Discriminando condições favoráveis a diferentes modos de tempo severo no leste da Bacia do Prata.

Abstract

This study investigates the mean synoptic conditions observed during situations of convectve storms that produce phenomena of high social and economic impact in southsoutheast Brazil (SSB) and the neighboring areas of Argentina and Uruguay. Emphasis was placed on the evaluation of possible discrimination between the synoptic conditions favorable to tornadoes (TOR) and those conducive to heavy rain (CHU) and non-tornadic convectively-induced damaging winds (RAJ). Episodes of these three forms of severe convective weather observed between 1991 and 2018 in SSB and neighboring regions were selected for study, totaling 40 events for each category. The events were purposedly selected in such a way that all samples share same similar seasonal and spatial distribution within the area of study. The meteorological analysis is performed by contrasting the mean synoptic composites for each category, and by comparing the respective percentiles of convective parameters that are often employed in severe weather analysis and forecasting. In addition, an analysis of the most frequent synoptic patterns prevailing in each class is conducted. The data source for the meterological analysis is the Climate Forecast System Reanalysis and Climate Forecast System version 2 (CFSR-CFSv2), from which data were extracted for the time closest to the severe weather occurrence as well as for the 24 hours preceding the episodes. Data are sampled for an adaptive domain of 30° latitude and 30° longitude always centered at the location of the weather event. Mean synoptic composites for this domain were produced for the three categories, and convective parameters were computed from the tropospheric profile corresponding to the central grid point. The synoptic composites indicated subtle differences between the three severe weather categories, but all classes confirm the important role played by a low pressure system that extends from the northeast of Argentina/Paraguay into southern Brazil in conditioning the synoptic environment that favors severe convective weather. There was a tendency for RAJ events to occur, in average, under environments that were more baroclinic than the TOR and CHU counterparts. Overall, the convective parameters showed little discrimination between the TOR and RAJ categories, but the storm-relative helicity and bulk wind difference computed for the effective inflow layer provided good discrimination between these classes. The indices Supercell Composite Parameter and Significant Tornado Parameter also displayed good performance in discriminating the categories of severe weather, especially when comparing the TOR and CHU samples. By providing a conceptual model of the synoptic conditions that prevail in TOR situations and the statistics of convective parameters in severe weather events, the results of this work can be applied in the forecast of severe storms in the eastern part of the La Plata Basin.