Reconstruction of historical datasets for analyzing spatiotemporal influence of urban morphology on microclimate, a study of Kowloon Peninsula

Abstract

The urban landscapes and morphologies of Hong Kong changed dramatically between 1960 and 2010, for pursuing an urban form with the high-rise and high-density urban settings, especially over the city core areas of the Kowloon Peninsula. This kind of "compact city" can influence the microclimate resulting in lower living quality. Previous studies have analyzed the relationships between built environment and microclimates, by either a temporal study or a spatial approach, while a strategy for investigating the spatiotemporal relationship has yet to be developed. This study initiated a set of innovative strategies to map the historical built environment and microclimates of a compact city, with a spatiotemporal approach to analyze the relationships between building structures and urban climates, for developing a sustainable protocol in future urban planning. Three major components were reconstructed, including 1) the annually averaged land surface temperature (LST) for determining the relative temperature; 2) 3D building datasets for representing the building morphology; and 3) sets of urban morphological data derived from building datasets for analyzing the relationship between urban microclimates and building structures. Then, the spatiotemporal relationship between urban microclimates and built environment was analyzed, and two rule-based models were conducted to investigate the influence of air ventilation and shading from high-rise buildings on reducing surface temperatures for sustainable urban designs. Results showed that the maximum number of pixels with the relative temperature of >8°C were observed in 2010 (1580 pixels), and the number of pixels with relative temperatures of > 8°C and 7-8°C increased significantly from 1996 to 2010. In addition, the centroid shift of the relative temperature of ≥5°C was mainly caused by increases of high-rise buildings, land reclamation, and tree planting for urban landscaping. A trend of increasing building heights across the Kowloon Peninsula has also been observed in this study, in which the averaged building height has increased by 1.95 m for every ten years since 1964. Results of the trend analyses showed that the annually averaged LSTs in the center of this study area were lower than those of the surrounding districts, and density of built-up area was larger in the central Kowloon Peninsula. The comparison of spatial trends of urban morphologies and microclimates showed the potential correlation between the two. A statistical analysis was further used to determine the correlations between urban morphologies and microclimates. Strong positive correlations were observed between the relative temperature and global solar radiation, sky view factor, whereas intense negative correlations were found between the relative temperature and building height, frontal area index, wind speed, density of built-up area. An analysis of changes in the major ventilation paths across the Kowloon Peninsula between 1988 and 2010 indicated that unprecedented urban growth resulted in the decrease of ventilation in urban canyons. The building arrangement with E-W street canyons is parallel with the prevailing wind direction and contributes to the increase of ventilation for mitigating the surface temperature. Sunlight in the morning is obstructed by buildings at N-S street canyons in Mong Kok district; these buildings create more cast shadows than the NE-SW oriented buildings, e.g., in Sham Shui Po district. Spatiotemporal assessment of the effects of urban morphology on the daytime surface temperature showed that relative temperature decreased by approximately 0.30°C per 10 meters raises in building heights. The relative temperature in a high-rise building was 0.49°C, which was higher than in Kowloon Park (-0.12°C) but lower than in a small green area with the effect of shadow from buildings (0.86°C). In addition, the spatiotemporal estimation of air ventilation and shading effect indicated that areas with improved air ventilation through years can significantly reduce an additional 0.12°C-1.09°C than the areas without improvement, while areas with an increase in shades through years have 0.6°C to 0.76°C higher reduction of relative temperature. This study elaborates a design for improving the urban thermal environment in high-rise and high-density urban morphologies. Because shading effects and influence of air ventilation should be the keys to reducing surface temperatures, a suggested urban design for improving the urban thermal environment should be in the arrangement of wide and continuous N-S and E-W street canyons between high-rise buildings. The sustainable planning protocols such as urban greening, which is useful for reducing thermal discomfort, is also planned in surrounding areas of high-rise buildings. This analysis will facilitate decision makers to tackle the issue of urban thermal environment for developing a sustainable protocol in future urban planning.