A modeling study of the impact of stratospheric intrusion on ozone enhancement in the lower troposphere over the Hong Kong regions, China

Stratospheric intrusion is of great importance to transporting ozone (O-3)-enriched air from stratosphere into the middle and lower troposphere. The Weather Research and Forecasting model coupled with Chemistry (WRFChem) was employed to simulate a typical stratospheric intrusion episode occurring on March 24, 2010 and examine the impact of stratospheric intrusion on the O-3 enhancement in the lower troposphere. An upper boundary condition scheme was applied to the simulations to overcome the limitation of WRF-Chem in which O-3 chemistry in the stratosphere is not included. The simulations were evaluated with various observational data including O-3 sounding, surface O-3, as well as the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim reanalysis data. The WRF-Chem showed reasonable performance on simulating the meteorological processes that control the stratospheric intrusion. Several findings were identified from the detailed analyses of the modeling results. First, the subtropical high-level jet was responsible for this stratospheric intrusion event. When Hong Kong was located at the south side of the subtropical jet where subsidence was dominated, the O-3-enriched air was transported from stratosphere to troposphere. Second, the stratospheric intrusion caused substantial enhancement of O-3 with the maximum O-3 concentration higher than 80.0 ppbv in the lower above the atmospheric boundary layer in the Hong Kong region. Finally, the process analyses (PA) method is performed to quantify the impact of stratosphere intrusion on the O-3 enhancement in the lower troposphere. The PA result illustrates that a substantial increase in the lower troposphere O-3 is highly associated with vertical transport from the upper troposphere. These results indicated important role of stratospheric intrusion on O-3 enhancement events and more observations are advised to better constrain and quantify its effects.

Automated summary

Stratospheric intrusion plays a crucial role in transporting ozone-rich air from the stratosphere to the lower troposphere. The study utilized the WRFChem model to simulate a stratospheric intrusion event and found that high-level jet streams were key factors in the intrusion process. The results showed a significant enhancement of ozone in the lower troposphere, with both horizontal and vertical transport playing important roles in this enhancement.