Study on the Stable Components of Atmospheric Circulation during the Continuous Heavy Rainfall of Meiyu in 2016

There are significant precipitation stages that bring changes during the rainy period known as Meiyu. It is important to study the key stable circulation components that lead to the continuous heavy rainfall of this period to enable the analysis and prediction of Meiyu precipitation. Using NCEP/DOE Reanalysis II data from 1979 to 2016, the key stable components of the circulation system during the 2016 Meiyu period were extracted from the geopotential height field, wind field, and relative humidity field. The spatial structure, evolutionary characteristics, and longer-term background factors were analyzed to provide a basis for extended period forecasts of the periodic heavy rainfall in the Meiyu area. The results suggest that (1) the “tri-pole” pattern is the key system that maintained the continuous heavy rainfall in the Meiyu area in 2016. This “tri-pole” pattern in the stable components of the geopotential height field corresponds to the Ural Mountains blocking high, the Okhotsk Sea blocking high, the Southeast Asia high, and the West Pacific subtropical high. The structure of this configuration is conducive to the convergence of warm and cold air in the Meiyu area to generate persistent heavy rainfall. (2) In the middle and high latitudes, there is a relatively deep negative anomaly between two deep positive-anomaly areas. On one hand, this is conducive to the southward transport of cold air. On the other hand, the stability of the relative humidity field indicates that the water vapor transport and convergence of the northern branch of the air flow play an important role in the continuous precipitation of the Meiyu area. (3) The positive-anomaly area in the middle and low latitudes mainly occurs in the middle and high troposphere with zonal distribution features. However, there are two positive areas on both the eastern and western sides of the Meiyu area, which continue down to the lower level and may strengthen the convergence and transport of warm and humid air in the south of the Meiyu area. (4) The analysis of the evolution of the stable component revealed that the establishment and evolution of the “tri-pole” pattern led to different precipitation-stage characteristics in the Meiyu area. (5) The stable circulation components on a longer time scale (60 d) provide important background circulation for the key stable components of the “tri-pole” pattern during the period of continuous heavy rainfall.