Linking satellite images and hand-held infrared thermography to observed neighborhood climate conditions

Abstract

While satellite images effectively show surface urban heat islands in urbanized areas, linking surface temperatures to actual ambient temperatures remains a research challenge. Microclimates in urbanized settings can vary tremendously in very short distances, making adequate climate interpolations across a large metropolitan area difficult, at best. This study links the coarse scale of satellite (ASTER) images to the fine scale of hand-held thermography as part of an in-depth suburban neighborhood climate study to determine if ASTER imaging can be used to adequately estimate neighborhood climate conditions in an urbanized area. The study utilizes day and night remotely-sensed and ground data from June, 2004 for Phoenix, Arizona. Microclimate conditions of three urban fringe neighborhoods with varying amounts of natural vegetation and development density were studied, along with ASTER remote sensing data, mobile climate transects, and spot infrared thermographic images.

These neighborhoods, though variable, showed only minor differences, and the study indicates that daytime images (11:20 am) do not adequately rank observed conditions within these neighborhoods — the highest ASTER surface temperatures were recorded for the least-dense neighborhood with a natural desert landscaping, though lowest ambient temperatures were measured there. Daytime mean surface temperatures versus air temperatures were 50.4 °C (30.8 °C air temp); 53.5 °C (29.7 °C); and 50.6 °C (31.9 °C). It was found that nighttime (10:40 pm LST) differences among neighborhoods of surface and air temperatures were relatively consistent, with the most densely developed neighborhood having the highest ASTER surface temperatures (29.0 °C) and transect-derived air temperatures (30.0 °C). Issues of view angle, shadowing, emissivity, resolution, and wind conditions for daytime results with their relatively small mean differences observed across the neighborhoods may explain why the rank of ASTER thermal conditions versus observed ambient conditions was poor. However, following sunset, these issues of view angle, etc., are much less problematic.

Introduction

Research on urban heat islands (e.g., Bonan, 2000, Oke, 1973, Svensson, 2004) suggests even settlements with a small population can have a substantial heat island impact. The metropolitan area of Phoenix, Arizona, USA with a population of over 3 million, has a well studied urban heat island (UHI), characterized by higher urban vs. rural temperatures, and a historical trend of significantly increasing temperatures in the city over the past 50 years (e.g.: Baker et al., 2002, Brazel et al., 2000). Urbanization is rapidly expanding into the open desert and the resultant suburban residential land may be contributing a positive feedback to the spread of the urban heat island. The heterogeneity found within the urban fabric in metropolitan Phoenix creates a complexity of microclimates with significant local variability within the metropolitan area, which is difficult to assess adequately.

Remote sensing is a key to mesoscale modeling through specification of land cover distributions and creating spatial products of moisture, reflectance, and surface temperatures (Zehnder, 2002). Understanding the relationship between the “coarse” scale satellite remotely-sensed data, to “fine” scale hand-held thermography, and then to the observed ambient temperatures at the neighborhood microclimate scale, could allow further extrapolation to estimate the impact of urbanization across the metropolitan area for input into mesoscale models, and enhance prediction of UHI expansion through assessing the microclimate impact of growth and neighborhood design.

While there have been many studies that have highlighted temperature variation in urban areas through the use of high resolution remote sensing imagery (e.g., Eliasson, 1992, Quattrochi and Ridd, 1998), our study also adds the technique of ground thermography surveys and in situ surface climate observations to describe the residential environments. This method quantifies the thermal variation based on urban land use type, using the techniques of thermography, processing of remote sensing data, and utilization of Geographic Information Systems (GIS). These techniques can be easily applied to other arid cities and utilized by non-modeling researchers to describe the thermal environment of residential neighborhoods on the periphery of the city, and generate results that can be provided to urban planners and policy makers.

Connections between urbanization and climate in the region have been studied previously (e.g. Lougeay et al., 1994), but it is not known to what extent new residential developments beyond the urban fringe have an impact on local climatic conditions. Previous mobile transect sampling through urban, residential, and rural areas has shown on clear, calm days, the existence of very large thermal gradients across the urban fringe zone into rural areas, but the details of rural thermal patterns, associated with new developments, has not been fully investigated (Brazel et al., 1999, Hawkins et al., 2004, Hedquist, 2002, Stabler et al., 2005). In order to generate accurate models of Phoenix's mesoclimate, the impacts of large acreage tract residential developments on the urban fringe should be understood.