Assinaturas polarimétricas em tempestades supercelulares no Sul do Brasil

Abstract

Southeastern South America, which includes portions of southern, west-central and southeastern Brazil, is one of the world’s hot spots for severe convective storms. Among these storms, supercells, which are commonly responsible for the generation of large accumulations of or large hail, damaging wind gusts, and tornadoes, are not rarely observed in that part of the world. The installation of dual-polarization Doppler radars in the state of Santa Catarina (southern Brazil) in recent years, creates the opportunity for the analysis of the microphysical and dynamical features of supercells in Brazil through their polarimetric signatures. These include the ZDR arc, KDP foot, polarimetric hailfall area in the forward flank-downdraft, ZDR column, KDP column, and the ZDR and ρHV rings, which still are poorly documented storm features outside of northern hemisphere. This study represents a first documentation of the aforementioned polarimetric signatures in Brazil over a relatively large number of storms, by analyzing continuous volumetric data from 15 supercell thunderstorms that occurred over a domain covered by the Chapecó S-band dual-polarization Doppler radar, located in western Santa Catarina state, collected during a period of four years between 2017 and 2021. Most of these storms are associated with verified severe weather reports made available by the recently created PRETS severe weather database in Brazil. Additionally, an in-depth analysis is performed for two events in particular. The first case is described through a quantitative and qualitative analysis of the evolution of midlevel and base-level signatures of a prolific long-lasting hail producing supercell that traversed the entire radar surveillance domain, causing significant damage to farmsteads and urban areas. The second case refers to a convective cluster resulting from the merging of two tornadic supercells, that spawned a brief tornado. Results from the quantitative analysis of the first event agrees with previously observed features in the literature, such as the time-lagged correlation between increases in the ZDR column volume and the occurrence of heavy precipitation and hail at the surface. Moreover, a correlation has also been found between precipitation fallout and KDP column metrics, also pointing to the potential application of this signature in nowcasting heavy precipitation and hailfall. However, one period of hail occurrence was marked by the maintenance of the ZDR column, at the expense of the KDP column, something that has not been previously documented. Furthermore, analysis of the tornadic case points to the importance of the employment of polarimetric variables concomitantly with conventional radar moments for nowcasting the tornadic potential even in seemingly weaker convective cells, as well as confirming tornado occurrence.