A remote sensing study of urban heat island effect in Lanzhou city, northwest China

As a promising application, quantitative remote sensing of urban heat island (UHI) can facilitate our understanding of urban/suburban environment and its relationship with urbanization. This paper investigates the urban heat island effect of Lanzhou, China, a densely built up city in a valley, based on Landsat ETM+ image acquired on April 22, 2000, whose spatial resolution is fitly sufficient for measurement of some important environmental parameters. For better quantification, Land surface temperature (LST) was retrieved using the mono-window algorithm, vegetation fraction was derived using vegetation-impervious surface-soil spectral mixture model, and Normalized Difference Vegetation Index (NDVI) was also derived from the corrected image. Then the relationship between LST and NDVI as well as vegetation fraction was estimated. Results show that Lanzhou city's urban heat island effect is significant, which could be visually characterized by the spatial pattern, extent, heterogeneity and intensity of retrieved thermal properties and the maximum urban/suburban temperature difference approximately reaches 10K. Moreover, by analyzing urban composition, it is revealed that LST possessed a strong negative correlation with the vegetation abundance and suggested that vegetation is a key factor controlling the spatial distribution of land surface heat flux. Particularly, due to the scarcity of vegetation, some hotspots are bare soil distributing on suburban surrounding hill, the surface temperature of which is even slightly higher than downtown. These results can help us develop countermeasures to thermal environmental problems in urban areas.