New Perspectives on Longwave Imaging of Urban Heat Islands: Middle Infrared to Microwaves

Abstract:

Urban populations are projected to increase throughout the century. As urban areas expand and increase in density as a result of population pressures, urban heat islands (UHIs) will grow and intensify. Characterization of UHIs using remote sensing has focused primarily on the use of thermal infrared (TIR) sensors. Remote sensing of TIR measures, at 1 km spatial resolution or coarser, land surface or skin temperatures to reveal the surface UHI (sUHI) in contrast to in situ approaches that measure air temperature at points in networks or along transects. The city, its suburbs, and rural matrix all emit longwave electromagnetic radiation at wavelengths both shorter and longer than TIR. Here we introduce the use of two other wavelength regions for UHI characterization. Our prior work has shown that the middle infrared (MIR) in the atmospheric window from 3-5 microns offers some advantages to imaging urban areas. We demonstrate a new method for characterizing the intensity and the seasonality of sUHI in the MIR for selected cities in North America and Europe using MODIS band 23. In contrast to MIR and TIR sensors, microwave radiometers (MWR) can retrieve both air temperature and water vapor, albeit at coarse spatial resolution (~25 km) relative to most urban areas. Using the AMSR-E/AMSR2 product time series, we show both the advantages and limitations of using MWR data to characterize UHIs in the megacities and major conurbations of North and South America. These new approaches to UHI characterization complement the traditional TIR methods to reveal other impacts of cities on their environment.