Microclimate in High-Rise Central Business Districts

Abstract

Study intent Central Business district features intensive land use, diverse building form and functions, high volume of vehicle traffic, lower greenery ratio, and of course taller buildings, compared to high-rise residential districts. Thermally comfortable outdoor environment would bring economical and social benefits to the area; so, it is important to understand and evaluate the impact of development on its microclimate and pedestrian comfort. We developed a thermal atlas methodology for this purpose. The thermal atlas is based on empirical modeling, digital elevation model data processing, and spatial analysis. Key morphological indicators were used to account for six aspects of urban climatic impact, i.e., building density, land parcel use, anthropogenic heat, greenery, ventilation potential, and heat sink. The resultant thermal atlas comprises the following components: urban morphological maps (or sub-layers); empirical models (for sub-layer weighting and model validation); thermal comfort indices maps; and thermal zoning and design recommendation maps. Shanghai Lujiazui CBD, the culmination of CBD development in China, is assessed as a case study. The second case studied the Lujiazui Elevated Walkway (LEW) to complement the application of thermal atlas system at the ground level. In high-rise commercial districts, elevated walkway system is becoming an indispensable way to connect plots and buildings separated by driveways and avenues. Assuming more breezes on elevated levels compared to sidewalks at the ground levels, walking high can be, however, exposed to higher solar radiation and thus higher radiant temperatures without proper shading. The case study aims to gain an empirical understanding of the overall effect of changing in elevation on pedestrian summertime comfort. Results and discussion Based on the results, design suggestions are made for Lujiazui CBD, i.e., providing opaque shading devices for major pedestrian spaces at century walkways and waterfront esplanade; reducing the size of street blocks; dividing massive single buildings into building clusters with smaller spacing; and improving the accessibility to the heat sinks. The thermal atlas can rapidly analyze and visualize urban microclimate variations as affected by different urban design scenarios, thus a useful decision-support tool. For the LEW case study, data analysis based on the biometeorological measurements and guided questionnaire survey indicates that, the LEW was more uncomfortable than the ground level during the measured period: air temperature was higher, but wind velocity is lower on the skywalk level than on the ground level, which is counterintuitive. It could be due to the convection enhanced by buoyancy between shaded and unshaded places. The resultant thermal comfort index indicates warm conditions on the ground level (when shaded) whereas hot conditions on the skywalk level. Countermeasures of various shading design and evaporative (mist) cooling design are discussed so as to improve thermal comfort level.

The content of this chapter is published in: Yang and Chen (2016), Yang et al. (2016).