Supercells and bow echoes in Rio Grande do Sul state, southern Brazil: selected case studies

Among the modes of local convective storms, supercells and bow echoes outstand for being prolific producers of severe weather phenomena such as large hail, tornadoes, and damaging straight-line winds. The atmospheric ingredients necessary for the development of supercells and bow echoes are related to fundamental concepts of Fluid Mechanics that are valid universally (Bluestein, 2007). Nevertheless, the atmospheric processes responsible for gathering and modulating such ingredients may show some regional idiosyncrasies that are relevant for severe weather forecasting in different regions around the world.

This study is part of a larger initiative aimed at describing the atmospheric environment and storm evolution of local convective systems that produce severe weather in subtropical South America. Emphasis is placed on highlighting similarities and distinctions between the South American severe weather environment and the well documented North-American counterpart. To that end, a multi-scale analysis is conducted of selected episodes of supercells and bow echoes observed from 2009 to 2014 in Rio Grande do Sul state, extreme southern Brazil. For this study only situations for which radar data are available were chosen, which are: 07 September 2009 (severe weather outbreak; bow echo and supercells); 18 September 2012 (supercells); 29 May 2013 (bow echo; a likely derecho event); 10 November 2013 (supercells); 16 March 2014 (supercells); 03 July 2014 (supercells).

Sources for meteorological data -- other than radar scans from the Brazilian Military Weather Service -- include proximity soundings from Santa Maria (SBSM) and Porto Alegre (SBPA), hourly observations from the AWS network operated by Brazilxs National Meteorological Institute and METAR reports, visible, infrared and water vapor imagery from GOES satellite, and numerical analysis from the GFS-NCEP model. Damage reports obtained from Rio Grande do Sulxs Civil Defense System and media clipping are also included when available.

The two episodes of bow echoes were observed in the morning hours; both were pre-frontal systems displaying strong west to east propagation. Based on the areal coverage of wind gust and damage reports, the 29 May 2013 event marginally satisfied the definition of a derecho. This same storm system also displayed a cyclonic MCV in the southern flank of the convective line. The supercell storms occurred from mid-afternoon to late evening hours. Large hailstones or significant hail accumulation, in addition to damage caused by winds, were reported for all supercells. A tornado was visually confirmed in one of the events. The six convective weather events occurred under moderate or strong northwesterly flow at 850hPa, rich in moisture. In all cases the strengthening of the 850hPa flow was related to low-level height falls associated with a low pressure system over northern Argentina that produced an inverted trough over Rio Grande do Sul state. In some cases the inverted trough interacted with a developing extratropical cyclone (EC) located further to the south, just off the Argentinean or Uruguayan coast, producing a deformation field that favored frontogenesis. However, in three events (all of them involving supercells, but not bow echoes), severe convection developed without a deepening or mature EC. In contrast, all cases occurred with the northern Argentina low pressure system in place, which in some cases (especially for the supercell events) induced northeasterly flow at the surface that enhanced the low level directional vertical wind shear. This general pattern differs from the classic conceptual model for the synoptic configuration favorable to severe weather in the Central Plains of North America under strong forcing.

Additional findings will be included in the extended version of the Abstract.

Bluestein, H., 2007: Advances in applications of the physics of fluids to severe weather systems. Rep. Prog. Phys, 70, 1259-1323.