Characterizing Urban Heat Island Effect at Global Settlements Using MODIS and Nightlight Products

Impervious surface area (ISA) from the National Geophysical Data Center (NGDC) and land surface temperature (LST) and Normalized Difference Vegetation Index (NDVI) from MODIS averaged over three annual cycles (2003-2005) are used in a spatial analysis to assess the urban heat island (UHI) skin temperature amplitude and its relationship to development intensity, size, and ecological setting for more than 3000 urban settlements over the globe. Development intensity zones based on %ISA were defined for each urban area emanating outward from the urban core to the non-urban rural areas nearby and used to stratify sampling for land surface temperatures and NDVI. Sampling was further constrained by biome and elevation to insure objective intercomparisons between zones and between settlements in different biomes. Stratification based in ISA permits the definition of hierarchically ordered zones that are consistent across urban areas in different ecological setting and across scales. We find that ecological context and settlement sizes significantly influence the amplitude of summer daytime UHI (urban-rural temperature difference). An average of 3.8 C UHI is found in cities built in biomes dominated by forests; 1.89 C UHI in cities embedded by grass/shrubs; and only a weak heat island or sometime heat sink in cities in arid and semi-arid biomes. Overall, the amplitude of UHI is significantly negative correlated (R=-0.66) with vegetation activity difference between urban and rural represented by MODIS NDVI. The average UHI is 4.7 C for global settlements larger than 500 km2, compared to 2.5 C for global settlements smaller than 50 km2 and larger than 10 km2. For all global settlements, the average amplitude of the UHI is 2.6 C in summer daytime and only 1.4 C in winter and impervious surface area is one of the primary drivers for increase in temperature with variations in different ecological context and latitudinal zones. More than 60% of the total variances in LST can be explained by ISA for urban settlements in forests at mid-to-high latitudes. The percentage will increase to more than 80% when only USA settlements are examined. Overall, our results indicate a possible way to examine and compare the amplitude as well as the driver of the UHI over global settlements. The relationship between UHI and key factors indicates that the energy consumption required for cooling is likely to increase with urban growth especially during summertime and in larger urban areas.