Temporal characteristics of thermal satellite images for urban heat stress and heat island mapping

Abstract

The reconstruction of urban climate is still challenging to climatologists in spite of over five decades of research including direct data measurement and model building. Methods for measuring and monitoring urban climate have strengths and weaknesses depending on the application. The mapping of patterns of urban heat stress over a city is not useful if the patterns depicted apply only to the time of data acquisition. Since thermal satellite sensors can now provide detailed temperature data covering whole cities and beyond, their adoption in urban planning depends on demonstrating their relevance to commonly prevailing conditions. This research investigates and presents a methodology based on four summertime ASTER thermal satellite images of Hong Kong for urban heat stress mapping at detailed level. It demonstrates that satellite images obtained under certain climatic conditions, and accompanied by adequate ‘in situ’ ground data, can provide a basis for an operational heat stress mapping system. The temporal limitation of thermal satellite images is examined for both day and nighttime images by comparison of image-derived air temperatures with ground data representing extended periods and other hot days and nights outside the image acquisition times. The nighttime images were found to be more representative of air temperature at other times than the daytime images, due to a more stable boundary layer, with lower wind speeds and temperature inversion at night. The nighttime images showed high and significant correlations with ground level air temperatures for an average 13-h period surrounding the image time 10.42pm, from 6pm to 4–8am the next day. Additionally they were highly and significantly correlated with ground air temperature distributions on 93% of all hot summer nights in the same years. Therefore the nighttime images can be considered representative of a commonly occurring summer nighttime situation in Hong Kong, and can be used to determine the locations of areas where temperatures commonly exceed hot weather warning thresholds. Notably, the images were better able than climate stations to represent areas in the urbanized Kowloon Peninsula and several smaller satellite towns which exceeded hot weather warning thresholds. Many areas exceeded the thresholds, even when no hot weather warning was in force, due to the unrepresentative location of climate stations. The images were also more able than climate stations to indicate the hottest and coolest areas over the Hong Kong territory, thereby enabling measurement of the magnitude and extent of the urban heat island.