A 12-year climatology of severe weather parameters and associated synoptic patterns for subtropical South America

This study investigates atmospheric profiles from subtropical South America with the goal of documenting the local climatology of meteorological conditions conducive to severe thunderstorms. Following an ingredients-based approach, several convective parameters are computed from thermodynamic and kinematic profiles obtained from 00Z and 12Z soundings from southern Brazil and northeastern Argentina spanning the 12-year period from 1998 to 2009. In addition, data from the NCAR-NCEP Reanalysis Project for the same period are included in the analysis to augment the sample size by adding Reanalysis-derived profiles valid at 18Z --- more representative of mid-afternoon hours in that part of the world. Six upper-air sounding sites were chosen for this study: Foz do Iguassu, Curitiba, Porto Alegre and Florianopolis in southern Brazil, and Ezeiza (Buenos Aires) and Resistencia in northeastern Argentina.

For each location, the annual and monthly distributions of convective parameters that diagnose the main ingredients for severe convection --- moisture availability, conditional instability and vertical wind shear at distinct layers --- are evaluated with the aid of scatterplots (for the analysis of relevant parameter spaces), boxplots (for medians, 10%, 25%, 75% and 90% percentiles of the parameters) and other basic statistics. The large-scale atmospheric patterns observed during days when the profiles displayed extreme values of convective parameters (namely, at and above their corresponding 90% percentiles) are studied in order to assess the main synoptic patterns that lead to such extreme conditions and to compare with the well-documented counterparts in North America.

From the total number of sounding-derived profiles (approximately 20000) around 1% was flagged as clearly conducive to severe weather. It is argued that a higher percentage may be found after completing the assessment of the Reanalysis-derived profiles valid at 18Z. Strong seasonality was found for some of the parameters such as: surface-based and 100hPa mean-layer CAPE, which, as expected, highlighted conditions of enhanced instability during the warm season; LCL heights, which displayed lower values during the winter; stronger 0-6km bulk shear during the cold season. In turn, the mid-level lapse rates and the 0-1km bulk shear displayed weaker seasonality. The LCL heights were found to be lower than those typically observed on North America, while the magnitude of the shear parameters are comparable to the North American counterparts. Similarly to what is found in the US South Central Plains, Spring and Fall are the seasons when the combination of moderate-to-high conditional instability and vertical wind shear are more frequently observed. The synoptic patterns conducive to severe weather most often display a northerly low-level jet under a migratory baroclinic system, but the location where severe convection is most favored differs somewhat from what is observed in North America.