Contrasting boundary-layer energy budgets during daytime, nighttime, and compound heatwaves in eastern China

Heatwaves over the Yangtze River Delta (YRD) have intensified in frequency and diversity amid rapid urbanization and climate change, yet their underlying boundary-layer processes remain insufficiently understood. This study examines boundary-layer energy budgets of three heatwave types across the YRD using observations from 186 meteorological stations and reanalysis data covering 2002-2022. Daytime-only heatwaves exhibited reduced cloudiness (-37%), enhancing shortwave radiation (+201 W/m²), while efficient nocturnal cooling limited nighttime warming. Nighttime-only events were characterized by greater cloudiness (+6%) and humidity, increasing longwave radiation and suppressing cooling, with rice paddies buffering daytime temperature extremes more effectively than forests. Compound heatwaves combined both mechanisms-enhanced daytime solar heating and inhibited nocturnal cooling-producing the most intense temperature anomalies (>10 °C) concentrated in coastal-urban areas. Enhanced thermal turbulence deepened the boundary layer during daytime and compound events, whereas mechanical turbulence weakened under subsidence. These findings clarify heatwave boundary-layer processes and support region-specific climate adaptation strategies.