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

Abstract. 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 summer and autumn of 2009–2011. A total of eight HPEs were identified, mainly happening in August and September. K-means clustering was applied to group the HPEs into three clusters based on their characteristics and mechanism. We found three HPEs were driven by weak subsidence and convection induced by approaching tropical cyclones (TC-HPE), two HPEs was controlled by calm conditions (ST-HPE) with low wind speed at the lower atmosphere, whereas the remaining three HPEs were driven by the combination of both aforementioned systems (HY-HPE). TC-HPE and ST-HPE had both positive synergistic effect between HPE and UHI (~1.1 °C increase); 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.