Exploration of the thermal behaviour and energy balance of urban canyons in relation to their geometrical and constructive properties

Abstract

It is well established that the thermal urban environment differs from the countryside. The three-dimensional physical obstacles, the different thermal and radiative properties of urban surfaces, and human activities influence all terms of the urban energy balance such as net radiation Q* (shortwave and longwave), turbulent sensible and latent heat fluxes QH and QE respectively, as well as the heat storage ΔQS amounts and temporal cycles. This mostly leads to increases in air temperatures known as microscale urban heat islands, but could also result in temperature decreases, termed cool islands. These patterns vary in time and magnitude depending on the interactive effects of manifold urban and building parameters.The present paper reports on an extensive study using the TEB model to explore these relationships. Urban canyon geometry and building construction (thermal insulation, thermal inertia) are modelled following a systematic design of experiments plan DOE. The temperate mid-European location of Mannheim (TRY12) was used as a case study. The results report daily, monthly and yearly cycles of urban air temperatures, surface temperatures, and energy balances.Urban canyons bounded by massive or lightweight constructions show different daily sensible and heat storage cycles and consequently different patterns of heat release and periods of in-canyon warming. Massive construction causes nighttime warming whereas lightweight construction leads to daytime warming, both exacerbated as the canyon gets deeper. Thermal insulation also proved to have thermal effects outdoors. This research points out the need to consider urban microclimates in building energy studies under such modified boundary conditions.