Analysis of two derecho events in Southern Brazil.

Two long-lived bow echoes that produced extensive swaths of damaging winds in extreme southern Brazil are investigated; the first event occurred on 29 May 2013 (austral fall) and the second one on 18–19 October 2014 (austral spring). Three main aspects are addressed: analysis of the atmospheric environments in which the convective systems developed; documentation of the main morphological features accompanying the bow echoes; verification of whether the severe weather events satisfied the definition of a derecho. The evolution of the bow echoes is described utilizing data from an S-band Doppler weather radar from the Brazilian Air Force. Hourly wind gusts from surface automated weather stations operated by Brazil´s National Institute of Meteorology combined with damage reports obtained from the Civil Defense System of Rio Grande do Sul state and media clipping are employed to characterize the progression, intensity and extension of the damaging winds. Both events exhibited typical characteristics of derecho-producing bow echoes, including intense rear-inflow jets, rear-inflow notches in the reflectivity field trailing the bowing segments, and mesoscale convective vortices in the polar flank of the convective systems. One of the convective systems also developed the bow-and-arrow convective structure. The lack of an extensive post-storm damage survey precluded a more rigorous analysis of the concentration of damage reports. However, the chronological progression of the severe wind gusts and damage reports followed the path of the bow echoes for more than 400 km, and it is concluded that most likely both events represented the manifestation of derechos. The large-scale environment is analyzed with Climate Forecast System version-2 gridded data, whereas the near-storm environments are assessed with the aid of proximity soundings. [ABSTRACT FROM AUTHOR]