Severe Storm Signatures Observed by Doppler Radar in the State of São Paulo, Brazil: From Synoptic Scale to Microburst

The structure of severe storms which occurred on three different days in the central area of the State of São Paulo, Brazil, are described, based on observations from the S-band Doppler radar in Bauru. On 14 May 1994, at least one long-lasting storm could be classified as a super-cell storm (radar reflectivities exceeding 60 dBZ for five hours), which was embedded in a frontal system extending from an Atlantic Ocean Low across the southern States of Brazil. An associated cut-off low over the State of São Paulo induced strong convergence at the surface which triggered and maintained this supercell storm. A similar frontal system, with a surface trough extending over the south-south-eastern States of Brazil on 17 October 1999, resulted in a line of precipitation over the State of São Paulo. On both days severe hail falls were reported from several individual storms and extensive damage to property and life was reported. Frontal systems are the more common mechanism for triggering severe thunderstorms in the south-eastern parts of Brazil. However, the synoptic situation for the third study, on 22 November 1999, can be described as post-frontal convection, induced by a strong Bolivian High, resulting in medium-sized, multicellular storms developing at random throughout the radar range from late morning till evening. Although most of the storms on this day did not exceed 50 dBZ with echo tops below 10 km, some were severe in terms of extremely strong gust fronts and short, but heavy downpours.

When the radar reflectivity structures were analyzed in conjunction with the radial velocity fields, one could identify signatures that can be related to the observed severity, viz. rotation (14 May 1994), intense convergence (17 October 1999) and strong divergence fields displaying typical microburst features coinciding with the collapse of the echo core (22 November 1999). This highlights the importance of Doppler radar observations, but the possibility for the development of extremely severe rain, hail or wind storms with the potential to cause destruction or even devastation, preferably needs to be already identified at the synoptic scale prior to the formation of radar echoes in order to become a useful tool for Nowcasting techniques.