Characteristics of Moist Potential Vorticity over the Middle and Lower Reaches of the Yangtze River during the Meiyu Season and Its Relationship with the Tibetan Plateau

The ERA-Interim reanalysis data provided by the European Center for Medium-Range Weather Forecasts and the FGOALS-f3-L coupled model were used to analyze the characteristics of the moist potential vorticity (MPV) over the middle and lower reaches of the Yangtze River during the Meiyu period from 1980 to 2017 and their connection to the Tibetan Plateau. The moist isentropic surfaces are found to tilt northward with increasing height over the Meiyu area. The belt of the barotropic term (MPV₁) and baroclinic term (MPV₂) of the MPV lie along the sloping moist isentropic surfaces, which move northward when the rain belt moves northward. MPV₁ and MPV₂ are located to the north of the Meiyu rain belt in the middle troposphere. However, in the lower troposphere, the negative value belt of MPV₂ corresponds better to the Meiyu rain belt. This better correspondence is mainly due to the distribution of MPV₂ satisfying the necessary condition of the development of slantwise vorticity, which is conducive to substantial development of the vertical vorticity of the upsliding warm and wet air, resulting in the development of precipitation. Further analysis reveals that the MPV₂ belt over the Meiyu area starts from the Tibetan Plateau in the west and extends eastward to the Northwest Pacific. The FGOALS-f3-L output without Tibetan Plateau topography indicates that the Tibetan Plateau has an important influence on the distribution of MPV₂ downstream. In the no Tibetan–Iranian Plateau simulations, the negative value belt of MPV₂ is substantially weakened and even disappears along the Yangtze River.