华南地区春季平流层入侵对对流层低层臭氧影响的模拟研究

A Modeling Study of the Impact of Stratospheric Intrusion on Ozone Enhancement in the Lower Troposphere in South China

利用中尺度大气化学模式WRF/Chem对2013年3月6日华南地区一次平流层入侵事件及其对对流层低层臭氧的影响进行模拟研究。通过加入UBC(Upper Boundary Condition)上边界处理方案,弥补WRF/Chem模式未考虑平流层臭氧化学反应的不足。结合臭氧探空廓线资料、地面O3、CO、NOx、相对湿度、温度和风速等观测资料以及再分析资料对模拟结果进行定量评估,结果表明模式能较为真实地模拟本次平流层入侵过程。模拟分析进一步揭示:(1)副热带高空急流是本次平流层入侵的主要原因。当华南地区处在副热带急流入口区左侧下沉区域时,平流层入侵将富含臭氧的干燥空气输送到对流层中低层。(2)本次平流层入侵对对流层低层臭氧收支有重要影响,导致香港地区近地层臭氧体积混合比浓度明显上升,如塔门站夜间臭氧浓度升高21.3 ppb(1 ppb=1×10-9)。地面气象场和化学物种的分析进一步确认了平流层入侵的贡献。(3)采用动力学对流层顶高度时零维箱式模型和Wei公式计算得到的平流层入侵通量相当,分别为-1.42×10-3 kg m-2 s-1和-1.59×10-3 kg m-2 s-1,这一结果与前人研究相吻合,且与采用热力学对流层顶高度计算所得到的结果具有可比性。

The Weather Research and Forecasting model coupled with Chemistry(WRF/Chem) is used to simulate the stratospheric intrusion event occurring on 6 March 2013 and investigate the impact of stratospheric intrusion on the enhancement of ozone in the lower troposphere. An upper boundary condition scheme is employed in the simulations to overcome the problem that WRF/Chem does not have the capability to simulate O3 formation in the stratosphere. A series of observational data including ozone sounding data, surface O3, CO, NO, relative humidity, temperature, winds together with ECMWF(European Centre for Medium-Range Weather Forecasts) Interim data are used to evaluate the model results. The WRF/Chem shows reasonable performance on simulating the meteorological processes that control the stratospheric intrusion. Several findings are identified from the detailed analyses of the modeling results. First, the subtropical jet at high altitude was responsible for this stratospheric intrusion event. When Hong Kong was located at the left side of the entrance of the subtropical jet zone where subsidence was dominated, the ozone-enriched air was transported from the stratosphere to troposphere. Second, the stratospheric intrusion caused substantial enhancement of ozone in the lower troposphere over the Hong Kong region. For instance, the surface ozone increased by 21.3 ppb(1 ppb=1×10-9)(nighttime) at Tap Mun station, Hong Kong. The analyses of surface observational data including winds and CO further confirmed the contribution of stratospheric intrusion to the increase in surface O3. Third, the stratosphere–troposphere exchange fluxes of ozone calculated with a box model and the Wei formula are comparable to each other(-1.42×10-3 kg m-2 s-1 versus-1.59×10-3 kg m-2 s-1), and consistent with the findings of other studies.Furthermore, the results are comparable when the dynamic and thermodynamic tropopause heights are utilized in the flux calculations.