Convectively-Induced Severe Wind Gusts in Southern Brazil: Surface Observations, Atmospheric Environment, and Association with Distinct Convective Modes

This study investigates the occurrence of severe wind gusts (SWG; gust speeds equal to or above 25 m/s) detected by surface hourly data from the network of 94 automated weather stations (AWS) operated by Brazil´s National Meteorological Institute (INMET, in portuguese) located in the southernmost section of the country. Only those events unequivocally associated with deep convective storms, as characterized by the analysis of thermal-infrared geostationary satellite imagery, were selected. Hourly time-series of surface pressure, air temperature and dew points around the time of the SWGs were also investigated in order to assess the capability of INMET´s AWS synoptic network in detecting convectively-induced surface features that are closely associated with gust fronts, such as surface cold pools and mesohighs.

A total of 266 SWGs generated by local deep convection were identified from INMET data for the period from January 2005 to December 2015. Results show that the highest frequency of SWGs in southern Brazil is in the Spring an Summer months, which is consistent with the known climatology of convective storms in the region. Most SWGs were detected from the late afternoon to the overnight hours. Fewer events were identified during the morning and early afternoon. Overall, the western portion of Brazil's south displayed the largest frequency of SWGs in our sample. This result suggests two main things: first, nocturnal mesoscale convective systems (MCSs) that commonly form over central-northeastern Argentina and cross the border with Brazil may be the source for many of such SWG events; second, a role played by the synoptic-scale northern Argentina (inverted) trough that often extends into western sections of southern Brazil (with an attending northwesterly low-level jet stream) in setting up the environment for the development of severe thunderstorms.

Further analysis of the hourly time series showed, for all SWG events, a jump [drop] in surface pressure [air temperature] within the same hour of the SWG detection. Such variations in temperature and pressure were sharper than the synoptic-scale evolution of these variables for the hours preceding and following the SWG, being consistent with the conceptual model of a cold pool and mesohigh following the convective gust. The median value of the pressure jump was +3.3 hPa, with highest value reaching +7.8 hPa, while for the decrease in air temperature a median value of -5.2 K was found, with the strongest drop reaching -12.7 K. A drop in dew point temperatures was also present in all cases. No significant correlation between gust speed and the corresponding 1-hr pressure/temperature variations was found.

For only four cases out of the 266 SWG events were there available upper-air observations in the vicinities of the AWS that met all the criteria for a proximity sounding. As expected, the vertical profiles from these soundings revealed a conditionally unstable environment, with surface-based [most unstable] CAPE ranging from 130 J/kg to 1791 J/kg [1515 to 2365 J/kg], and downdraft CAPE (DCAPE) ranging from 600 J/kg to 1685 J/kg. Three of the proximity soundings displayed differences greater than 20 K between the surface equivalent potential temperature (theta-e) and the lowest theta-e aloft. The sounding with the lowest theta-e difference, however, was the one with strongest 0-6 km bulk vertical wind shear (32 m/s) and also an intense 0-3km bulk shear (22 m/s), suggesting a more prominent role played by vertical momentum transport in generating the SWG. Conversely, the profile that displayed the weakest bulk shear (for both 0-3km and 0-6km layers) displayed a strong theta-e difference (25K), suggesting that the thermodynamic forcing may have prevailed in generating strong downdrafts in that case.

Among the 266 SWG events, base reflectivity fields from nearby S-band weather radars were available for 30 episodes. These were utilized to identify the (main) convective mode of the storms that produced the SWGs, being classified as individual cells, multicell clusters, linear MCSs (i.e., squall lines/bow echoes) and nonlinear MCSs. One event was classified as individual cell, while seven were classified as multicell cluster, three as nonlinear MCS, and nineteen as linear MCS. This preliminary result indicates that a large number of the SWGs over southern Brazil are generated by squall lines (including bow echoes).