An Observational Analysis of Two Derecho Events in Southern Brazil

Bow echoes are capable of producing extensive swaths of damaging winds that, in extreme situations, characterize a derecho event. While quite well documented in North America (and, more recently, in Europe), studies addressing derecho occurrences in Brazil are still scarce, even though the southern section of the country is well known for
hosting severe thunderstorms and tornadoes. This study describes two derecho events in the state of Rio Grande do Sul (RS), in extreme southern Brazil, observed during the (austral) Fall and Spring seasons.

The first event developed during overnight and early morning hours of 29 May 2013 with deep convection initiating over far northeastern Argentina and displaying fast upscale growth, while developing a large (single) bowing segment after crossing the brazilian border into RS state. Analysis of S-band Doppler radar data from Santiago (SNTG), located in mid-western RS, indicated that this fast-moving storm system developed all characteristics usually associated with derecho-producing bow echoes, such as a rear-inflow jet (RIJ) (with radial velocities reaching 40 m/s at approximately 2 km AGL), a rear-inflow notch (RIN) in the reflectivity field just behind the convectively active portion of the bowing segment, and a mesoscale convective vortex (MCV) on the southern flank of the convective system. Damage as strong as F2 was reported in a progressive way along the storm´s path associated with strong surface winds that, at least on two locations, were clocked above 26 m/s by surface automated weather stations (AWS) from Brazil´s National Meteorological Institute. A continuous path more than 500 km long with a west-east orientation was characterized across RS state with this event.

The overall evolution of the second episode, that developed from the late night hours of 18 October 2014 to the early morning hours of the following day, was fairly similar to the 29 May 2013 case, with convective initiation over far northeastern Argentina and full bow echo development occurring further downstream, over RS. This second event, however, was stronger and longer-lived. Data from SNTG radar indicated that the bow echo displayed a descending RIJ with radial velocities approaching 50 m/s around 1,5 km AGL, and a line-end MCV that was very well defined in both reflectivity and radial velocity fields. Behind the trailing stratiform precipitation sector of the MCS, zonally-aligned convective
cells trailed the system, characterizing a bow-and-arrow mesoscale convective structure in the reflectivity field. At the surface, wind damage reports were more extensive than in the previous case, and a larger number of AWS reported wind gusts above 26 m/s. The damage reports and gust measurements followed closely the progression of the bow echo across RS state, characterizing a path of approximately 600 km from west to east.

Both events met the traditional derecho criteria established by Johns and Hirt (1987). If considering the updated and more stringent derecho criteria proposed by Corfidi et al. (2016), the derecho definition is clearly [marginally] met for the second [first] event. On both cases, the atmospheric profiles, as sampled by proximity soundings in RS, were very moist and displayed strong vertical wind shear (0-3km bulk shear of 22 m/s for case 1, and 17 m/s for case 2; 0-6km bulk shear of 32 m/s for case 1, and 25 m/s for case 2). The highest value of (most unstable) CAPE was found in the second case, reaching 2764 J/kg for the Uruguaiana proximity sounding, as computed by SHARPPy tool. A detailed analysis of the synoptic environment was also carried out, indicating that the 29 May 2013 event developed under moderately strong synoptic forcing, while the 19 October 2014 case evolved under a comparatively weaker forcing.