Characteristics of stratosphere-troposphere exchange during the Meiyu season

Characteristics of stratosphere-troposphere exchange (STE) during the Meiyu season in the Yangtze-Huaihe valley, China, is investigated using the European Centre for Medium-Range Weather Forecasts interim reanalysis data, National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data, Meiyu records from the National Climate Center of China, the data from a trajectory model, and a general circulation model (GCM). Results show increases in potential vorticity and decreases in specific humidity in the upper troposphere and lower stratosphere (UTLS) before Meiyu onset, suggesting a strong downward transport of air masses around the tropopause that can be attributed to frequent tropopause folds over the Meiyu area. The minimum tropopause height occurs 3 days before Meiyu onset and then rises until about 6 days afterward. The downward cross-tropopause mass transport (CTMF) is evidenced before Meiyu onset, which is mainly caused by the sharp meridional gradients in the tropopause pressure over the Meiyu area. After Meiyu onset, the upward cross-tropopause transport intensifies due to enhanced convections. The analysis also suggests the strongest upward transport in the UTLS occurs northeast of the Meiyu region, within the core of the upper tropospheric westerly jet. Results from a trajectory model indicate that the lower stratospheric air intrudes into the troposphere before Meiyu onset. The significant upward movements of the middle tropospheric air are notable after Meiyu onset. As convections are weak and the upper level westerly jet is located far to the Meiyu area in poor Meiyu years, the upward CTMF over the Meiyu region is weaker during the Meiyu season compared with that in rich Meiyu years. Citation: Luo, J., W. Tian, Z. Pu, P. Zhang, L. Shang, M. Zhang, and J. Hu (2013), Characteristics of stratosphere-troposphere exchange during the Meiyu season, J. Geophys. Res. Atmos., 118, 2058-2072, doi: 10.1029/2012JD018124.