A Left-Moving, Cyclonic Supercell in Southern Brazil: Radar Data Analysis and Atmospheric Environment

The storm-scale evolution of supercell thunderstorms in southern Brazil is still poorly documented, partially because of the few number of events of supercells well sampled by the operational radar network in that part of the world. This study aims at investigating the life-cycle of a left-moving supercell thunderstorm (i.e., a cyclonic cell in the Southern Hemisphere) that developed in the evening and night hours of 6 October 2015 in the state of Rio Grande do Sul (RS) in extreme southern Brazil. Emphasis is placed on the analysis of the storm evolution as characterized by Doppler radar data, including complex interactions among neighboring cells in the early stages of development of the storm. During most of its life-cycle the supercell evolution was sampled within a range of less than 60 km from Santiago´s (SNTG) single polarization S-band Doppler radar situated in mid-western RS and operated by the Brazilian Air Force.

The early development of the storm was characterized by a rapid intensification in the reflectivity field observed shortly after the merging of two (originally distinct) convective cells, in a fashion that resembled the conceptual model of updraft collision proposed by Hastings and Richardson (2016). Reports of very large hail and damaging winds accompanied the resulting discrete left-moving cell that rapidly displayed supercell characteristics as it continued in its trajectory, first to the east and, later, to the east-northeast (i.e., further deviating to the left of the prevailing northwesterly 0-6 km mean wind). Despite no visual confirmation, a tornado occurrence could not be ruled out given the storm-scale structures displayed by the cell in the radar scans during its mature stage. These included a well defined hook-echo collocated with a cyclonic velocity couplet at lower scans, and a rear-flank downdraft (RFD) characterized in the base reflectivity also in the lower scans. In addition, the mature storm displayed a mid-level bounded weak echo region (BWER) and a V-shaped structure in the reflectivity field. The authors believe that the scientific documentation of all these features, simultaneously, for the same storm is unprecedented for a supercell in Brazil.

As the storm continued to move east-northeastwards, the hook echo was gradually displaced to the forward region of the storm and a new hook-like appendage formed on the supercell's rear flank although no clear-cut velocity couplet was evident with this new feature. Interestingly, after approximately 3-hr of evolution, the storm displayed a rapid decay, which was also confirmed by GOES-13 thermal infrared imagery and by the quick reduction in the rate of lightning flashes as detected by Brazil´s Integrated Network of Lightning Detection (RINDAT; acronym in Portuguese).

The synoptic-scale environment in which the storm developed was assessed based on two available soundings (00Z of 07 October 2015 [9PM LST 10/06/2015], from Uruguaiana (SBUG) and Santa Maria (SBSM)), surface observations from INMET´s (National Meteorological Institute, in Portuguese) network of automated weather stations (AWSs), and on map analysis using NCEP Climate Forecast System Version 2 (CFSv2) data. The prevailing environment on 6 October 2015 featured an approaching 500-hPa short wave trough over central-northern Argentina, with RS state under strong upper-level flow. Geopotential height falls in the lower troposphere were observed over northeastern Argentina, leading to the deepening of a NW-SE-oriented (inverted) trough at the surface, which extended towards the RS-Uruguay border.

A northwesterly low-level jet stream (LLJS) developed in response to the tightening in the horizontal gradient of geopotential heights in the 850-700hPa layer. The LLJS was responsible for promoting the advection of warm and moist air from the lower latitudes into southern Brazil. Surface AWSs in RS reported temperatures and dew points as high as 30 °C and 20 °C, respectively, around the time of the storm development. SBSM sounding indicated that the air mass was capped by cold, dry air aloft, resulting in high values of most unstable [surface-based] CAPE of 2913 J kg^-1 [1571 J kg^-1]. The capping inversion prevented widespread storm initiation, with only a few storms developing across western RS, just to the north of the surface (inverted) trough, in the warm sector. Given the presence of upper- and low-level jet streams, strong speed and directional vertical wind shear was in place over central-western RS, as confirmed by a highly curved hodograph in the SBSM sounding, with 0-6 km [0-1 km] bulk shear reaching 20 m s^-1 [11 m s^-1], accompanied by 0-3 km [0-1 km] storm relative helicity of -423 m² s^-2 [-230 m² s^-2]. The general environment in which the cell developed displayed the same (well documented) atmospheric ingredients considered necessary for supercell storms in North America. However, this event exemplified a South American synoptic-scale setting that, albeit leading to a similar combination of ingredients that favor supercells, differs from the classic North-American synoptic-scale pattern.

Reference:

Hastings, R., and Richardson, Y., 2016: Long-Term Morphological Changes in Simulated Supercells Following Mergers with Nascent Supercells in Directionally Varying Shear. Mon. Wea. Rev., v.144 , 471-499, doi: http://dx.doi.org/10.1175/MWR-D-15-0193.1.