Attribution of Uhi Intensity for Cities in North America

Abstract:

The urban heat island is a result of perturbation to the surface energy balance caused by conversion of natural to urban land cover. In this study, we quantify the various contributions to the UHI intensity for cities in the US and in Canada using the NCAR’s climate model. In the modeling system, the UHI intensity is defined as the difference in the surface temperature between the urban and rural land unit within the same model grid. A surface temperature solution, first derived for temperature perturbation caused by local-scale deforestation, is used to partition the UHI intensity to various contributing factors, including reduction in evaporation, anthropogenic heat release, and changes in surface albedo, heat storage and convection efficiency. Results show that in the daytime, the UHI intensity variations among the cities are explained primarily by changes in convection efficiency between urban and non-urban land. Here the convection efficiency is determined by the aerodynamic resistance to heat diffusion from the earth’s surface to the atmospheric boundary layer. This convection effect depends on the local background climate, increasing the daytime UHI by an average of 3 K in humid climates but decreasing it by 1.5 K in dry climates. The model simulations also reveal evidence of higher UHI in drier years for cities in the humid southern United States. These results offer useful guidance for the development of UHI mitigation strategies.