Abstract
The thermal textures of satellite sensed images, caused by homogeneity and heterogeneity of urban canopy layers, indicate the objective existence of local urban heat islands or cool islands in a city. Extracting and analyzing such textures helps us to identify the thermal environment on a community scale, and take countermeasures against urban heat islands that better suit the local context. Conventional research with satellite thermal images has paid most attention to energy exchanges between canopy layers and the above atmospheric layers in pixel-based ways, but missed features appearing on the images and the mechanism by which they are formed.
Instead of a statistical comparison of brightness temperatures and environmental conditions pixel by pixel, this study extracts thermal features from satellite sensed images by a Rater-Patch-Cluster Scheme (RPCS), and discusses their thermal characteristics on a local scale. This scheme is applied to Yokohama City with ASTER/TIR observed on October 30, 2003 to identify the spatial structure of urban heat islands and the characteristics of local thermal features in the late autumn. As a result, the following knowledge is obtained.
1) The Rater-Patch-Cluster scheme is an efficient procedure for abstracting the spatial structure of urban heat islands over a wide area, and assessing the intensities of heat islands or cool islands locally.
2) There exist significant logarithmic relationships between feature areas and local heat island intensities and significant linear relationships between feature areas and local cool island intensities. That is, the larger the area of heat islands (cool islands), the stronger the intensity of heat islands (cool islands) within a city.
3) The differences in brightness temperatures in the city were complicated by two dominant factors of urban canopy layers : land use and topographical characteristics. It was observed through the thermal image that the cold air generated by grasslands and bare land after sunset could flow down along valleys or slopes of hills so that the spatial extent of cool islands were expanded.
The above conclusions show that thermal features are important elements of the local environment, and thermal images such as ASTER/TIR were useful thermal information resources for land use planning or natural land conservation.
