals the absorbed long wave radiation by the sky, while the conductive heat flux is increasing relative to the absorbed radiation. The sensible heat flux process shows different behaviour for different H/W ratios. For H/W ≤ 1.0, one single vortex is formed spanning the whole street canyon. For deeper canyons, this vortex only spans the upper part of the canyon and a stably stratified flow is formed in the lower parts of the canyon, reaching very low air temperatures in these regions. This shows that there is a subtle and complex interplay between all processes and reveals the necessity to represent all physical processes as accurately as possible.