A climatology of quasi‐linear convective systems and associated synoptic‐scale environments in southern Brazil

Abstract

This study presents a 9-year climatology and composite analysis of quasi-linear convective systems (QLCSs) in southern Brazil (SB). QLCSs are identified using radar imagery and defined as a 100-km long, 40-dBZ convective line that lasts for at least 1 hr. QLCS cases associated with at least three severe wind reports are classified as severe. Composites of the synoptic-scale environments and severe convective weather parameters are constructed for severe and non-severe cases of three synoptic patterns found as the most recurrent in QLCS cases. QLCSs are more frequent and more likely to be severe during spring, while very few cases occur during winter. Most cases occur during late night and morning, corroborating previous research of mesoscale convective systems in this area, but the percentages of severe cases are higher in late afternoon and early night. Faster QLCSs, particularly those with velocities greater than 50 km/hr, have higher probability of being severe in comparison to slower systems. The three synoptic patterns more often related to QLCS occurrence in SB are characterized by a mid-level trough upstream of SB (type 1), a mid-level trough west of the Andes (type 2) and predominance of zonal flow over SB (type 3). In general, there is a low-pressure system over northwestern Argentina extending a trough to SB, where low-level moisture flux convergence and warm advection occur. In severe cases, the sea-level pressure is lower in northwestern Argentina and there is greater low-level northwesterly flow over SB. The type 1 is the most frequent pattern when QLCSs are observed in SB and is also the type with higher percentage of severe cases. Convective available potential energy (CAPE) and downdraft CAPE are good discriminators between severe and nonsevere cases in type 1 and 2 environments, while the 1,000-500-hPa bulk wind difference is a better predictor in type 3 cases. K E Y W O R D Squasi-linear convective systems, severe thunderstorms, southern Brazil, synoptic climatology