Integrated damage, visual, remote sensing, and environmental analysis of a strong tornado in southern Brazil

Abstract

On 20 April 2015 a strong tornado struck the city of Xanxerê located in western Santa Catarina (SC) state, in southern Brazil. The tornado destroyed several buildings, causing almost 100 injuries, four fatalities, and R$ 104 million in damage (approximately USD$ 34.6 million according to 2015 dollar quotation), making it one of the most damaging tornadoes in Brazil's history. This study provides an integrated damage, visual, remote sensing, and environmental analysis of the tornado event. Medium-resolution satellite imagery and in situ photographs of the damage show that the tornado caused high-end F2 damage on the Fujita scale across a 4.9-km-long path in the northern portion of Xanxerê. Videos of the tornado and its parent storm reveal the existence of a low-level mesocyclone and rear-flank downdraft, indicating that the storm was a supercell. Horizontal vortices, which are occasionally observed in strong tornadoes but rarely documented in Brazilian tornadoes, were also observed in the videos. The supercell developed in a warm and moist air mass characterized by moderate buoyancy and weak mid-level lapse rates. The mid- and upper-tropospheric forcing for synoptic-scale ascent was weak but the prevailing zonal flow was moderately strong, providing sufficient deep-layer shear for supercell development. A key component of the synoptic-scale environment was the presence of a northerly low-level jet stream that promoted warm moist advection into western SC in addition to increasing the low-level vertical wind shear in the region. On the mesoscale, the presence of a broad cloud shield from a decaying mesoscale convective system (MCS) preceding the supercell formation hampered daytime radiative heating over western SC resulting in low dewpoint depression and, thus, lower lifting condensation levels around Xanxerê. Remote sensing and surface data suggest that the MCS-induced outflow boundary may have been responsible for the initiation of the supercell. The complex subtropical environment of the Xanxerê tornado differs in many aspects from well-documented North American tornadic environments and highlights the need for more studies addressing distinctions between the atmospheric conditions that are conducive to tornadic activity in the two continents.