Comparative analysis of characteristics and causes of two heavy rainfall periods in the 2023 quasi-stationary Meiyu Front rainstorm in Suzhou

The Meiyu front oscillated north-south, resulting in two quasi-stationary periods on the morning of June 18, 2023 and in the evening of June 19, 2023. Affected by this, the first stage of heavy rainstorm process occurs in Suzhou, causing severe waterlogging in the main urban area, affecting the lives of 858 000 residents. This paper uses multi-source data such as Jiangsu automatic station data, radar and wind profile data and European Centre for Medium-Range Weather Forecasts ERA5 (ECMWF Reanalysis v5) reanalysis data. Through mesoscale analysis and physical quantity diagnosis methods, it compares and analyzes the similarities and differences between the two quasi-stationary stages in terms of precipitation characteristics, circulation background, environmental conditions, frontogenesis mechanisms, radar echo evolution, and the causes of quasi-stationarity, and establish a conceptual model of Quasi-Stationary Meiyu-Front heavy rainstorms in Suzhou. The results are as follows. (1) Both stages of heavy rainfall have the characteristics of concentrated heavy rainfall periods, high rainfall efficiency, and large cumulative rainfall. The first stage of heavy rainfall is quasi-stationary frontal rainfall of the plum rain front, with high spatial overlap and wide impact range. The second stage occurs during the process of weak cold air infiltration behind the upper trough and the southward pressure of the low-altitude cold shear line. Mesoscale convective systems are active and local rainfall intensity is large. (2) In the first stage, multiple β mesoscale convective systems (MCS) propagate eastward and southwards on the east-west belt shaped quasi stationary Meiyu front, forming a train effect under the guidance of strong westerly winds at high altitudes. During the second stage of the Meiyu front cloud system, multiple MCSs organize into a mesoscale convective complex. New convective cells continuously develop on the trailing side of the MCS, resulting in back-building propagation. The intense radar echoes exhibit a low centroid, steep gradient, and compact structure. The strong echo centroid is low, the gradient is large, the structure is dense. The low-level jet stream propagation and mid-level wind field disturbance on the wind profile radar have a certain warning effect on extreme hourly rainfall intensity. (3) The main reasons for the formation of the first stage of the "train effect" quasi-static rain belt are highly organized MCS, ground quasi-static Meiyu front, low altitude shear, and enhanced inflow leading to the generation of divergence fronts. The persistent southwesterly jet continuously transports moisture and energy, which, together with the intense ascending motion over the shear line and the ascending branch of the secondary circulation, jointly triggers the formation of a surface mesoscale low-pressure system. This dynamic configuration establishes an environment conducive to convective development, resulting in MCSs during the second stage exhibiting quasi-stationary characteristics with back-building propagation.