Atmospheric boundary layer heat budget and its urban-rural differences in Beijing during a heatwave event

This study investigates the atmospheric boundary layer (ABL) heat budget over the urban and rural areas in Beijing during a heat wave (HW) event in Jun 2020, using potential temperature tendency outputs from the Weather Research and Forecasting (WRF) model. Validations against observational and reanalysis data show that the WRF model effectively captured the HW event and the urban heat island (UHI) effect, and well simulated variations in ABL temperatures and winds. It is found that the turbulent diffusion primarily contributes to the daytime ABL warming, while the larger-scale advection dominates in the nighttime ABL cooling. Both the daytime turbulent diffusion and the nighttime large-scale advection are generally stronger in the urban region than in the rural region. Besides, the radiation effect also plays a significant role in the daytime ABL warming especially in the rural area. Under HW conditions, the daytime warming effect of turbulent diffusion is notably amplified, while at night, the large-scale advection cooling is substantially weakened, and the turbulent diffusion intensifies to become the dominant term, particularly in the rural area. Surface sensible heat flux responds differently to the HW event compared to the turbulent diffusion term, as the latter also includes other turbulent processes like entrainment and latent heat flux. The large-scale advection term closely matches the horizontal temperature advection, indicating that the large-scale subsidence could be negligible in the ABL heat budget compared to horizontal advection over complex urban-rural terrain.