The relationship between remotely-sensed surface parameters and urban heat islands in the USA

Impervious surface area (ISA) from Landsat TM and land surface temperature (LST) from MODIS are used in a spatial analysis to assess the urban heat island amplitude and its relationship to urban structure. The fractional area of impervious surface is correlated to land surface temperature anomalies defined relative to nearby non-urban areas for 80 U.S. cities grouped by eco-climatic region. MODIS LST data over three annual cycles (2003-2005) are analyzed for ISA zones emanating outward from the urban core in decreasing ISA increments. Areas with the highest fractional ISA define the urban core and zones are identified outward until the ISA dropped below 0.1% defining the non-urban fringe near each city. For statistical sampling, ISA zones are constrained by eco-climatic region and elevation to remove biases from those sources. We find that for the largest U.S cities, ISA is the primary driver for increases in LST. When assessed over the continental U.S, the fraction of ISA explains 70% of the total variance in LST. These correlations are highest in the northeastern U.S where cities are embedded in temperate forests and where the increase in ISA explains a large part (92%) of the variation in LST. Except for desert cities, urban areas (ISA > 25%) are, year around, substantially warmer than the non-urban fringe by about 2.9 oC. The amplitude of the urban heat island is remarkably asymmetric: it is larger during summer where it reaches 4.3 oC, while during winter the excess heat due to urbanization is only 1.3 oC. In desert environments we find that the LST response to ISA is bowl-shaped. Zones with moderate ISA are cooler than the surrounding desert but as ISA increases above 75% the LST becomes more like the non-urban desert fringe. These observational results are in line with previous studies and indicate an increase in the urban heat island amplitude with increase in city size that is consistent among cities across a broad climatic range. This relationship indicates that the energy consumption required for cooling is likely to increase with urban growth, especially during summertime.