A Multi-Scale Analysis of the Extreme Precipitation in Southern Brazil in April/May 2024

• Published in: Atmosphere Vol. 15; no. 9; p. 1123
• Main Authors: Reboita, Michelle Simões, Mattos, Enrique Vieira, Capucin, Bruno César, Souza, Diego Oliveira de, Ferreira, Glauber Willian de Souza
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 01.09.2024
• Subjects: Air temperature; Annual reports; Anticyclonic circulation; Barotropic mode; Circulation anomalies; Civil defense; Coastal plains; Cold; Cold front; Cold fronts; Disasters; Emergency warning programs; extreme precipitation; Extreme weather; Floods; Fronts; Landslides & mudslides; Lightning; Low-level jets; Maximum precipitation; Mesoscale convective systems; Mesoscale phenomena; Moisture content; Multiscale analysis; Natural disasters; Planetary waves; Precipitation; Radar; Rain; Rain and rainfall; Rainfall; Rossby waves; Satellite observation; Satellites; severe deep MCSs; Social networks; Soil moisture; Soil water; southern Brazil; Surface water; teleconnection; Tornadoes; Water infiltration; South America; Brazil; Rio Grande do Sul Brazil
• ISSN: 2073-4433; 2073-4433
• DOI: 10.3390/atmos15091123

Since 2020, southern Brazil’s Rio Grande do Sul (RS) State has been affected by extreme precipitation episodes caused by different atmospheric systems. However, the most extreme was registered between the end of April and the beginning of May 2024. This extreme precipitation caused floods in most parts of the state, affecting 2,398,255 people and leading to 183 deaths and 27 missing persons. Due to the severity of this episode, we need to understand its drivers. In this context, the main objective of this study is a multi-scale analysis of the extreme precipitation between 26 April and 5 May, i.e., an analysis of the large-scale patterns of the atmosphere, a description of the synoptic environment, and an analysis of the mesoscale viewpoint (cloud-top features and lightning). Data from different sources (reanalysis, satellite, radar, and pluviometers) were used in this study, and different methods were applied. The National Center for Monitoring and Early Warning of Natural Disasters (CEMADEN) registered accumulated rainfall above 400 mm between 26 April and 5 May using 27 pluviometers located in the central-northern part of RS. The monthly volumes reached 667 mm and 803 mm, respectively, for April and May 2024, against a climatological average of 151 mm and 137 mm for these months. The maximum precipitation recorded was 300 mm in a single day on 30 April 2024. From a large-scale point of view, an anomalous heat source in the western Indian Ocean triggered a Rossby wave that contributed to a barotropic anticyclonic anomalous circulation over mid-southeastern Brazil. While the precipitant systems were inhibited over this region (the synoptic view), the anomalous stronger subtropical jet southward of the anticyclonic circulation caused uplift over RS State and, consequently, conditions leading to mesoscale convective system (MCS) development. In addition, the low-level jet east of the Andes transported warm and moist air to southern Brazil, which also interacted with two cold fronts that reached RS during the 10-day period, helping to establish the precipitation. Severe deep MCSs (with a cloud-top temperature lower than −80 °C) were responsible for a high lightning rate (above 10 flashes km−2 in 10 days) and accumulated precipitation (above 600 mm in 10 days), as observed by satellite measurements. This high volume of rainfall caused an increase in soil moisture, which exceeded a volume fraction of 0.55, making water infiltration into the soil difficult and, consequently, favoring flood occurrence.