Increased urban heat island effect due to building height increase

Abstract

An increase in urban population with only limited space to extend urban development forces city planners to think upwards. Adelaide, South Australia is one example where height restrictions in the central business district (CBD) were recently lifted to allow for a projected increase in urban population. The city provides a unique study area for the analysis of the urban induced heat variation. The main CBD is small compared to most cities around the world. The CBD covers only four square kilometres and is surrounded by a 500 metre wide parkland belt with sparse vegetation. Adelaide's temperature distribution has been monitored by twenty temperature sensors in the CBD and parklands since winter 2010. The observations exhibit a maximum temperature difference between the CBD and parklands of 6°C; with an average of 1.5°C. The maximum temperature difference occurs around sunrise and the winter UHI effect is more marked than in summer. Using a sky view factor which includes information of the wind (effective sky view factor, ESVF), the change in temperature depending on the height variation is in excess of 1°C. This, in connection with a projected temperature increase due to climate change, has major implications to energy use and carbon emissions in the future. A micro-scale urban model (ENVI-Met) has been employed to study the influence of building height on the surrounding UHI effect. The modeling of a simple street canyon displays the relation between building height and street width and hence confirms the results exhibited by the effective sky view factor. However, the simple street canyon simulations with ENVI-Met show that when building height reaches street width, the increase in temperature at street level stops and even reverses for higher building heights. The model may lack significant processes (e.g. heat transfer between inside and outside of the buildings). Observations show that shading of surface area and heat storage processes in the walls are important contributions to the atmospheric temperature. ENVI-Met Version 3.1 does not include heat storage in building walls. Heat transport between inside and outside of buildings is treated as an in situ process. This missing process is a possible explanation in the reversal of the diurnal temperature change in the simple cases when building height reaches street width. It is also likely the reason that a lower average temperature is modeled in a complex city setting when double building height is simulated and compared to the control case. Further investigation of the atmospheric processes and their effect on the near surface temperature is needed. Modeling here is still in progress and further simulations, especially for the complex Adelaide CBD, cases are necessary.