Hailstorms in Association with Cold-Core Lows in Brazil

Hailstorms are among the most frequent forms of severe deep convection observed in Brazil, especially, but not exclusively, in the southern and southeastern sections of the country. As in other parts of the world, these convective storms are often capable of inflicting significant damage to crops and residential roofing. Hence, a good anticipation by forecasters of the atmospheric conditions that favor their formation is desirable, particularly with regard to the contrast between these conditions and those that lead to heavy rainfall events. In this study, five episodes of cold core lows (CCLs) that produced hailstorms in distinct (subtropical and tropical) latitudes in Brazil are analyzed, as well as one case of CCL in which heavy rainfall and floods were observed without accompanying hail.

The 08-11 April 2011, 12-17 December 2011, and 04-06 August 2019 events represented subtropical cut-off CCLs that produced hailstorms close to their centers, or just downstream of their centers. The hailstorms from 21 December 2017 and 07 January 2019 in northeastern Brazil were more unusual, and occurred in association with tropical CCLs situated far inland and well dislocated from the coastal areas of northeastern Brazil where they are most often observed. These latter two events are particularly interesting given the very low latitude and non-mountainous topography of the regions where hail was reported. Finally, the 24-25 May 2019 was a faster-moving CCL that produced copious amount of rain in southern Brazil, but with no hail reports.

The main objective of this study is to highlight similarities and distinctions among the six events, with emphasis in the contrast of tropical versus subtropical environments, and in hail versus heavy rainfall cases. To that end, global reanalysis data from ERA-5 dataset will be employed for the analysis of a number of synoptic-scale attributes that help to characterize not only the nature and intensity of the CCLs but also the environments conducive to the hailstorms/heavy rainfall. These include: the general synoptic pattern, speed and direction of movement of the CCLs, temperature anomalies with the CCLs, location and intensity of the upper-level jets, the vertical vorticity distribution within the CCLs, height of the freezing level (i.e., depth of the warm cloud layers), as well as convective parameters extracted from the ERA5 vertical profiles, such as CAPE, CIN and lifted index for distinct air parcels (including CAPE in the -10°C to -30°C layer), lapse rates, vertical wind shear and storm-relative helicity computed for distinct layers, and the vertical moisture distribution. Whenever available, actual proximity soundings will also be analyzed. These parameters will be computed with the Sounding and Hodograph Analysis and Research Program in Python (SHARPpy; Blumberg et al., 2016). Geostationary satellite imagery will be used to characterize depth, duration and location (with respect to the CCL center) of the convective activity for each case.

Finally, results from this study will be compared to the general conditions associated with severe weather occurences in CCL situations in North America.

Blumberg, W. G., K. T. Halbert, T. A. Supinie, P. T. Marsh, R. L. Thompson, and J. A. Hart, 2016: SHARPpy: an open source sounding analysis toolkit for the atmospheric sciences, Bull. Amer. Meteor. Soc., p.1625-1636.