Boundary Layer Processes Distinguish Daytime‐Only and Day‐Night Compound Heatwaves in Southwest China

Abstract Compared to daytime‐ and nighttime‐only heatwaves, day‐night sustained compound heatwaves prove to be more impactful to human health, energy and agriculture sectors. Southwest China saw the most frequent occurrence of compound heatwaves across the country, and therefore needs heat type‐tailored forecasting and warning most. Achieving this goal, however, is hindered by unclear mechanistic understanding, especially the critical process that decides the daytime heat persisting into the night or not. Despite seemingly common large‐scale descending motion, reduced cloud cover and enhanced solar radiation during daytime‐ and compound heatwaves, their synoptic origins differ fundamentally. Daytime heatwaves in the region are primarily driven by isolated continental highs, whereas compound heatwaves are dynamically triggered by the well‐organized Western Pacific Subtropical High. Reduced cloud cover typically promotes strong radiative cooling at night and should interrupt the persistence of daytime heat into night, as seen in daytime heatwaves. The warm nighttime anomalies of compound heatwaves apparently violate this radiative cooling paradigm. We find that a near‐neutral boundary layer profile during compound heatwaves is conducive to turbulent mixing and entrainment between near‐surface air and adiabatically‐heated low‐level atmosphere. Preconditioned dry soil and the resulting soil moisture‐dominated land–atmosphere coupling help shape this unique boundary layer structure.