The São Borja Downburst Observed During RELAMPAGO: Evidences and a New Conceptual Model

During the RELAMPAGO campaign, a downburst was observed at São Borja, a Brazilian city close to Argentina. A mesoscale convective system, formed over the south of Paraguay during the night of the 27 November 2018, split in two supercells. Its anticyclonic circulation moved toward the southeast reaching São Borja, the downburst occurred at around 1400 UTC. The measured Vertically Integrated Liquid (VIL) changed from 235 kg/m2 to 105 kg/m2 in 10 minutes. This 130 kg/m2 of water corresponds to around 1.5 million tons of water falling in 10 minutes over 2000 km radius area. Data from the X band Dual Pol radar describes the processes leading to the downburst, which are associated with the formation of a mesocyclone and its collapse in few minutes. Based on these measurements, a new conceptual model is presented. The mesocyclone is located at 6000 m height, just above the maximum vertical velocity, the Bounded Weak Echo Region (BWER), and captured hydrometeors in its circulation. This explains the highest reflectivity observed there. In this circulation, hail can grow because constantly fed by the strong updrafts which bring air mixed with supercooled liquid water and ice as shown by the lower polar cross correlation inside the BWER. This conceptual model can also explain how large hydrometeors can grow, because staying for longer time confined to the mesocyclone circulation. Hydrometeors are caught in the rotation and sustained by the meso-region of upward field. The region where the downdraft begins inhibits the upward motion. Ice evaporates and downdraft increases. The downdraft in this part of the mesocyclone creates convergence of ice that stimulates aggregation, increasing hail size, and thus precipitating. Consequently, precipitation reaches the lower level and overcomes the surface convergence reducing the updraft. The BWER is the updraft pillar of the mesocyclone: if it collapses or moves away from the mesocyclone, the mesocyclone collapses reaching the surface. This hypothesis was tested using the MESO-NH model. The presentation will describe the observations and some preliminary results from the simulations. The RELAMPAGO campaign in São Borja was supported by FAPESP grant 2015/14497-0.