Response of urban heat island effects within the planetary boundary layer to heat waves and impact of horizontal advection over Shanghai

This study characterizes the variation of the urban heat island (UHI) within the planetary boundary layer (PBL) and investigates the horizontal advection effects on the UHI variation throughout a heat wave (HW) event over Shanghai municipality in August 2019 based on numerical model simulations. It is found that the UHI intensifies under HW conditions with the UHI intensity gradually weakening from the surface upwards. The daytime UHIs during the HW are 3.68 K (29.27 %), 1.41 K (33.52 %) and 1.04 K (36.97 %) higher than those during the pre-HW at the surface, near-surface and in the PBL, respectively, while the nighttime UHIs have no significant response to the HW at least for this case. The mean UHIs during the HW period at the surface, near-surface (2 m) and in the PBL are 6.83 K (2.68 K), 2.45 K (1.34 K), and 1.73 K (0.04 K) in the daytime (nighttime), respectively. The PBL UHI generally exists only in the daytime potentially caused by the thermal convective diffusion. The near-surface and PBL UHIs are significantly affected by the horizontal advection, resulting in different UHI intensities and variations against rural regions in different orientations. Cold advection from the Yangtze River (the ocean) in the riverside (coastal) rural region corresponds to the great UHI intensification, while the strong cold advection in the urban region well explains the fast UHI weakening. This study highlights that, besides local thermal factors, synoptic circulation also play an important role in the interaction between the UHI effect and HW events.