Aircraft observed diurnal variations of the planetary boundary layer under heatwaves

This study investigates the diurnal variations of the planetary boundary layer (PBL) during heatwaves (HWs) based on a decade-long data record of hourly profiles from the Aircraft Meteorological Data Reports (AMDAR) at 54 major airports over the Contiguous United States. The results are also corroborated by the surface observations from weather station data. Temperature differences between HW and non-HW periods show strong diurnal and vertical variations in the PBL. Under HWs, the daytime convective PBL becomes higher while the nocturnal residual layer becomes excessively hotter, which creates a positive feedback on HW intensity and duration by providing heat for the next day through entrainment. Changes in wind speed in the PBL during HWs are dependent on the relative location between the observational site and the center of the high-pressure system and do not always show decreases. The specific humidity often increases in humid regions mainly due to the higher evaporation demand which causes more surface evaporation, but decreases in arid regions due to the dry air entrainment and subsidence. In addition, moisture advection could also play an important role in modulating the change of PBL humidity. This study improves our scientific understanding of HW-related changes in the PBL structure and highlights their possible connections with the synoptic-scale atmospheric circulation patterns and land-atmosphere feedbacks. However, future research examining the role of PBL in the onset and demise of HWs and examining whether meteorological models can capture the observed response of the PBL to HWs is strongly needed.