Interactions between the summer urban heat islands and heat waves in Beijing during 2000-2018

The coexistence of urban heat islands (UHIs) and heat waves (HWs) enhances the stress on human society. The HWs can be classified into pure daytime type (daytime HWs), pure nighttime type (nighttime HWs), and compound daytime-nighttime type (compound HWs). However, the interactions between UHI and three HWs types and their mechanisms, using the regional climate model incorporated with the local climate zone (LCZ), are less known. In this study, the regional climate model ALARO-SURFEX (incorporated with LCZs) was used to estimate the UHI and energy balance before, during, and after three types of HWs during 2000-2018. Our results indicate that: (1) All types of HWs enhance the nighttime UHI during the HW period. The compound HWs and daytime HWs enhanced UHI of both daytime and nighttime, while the nighttime HWs suppressed daytime UHI. (2) The enhancement effects of the compound HWs on the UHI are caused by raising the urban-rural differences in sensible heat (H) and ground heat flux (GF), and decreasing urban-rural differences in latent heat (LE). The enhancement of daytime HWs on UHI is triggered by increased urban-rural differences in H and declined that in LE. But nighttime HWs altered the daytime UHI by less urban-rural differences in net radiation (RN). (3) The daytime HWs predominantly increase the nighttime UHI in the LCZ1, LCZ2 and LCZ4. (4) The historical urbanization increased the compound HW intensity from 0.65 degrees C (Miyun, LCZ7) to 2.47 degrees C (Changping, LCZ3), and the compound HWs' frequency from 1 time (Shijingshan, LCZ7) to 7 times (Daxing, LCZ2).