The 19 November 2015 Tornado in Southern Brazil: Dual-Pol Radar Data Analysis of the Parent Supercell and Videographic Documentation of the Complex Near-Surface Circulation

An investigation is presented of the evolution of a supercell storm that produced a significant tornado that struck the town of Marechal Candido Rondon located in western Paraná state (PR; southern Brazil) in the afternoon of 19 November 2015. Emphasis is placed on the storm evolution based on the data analysis of a dual-polarization S-band Doppler radar located in the town of Cascavel/PR, operated by Instituto Tecnológico SIMEPAR, that sampled the entire life-cyle of the supercell within a range of less than 150 km (less than 100 km during the tornadic stage). Footage of this event also is analyzed in tandem with the radar data in order to document the complex circulation of the low-level mesocyclone and the large tornado.

Initial convective storms formed near the Brazil-Paraguay border in a very moist environment around 1630 UTC (14:30 local daylight-saving time (LDT)), with one of the cells experiencing rapid development in both intensity and size after merging with smaller cells. This storm gradually acquired supercell characteristics as it deviated to the left of the 0-6 km mean wind and displayed a clockwise velocity couplet in the radial velocity field. At its mature stage the storm displayed typical characteristics of midlatitude tornadic supercells in the radar scans, including a well defined hook-echo collocated with a cyclonic velocity couplet at lower scans, and a mid-level bounded weak echo region (BWER). Around 1815 UTC (16:15 PM LDT), while approaching the town of Marechal Candido Rondon, the storm spawned a tornado. As the strong tornado moved over the southern sections of the town a tornadic debris signature (TDS) was identified in the polarimetric data through the combined occurrence of high reflectivity (exceeding 40 dBz) and low correlation coefficient (< 0.8) collocated with the cyclonic velocity couplet. The authors believe that this is the first documentation of a TDS in South America.

As the tornado tracked across the urban portions of Marechal Candido Rondon, it was recorded in several videos by local residents. In a couple of these videos the low-level circulation of the storm displayed a highly complex multiple-vortex structure, with the entire low-level mesocyclone being very close to the ground and exhibiting a behavior that closely resembled what is called a multiple-vortex mesocyclone (MVMC; Wurman and Kosiba, 2013). With time this circulation evolved into a large single-cell tornado.

The near-storm environment is assessed based on surface observations from the networks of automated weather stations (AWSs) belonging to Brazil´s National Meteorological Institute (INMET, in Portuguese) and to Instituto Tecnológico SIMEPAR, combined with soundings from Foz do Iguaçu upper air station (SBFI). Synoptic charts describing the prevailing large scale conditions are also produced utilizing data from the NCEP Climate Forecast System Version 2 (CFSv2). Overall, the results indicate that the supercell developed under a very moist environment with strong conditional instability. Also, given the combination of a northwesterly low-level jet and northeasterly surface winds, moderate speed and directional vertical wind shear was in place over western PR around the time of the event.

Reference:

Wurman, J., and Kosiba, K. 2013: Finescale radar observations of tornado and mesocyclone structures. Weather and Forecasting, v. 28, 1157-1174, doi: http://dx.doi.org/10.1175/WAF-D-12-00127.1