Characteristics and Evolution of Radar Polarization During Extremely Persistent Heavy Rainfall

Based on data of S-band dual-polarization Doppler radar, automatic weather station, disdrometer, 2-D lightning locator and Doppler radar wind field retrieval method, mesoscale structure and cloud microphysical characteristics of an extremely persistent heavy rainfall occurred in southwest Fujian on 27 May 2022 are analyzed. Results indicate that the event takes place under the southwest flow on the south side of the low-level shear. Sufficient water vapor, moderate unsteady convective stratification, low lifting condensation height, and convective condensation height over the rainstorm area are all favorable for producing high-efficiency heavy rainfall. Strong echoes (no less than 45 dBZ) persist over the rainstorm area during heavy rainfall. The strong echo center is concentrated on the windward side of the mountain, located at the contraction of the topography of the trumpet opening to the southwest. The wind field retrieval shows that the strong echo persists for an extended period at the convergence of wind speed and the convergence of southerly and northerly airflow. During the first two stages, a strong echo continuously moves into the rainstorm area from the west, generating the train effect of backward propagation. In the third stage, the strong echo moves southeast under the guidance of northeast winds at the middle and upper levels. This process is dominated by oceanic convective rainfall and warm rain. Heavy rainfall is primarily composed of raindrop particles with high concentration and small scale. The lower layer is primarily composed of raindrop particles with high concentration and smaller scales. Raindrop particles in the middle layer are larger than those in the lower layer. Due to strong upward motion, negative flashes occur when graupel particles above 0 ℃ layer collide with ice crystals during the second stage. Due to the ice phase process, large ice phase particles like graupel particles fall, melt, coalesce, and merge with smaller raindrops, resulting in the formation of larger raindrops and the production of highly efficient rainfall. The development of K_DP above 0 ℃ layer indicates an increase in rainfall, forecasting an advance of 6-20 minutes for the strengthening of relative surface ground rainfall. A large number of raindrop particles are primarily concentrated at the confluence of air currents. Hydrometeor accumulates here. The prolonged and intense echo causes the accumulation of water condensate over an extended period, ultimately causing heavy rainfall. There are high concentrations of raindrop particles in the middle and lower layers. The high value of Z_DR is mostly concentrated in the middle layer updraft area. The high-value Z_DR area and the high-value K_DP area do not completely overlap. The distribution of Z_DR is wider than that of K_DP. Large raindrops break into small raindrops as they fall, increasing the number of raindrop particles.