The impact of peripheral circulation characteristics of typhoon on sustained ozone episodes over the Pearl River Delta region, China

Abstract. It is widely reported that the peripheral circulation of typhoon favors for the formation of sustained ozone episodes. However, the process how it impact on the day-to-day ozone pollution levels during the episodes have not been clearly studies, which is crucial for better prediction of the daily ozone variation. In this study, the analysis of ground observation, wind profile data, and model simulation are integrated. By analyzing the wind profile radar observations, we found a weak winds depth deepening (WWDD; vertical depth of the weak winds increased), which is more correlated to the ground-level ozone variation than surface weak wind. Long-term statistical analyses show that the WWDD is a common weather phenomenon that occurs in the peripheral subsidence region of typhoons and was generally accompanied by ozone pollution episodes. WRF-Chem with process analysis simulation show that under the impact of the peripheral subsidence chemical formation (CHEM) and vertical mixing (VMIX) effects are two major contributors to the enhancement of ozone levels, while the advection (ADV) are always negative values. But regarding the daily variability of the daytime ozone levels during the episode, it do not determined by the daily variation of daytime CHEM and VMIX, but that of the ADV term. A detail day-to-day analysis show that weak subsidence associated with typhoon periphery provide the premise for the clear sky and warmer air, which is conducive for the ozone photolysis formation (CHEM) above the ground in planetary boundary layer (PBL) and compensate the ozone through the positive VMIX effects on the ground. The WWDD induced by the peripheral circulation of typhoon system provide the premise for the day-to-day positive contribution of ADV term to ozone enhancement, which play an important role in determining the day-to-day daytime ozone variation. These results indicate the important role of the WWDD in the lower troposphere for the formation of sustained ozone episodes due to the peripheral circulation of the typhoon, which helps to better predict the daily changes of daytime ozone levels.