Attribution of Heat Wave-Induced Urban Boundary Layer Warming

More than 50% of the global population now lives in urban areas and the urban population continues to rise, especially in developing countries. This mandates a better understanding of the impacts of extreme weather and climate events, including heat waves, on urban environments. While it is well recognized that urban areas are usually hotter than the surrounding rural areas (i.e., the urban heat island effect), how the urban heat island effect evolves under heat waves remains an underexplored topic. Moreover, most previous studies focused on the near-surface air temperature but the urban boundary layer is less studied. Motivated by these, we use a combination of numerical simulations with the Weather Research and Forecasting (WRF) model and aircraft measurements of temperature profiles to characterize the thermal conditions throughout the boundary layer in a heat wave event over the Phoenix metropolitan area. Based on the budget equation for temperature integrated over the urban boundary layer, the warming of urban boundary layer under heat waves is attributed to the effects of surface heating, urban-rural advection, mean subsidence and entrainment.