Synoptic patterns associated with heavy rainfall events in the metropolitan region of Porto Alegre, Brazil

Abstract

This study presents a synoptic classification of heavy rainfall events (HRE) in the metropolitan region of Porto Alegre (MRPA) in northeastern Rio Grande do Sul (RS) State between 2000 and 2022. A total of 166 HRE were identified, characterized by daily accumulations exceeding 34 mm. The classification revealed four primary synoptic patterns associated with HRE in MRPA. Two of these patterns were linked to the movement of a typical cold front, while the other two were associated with cyclogenetic processes. In three of the patterns, two days before the occurrence of HRE in MRPA, anomalous northwesterly winds at the 850 hPa level were observed, influenced by the presence of the North-Western Argentinean Low (NAL) or Chaco Low (CHL). These winds intensified the advection of warm and moist air over a significant part of the RS State, favoring in increased rainfall in the MRPA. The four synoptic patterns showed a convective system beginning one day before the occurrence of HRE in MRPA, with the synoptic pattern associated with cyclogenesis in Uruguay exhibiting the most intense Mesoscale convective system (MCS) around the MRPA. Finally, the synoptic pattern associated with cyclogenesis in the eastern part of Santa Catarina (SC) State featured anomalous easterly winds over the eastern RS State since Day -1. This pattern facilitated increased surface mass convergence, leading to the intensification of rainfall in the MRPA.