Urban heat island diagnosis using ASTER satellite images and ‘in situ’ air temperature

Abstract

This study demonstrates that thermal satellite images combined with ‘in situ’ ground data can be used to examine models of heat island genesis and thus identify the main causes of urban heat islands (UHIs). The models, although proposed over 30 years ago, have not been thoroughly evaluated due to a combination of inadequate ground data and the low resolution of thermal satellite data. Also there has been limited understanding of the relevance of satellite-derived surface temperatures to local and regional scale air temperatures. A cloud-free ASTER thermal image of urban and rural areas of Hong Kong was obtained on a winter night with a well-developed heat island, accompanied by a 148 km vehicle traverse of air temperatures. Over the whole traverse a high R² of 0.80 was observed between surface and air temperatures, with the two datasets showing a similar amplitude and general trend, but with the surface exhibiting much higher local variability than air temperature. Gradients in both surface and air temperature could be related to differences in land cover, with little evidence of large scale advection, thus supporting the population/physical structure model of UHI causation, rather than the advection model. However, the much higher surface and air temperatures observed over the largest urban area, Kowloon, than over any smaller urban centre with similar physical structure in the New Territories, would seem more indicative of the advection model. The image and ground data suggest that Kowloon's urban canopy layer climate is mainly influenced by local city structure, but it is also modified by a strongly developed, regional scale urban boundary layer which has developed over the largest urban centre of Kowloon, and reinforces heating from both above and below.

Introduction

The sustainability of tropical and sub-tropical cities such as Hong Kong where dense and high-rise buildings are accompanied by an intense urban heat island (UHI) effect may be threatened if predictions of global warming are realized.

Two different models have been proposed to explain the extent and magnitude of the UHI. These are (i) the advection model which relates heat island magnitude (ΔT(u − r)) to the distance of fetch from the rural–urban boundary, and thus to the accumulation of heat as rural air moves inwards (Oke, 1976, Summers, 1964) and (ii) the population/physical structure model whereby population is a surrogate for the intensity of urbanization. Thus factors such as building size, anthropogenic heat inputs, heat capacity and thermal inertia increase toward the city centre, and sky view factor and vegetation are reduced (Oke, 1976). These models are relevant because they may indicate future planning policies for heat island mitigation. For example according to the advection model, the magnitude of ΔT(u − r) increases with distance from the rural boundary Therefore this model may suggest that structures built at the edge of the city, irrespective of their height, building density, street geometry and building material used, would exhibit a minimal heat island effect, but conversely they should intensify ΔT(u − r) in the city centre by increasing the distance from the countryside. On the other hand, according to the population/physical structure model, if city centres were less densely built (though not necessarily less tall) with higher sky view factors, using material with lower heat capacities and thermal inertia, ΔT(u − r) would be significantly reduced. Furthermore, adding new buildings at the periphery and thus increasing the distance from the periphery to the centre, as for example in Hong Kong's harbour reclamation schemes, would not affect the overall UHI magnitude. These questions are important because of high-rise trends in modern cities due to considerations of energy and transport efficiency. Thus by 2030 China is predicted to have 15 high-rise mega-cities each with 25 m people.

Studies conducted by night-time mobile traverse (Goldreich, 1985, Eliasson, 1992) support the physical structure model with, for example, cooler air temperature at open street intersections (Eliasson, 1992). On the other hand Stoll and Brazel (1992) and Spronken-Smith and Oke (1998) observe that although the thermal properties of surfaces and their radiative geometry are dominant factors in heat island formation, the correlation between the surface and air is affected by advection from adjacent land uses, thereby supporting Summers' (1964) advection model. Thus both models may be relevant, but their influence may vary for different cities and climates. Traditional methods of UHI analysis, the use of fixed stations and/or vehicle traverse have been unable to address such basic conceptual questions because the data collected are spatially incomplete. Although the UHI has been observed from thermal satellite images for four decades, the studies have not contributed substantially to understanding of the causes of urban heat islands, and inferentially, their mitigation (Roth et al., 1989, Arnfield, 2003, Voogt and Oke, 2003).