Characteristics of surface energy balance and atmospheric circulation during hot-and-polluted episodes and their synergistic relationships with urban heat islands over the Pearl River Delta region

This study analyzed the nature, mechanisms and drivers for hot-and-polluted episodes (HPEs) in the Pearl River Delta, China. Numerical model simulations were conducted for the summer and autumn of 2009–2011. A total of eight HPEs were identified, mainly occurring in August and September. K-means clustering was applied to group the HPEs into three clusters based on their characteristics and mechanisms. We found three HPEs were driven by weak subsidence and convection induced by approaching tropical cyclones (TC-HPE) and two HPEs were controlled by calm (stagnant) conditions (ST-HPE) with low wind speed in the lower atmosphere, whereas the remaining three HPEs were driven by the combination (hybrid) of both aforementioned systems (HY-HPE). A positive synergistic effect between the HPE and urban heat island (UHI; ∼ 1.1 ∘C increase) was observed in TC-HPE and ST-HPE, whereas no discernible synergistic effect was found in HY-HPE. Total aerosol radiative forcing (TARF) caused a reduction in temperature (0.5–1.0 ∘C) in TC-HPE and ST-HPE but an increase (0.5 ∘C) in HY-HPE.