Contributions of Local Background Climate to Urban Heat Island

Urban heat island (UHI), the ubiquitous phenomenon of higher surface temperature in urban buildup land than that of its surrounding, represents one of the most significant human-induced changes to the earth's surface climate. Even though they are localized hot spots in the landscape, these islands have profound impact on the lives of urban residents who comprise more than 50% of the world's population. A barrier to UHI mitigation is the lack of quantitative attribution of various factors contributing to the intensity of UHI (ΔT, temperature of urban center minus rural temperature). A common perception is that reduction in evaporative cooling in urban land is the dominant driver of ΔT. Here we perform a factor separation analysis of surface radiative temperature with a climate model to show that, for cities across North America, variations in daytime ΔT are largely explained by changes in convection efficiency (associated with aerodynamic resistance) between urban and nonurban land. This convection effect depends on local background climate, contributing 3.0 ± 0.3 K (mean ± 1 standard error, s. e.) warming to daytime ΔT in cities in humid climate but causing 1.5 ± 0.2 K cooling in dry climate. In the humid eastern United States, there is evidence of higher ΔT in drier years. These relationships imply that UHI will exacerbate heatwave stress on human health in wet climates where high temperature effects are already compounded by high air humidity and in drier years when positive temperature anomalies may be reinforced by a precipitation-temperature feedback. Results also confirm that albedo management is a viable option for reducing ΔT on large scales.