Comparison of the characteristics and causes of the two quasi-static stages of the first round of heavy rainstrom in Suzhou during the Meiyu Season in 2023

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. 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. In the second stage of the Meiyu front cloud system, multiple MCSs are organized into mesoscale convective complexes, which continuously regenerate behind the moving direction, forming quasi stationary backward propagation. 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 the organized MCS, maintenance of the ground quasi-static plum rain front, low altitude shear, and enhanced inflow leading to the generation of divergence fronts. The strong upward movement above the shear line, the superposition of the rising branch of the secondary circulation, the maintenance of the southwest jet stream, and the favorable configuration of the generation of mesoscale low pressure on the ground make the second stage MCS exhibit quasi stationary characteristics of backward propagation.