Thermal remote sensing of urban surface temperatures

• Main Author: Voogt, James Adrian
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/7496

Thermal remote sensors have been used extensively to examine urban surface temperature patterns but without explicit consideration of the complex form of the urban surface. This research investigates the extent of possible biases which may be present due to the unique structure of the urban surface and is an attempt to address some of the issues raised by Roth et al. (1989). The research approach is primarily observational. An extensive measurement pro gramme integrating ground and airborne remote sensor platforms as well as fixed monitoring sites was designed and implemented to obtain information from three of the most common urban surface types: downtown Office/commercial, residential and light industrial. Both nadir and off-nadir high resolution thermal imagery was collected to obtain information on component surface temperatures, and to determine the extent of anisotropy in urban surface emissions at the land use scale. Vertical facet temperatures are shown to exhibit large diurnal temperature ranges which strongly control the anisotropic longwave emission of the urban surface. Airborne observations reveal strong anisotropy of surface longwave emissions over each of the sites at times of maximum temperature contrast between opposing canyon facets. The magnitude of the anisotropy is sufficient to demand consideration whenever daytime thermal remote measurements are made over urban surfaces. Mixed distribution modeling has been explored as a technique to recover component surface temperatures from composite temperature frequency distributions. A low number of component distributions can be used to successfully model the composite distribution, however, the successful extraction of individual component temperatures is more difficult and depends upon the nature of the surface, and the temperature contrasts present. Estimates of the complete urban surface temperature have been made for the first time by combining remote and surface-based estimates of horizontal and vertical facet temperatures. The complete surface temperature is cooler than nadir or off-nadir remote measurements and is shown to be best approximated by off-nadir measurements in the direction of the most shaded vertical facet. A simple two-dimensional geometric urban surface model (a slightly modified version of that orignally presented by Sobrino and Caselles (1989) and extended by Caselles et al. (1992) is used to investigate the nature of anisotropy over the study areas. Urban surface temperature components are specified from observed data but a modified version of the Myrup (1969) surface energy balance model is shown to be adequate for estimating surface temperatures in canyon environments for the extension of geometrical surface modeling to other times and locations. Results indicate that the two-dimensional representation of the urban surface is generally inadequate. The development of a three-dimensional surface geometrical model is recommended to better represent the structure of the urban surface.