Observational characteristics and causes analysis of an extreme heavy rainfall event in Tianjin

An extreme heavy rainfall event occurred in Tianjin Jinnan on July 3, 2022. Based on conventional observational data, weather radar, and reanalysis datasets, this study analyzes the observational characteristics and mechanisms of this event from three perspectives: large-scale circulation patterns, the initiation and development of meso-scale convective systems, and cloud microphysical scales. The results are as follows. This event is characterized by its long duration and high rainfall intensity. The pre-precipitation environment was characterized by high temperature, high humidity, and high energy features. These conditions, combined with a southerly airflow—formed by the confluence of southwesterly warm and moist flow preceding the 500 hPa upper-level trough and southeasterly flow north of Typhoon Chaba, provided favorable moisture and warm, moist environmental conditions for the event. The rainfall event was jointly triggered by the cold pool outflow from upstream precipitation, an easterly disturbance within the boundary layer, and a surface convergence line. Meso-scale convergence, and its interaction with the strong pre-trough wind speeds, generated a meridional mesoscale secondary circulation. This circulation further promoted the development and maintenance of vertical motion over the Jinnan area, consequently leading to the initiation and intensification of the heavy rainfall event. This event was characterized by sustained and stable large raindrop diameters and high number concentrations, exhibiting features typical of "continental" convective precipitation. The synergy of these multi-scale factors is a key driver behind the formation of this extreme short-duration heavy rainfall event.