An Examination of the Skin-level Urban Heat Island Effect Using Satellite Observations

This paper describes a new approach for calculating the urban heat island (UHI) using satellite data, leveraging its uniform coverage over urban and natural regions. The UHI is defined as the difference between surface temperatures over urban and rural regions. Traditionally, UHI is identified using ground-shelter-measured 2m surface air temperatures (i.e., the screen-level temperature) measured by a pair of weather stations sited in urban and rural locations, respectively. Screen-level UHI is affected by the location, distance, and geometric conditions of urban and rural stations. For example, choosing different rural sites and city sites results in different UHI magnitudes for the same city. To avoid this uncertainty, this paper examines UHI using satellite-observed land surface skin temperature measurements (i.e., skin-level temperatures) which have the advantage of high spatial resolution and broader coverage. Since 2000, the National Aeronautics and Space Administration (NASA) Moderate Resolution Imaging Spectroradiometer (MODIS) has provided measurements of global skin-level surface temperature at 5 km resolution and 4-times-per-day. An assessment of skin-level UHI from 10-years of MODIS observations reveals that skin-level UHI has clear diurnal and seasonal variations: the skin-level UHI is stronger during the daytime and warm season (July), respectively. In addition, urban regions have reduced surface albedo and surface emissivity, which directly contribute to UHI. Using skin-level UHI approach could be important for more uniform assessments of UHIs in global cities and representation of them in coupled weather-climate modeling systems.