Numerical Simulations On The Troposphere-to-stratosphere Transport Of Atmospheric Pollutants By A Severe Convective Weather Event In Beijing

Stratosphere-troposphere exchange (STE) has important impacts on atmospheric chemical composition and radiation balance. It is very important to investigate STE activities for further advancement on atmospheric chemistry, climate change, and stratosphere–troposphere coupling in atmospheric models. So the study of STE arouses general interest internationally. Troposphere-to-stratosphere transport (TST) is one of the aspects of STE. With the rapid urbanization, atmospheric pollutants could be transported to stratosphere in TST process caused by severe convective weather events, which attracts the attention of researchers. On the basis of successfully simulating a severe convective weather event in Beijing on 23rd June 2008, the tracer initialization module of WRF-CHEM was coupled to WRF model to investigate the space-time evolution of PBL tracer. The analyses of CO and water vapor observations were also carried on in order to prove that this convection event in Beijing indeed led to TST and had transported pollutants of high concentration to stratosphere. Results indicate that:1. The severe convective weather event was induced by high-altitude cold vortex. The convective cells coming from the west grew and merged in urban area and then moved southeastward, causing rain rate above 30 mm/h in Mentougou, downtown area, Fangshan, and Daxing etc. The 3 h cumulative rainfall bands were in urban area and southern region of Beijing. The intrusion of dry cold air at the rear of the cut-off low, combined with the easterly warm and wet flow near surface, contributed to the development of the strong convection, and greatly decreased the tropopause height.2. Utilizing Beijing Rapid Update Cycle system (BJ-RUC), both of the evolution of synoptic systems and radar echo were simulated well after assimilated the conventional and unconventional local observations. Except that the simulated radar echo evolution was 1-2 h earlier than observations, and the radar echo top was also simulated slightly lower than the observations. The distribution of simulated 3 h cumulative rain bands was in good agreement with observations, though the intensity was slightly lower.3. The evolution of concentration of PBL tracer represented the evolution of pollutants such as carbon monoxide. Both the simulated PBL tracer and calculated trajectories of PBL air using model output indicate that this convection led to TST with pollutants such as carbon monoxide transported to stratosphere and delivered long distance. The updraft of the convection played important role in transporting pollutants to stratosphere. The release of latent heat thermal energy produced and maintained the strong updraft by heating the air and producing buoyancy.4. The rapid transport caused by strong convection greatly contributed to sharp drop of CO concentration in PBL. The magnitude of drop was consistent with the model results. The increase of relative humidity and water vapor density in lower stratosphere was observed by radiometer in Beijing observatory, which provides an indirect evidence for TST caused by convection and the transport of pollutants to stratosphere. Both the model and observation indicate that TST caused by convection may be one of the mechanisms by which the pollutants enter the stratosphere in Beijing.