Impacts of urban canyon aspect ratio and roof albedo on heat fluxes and temperatures in four urban centers

The impacts of urban canyon aspect ratio (UCAR defined as the ratio of buildings height to street width) and roof albedo on the heat fluxes and temperatures in four North America urban centers are investigated using a high-resolution numerical model coupled with the Town Energy Balance scheme. Analytical results show that UCAR affects the sensible heat flux over urban centers in a nonlinear way. While the increase of UCAR leads to the reduction of sensible heat fluxes from roads and walls during the daytime, the nonlinearity can lead to the enhancement of sensible heat flux over urban centers under certain condition. The comparisons between benchmark and three scenario model simulation results show that the the increase/decrease of UCAR leads to the increase/decrease of heat fluxes and over urban centers at temperature at 1.5 m in the early morning and nighttime. During the daytime, only very large UCAR can cause the decrease of heat flux and temperature. The increase of UCAR leads to the increase of urban heat island intensity, but there is no obvious correlation between UCAR and the temperature at each grid cell. The comparison between the benchmark simulation and the simulation with a double roof albedo suggests that the increase of roof albedo in the four North America urban areas leads to 0.1 °C-0.2 °C decrease in temperature at 1.5 m.