Atmospheric Conditions Associated With A Flash Flood Of The Piratini River Em Pedro Osório/Cerrito Municipalities In Rio Grande Do Sul, Brazil, In April 1992

Southern Brazil is highly vulnerable to extreme precipitation events, particularly the state of Rio Grande do Sul, where severe flooding is favored by the frequent influence of cold fronts, convective systems, and extratropical cyclones. In April 1992, under the influence of the El Niño phenomenon, a historic flood affected Pedro Osório and Cerrito in the Piratini River Basin. River levels rose by nearly 17 meters, destroying much of the local urban and productive infrastructure. This study aimed to analyze the meteorological factors responsible for this extreme event using rainfall observations and atmospheric reanalysis data. Daily precipitation data from four stations of the National Water and Basic Sanitation Agency (ANA) and ERA5/ECMWF reanalysis fields at 0.25° resolution were used. The results indicated accumulations exceeding 300 mm between 11 and 14 April, reaching up to 460 mm by the end of the analyzed period. This period was marked by cyclogenesis over the state of Rio Grande do Sul, Brazil.On 11–12 April, a mid-level trough approached, intensifying a surface low-pressure system over northern Argentina. The low-level cyclonic circulation, initially over northern Argentina and later over Rio Grande do Sul, increased atmospheric instability by transporting warm and moist air from the north. This condition generated upward air motions that persisted from the afternoon of 11 April through 12 April. The mid-level trough enhanced the intensification of the surface system and the destabilization of the atmosphere due to strong advection of negative relative vorticity over the region. Upper-level diffluence east of the mid-level trough enhanced divergence and intensified atmospheric instability. Approximately 200 mm of rainfall was recorded during this period. From the night of 12 April, the cyclone entered its dissipation phase, when its occlusion became evident.. Even under the cyclone’s occluded area, the study region received over 100 mm of rainfall due to persistent upward motion and continuous moisture transport by the cyclone from the Atlantic Ocean. Persistent instability and moist air transport to the study region contributed to the extreme rainfall and the historic Piratini River flood.