GOES-16 Mesoscale Rapid Scan in Southeastern Brazil: A Hailstorm Case Study

During the last days of November 2017, the GOES-16 mesoscale rapid scan was focused on the SOS-Chuva field campaign area in Southeastern Brazil. In the afternoon of 29 November 2017, severe thunderstorms were responsible for multiple reports of hail in the region. The simultaneous occurrence of the SOS-Chuva field campaign and the GOES-16 rapid scans gives an unprecedented opportunity to study these severe thunderstorms in this part of the world and evaluate how the rapid scans can improve the satellite-based nowcasting techniques. Data from the BrasilDAT lightning detection network and the C-band polarimetric radar from CEMIG were also used in the analysis.

The storms formed under relatively strong (15–20 m s^–1) southwesterly flow at midlevels in the northwestern periphery of a midlevel trough, which promoted deep-layer shear favorable for severe storms. Convective initiation occurred after 1600 UTC (1300 local time) due to the increase in thermodynamic instability associated with surface radiative heating and cold advection at midlevels, with CAPE values in the 1200–1400 J kg^–1 range at this time. Several approaches were tested in the 1 minute time step to identify the severity and nowcasting the Hail event as: area expansion, bi and tri-spectral channel combination, cloud top trend and high resolution winds. In this study, the focus is on two storms responsible for severe weather reports. The first storm caused large hail (4–5 cm in diameter) and widespread damage to roofs and vehicles in the city of Caeté around 1730 UTC. The first overshooting top colder than 230 K (10.35-μm brightness temperature) appeared after 1640 UTC associated with lightning activity of 3–5 lightning strikes per minutes. With the GOES-16 1-minute resolution imagery, it is possible to note several different overshooting tops forming consecutively in the area of more intense convection. The storm maintained the characteristics until nearly 1710 UTC, when a new overshooting top with temperatures below 215K formed and lightning activity increased to nearly 8-10 strikes per minute. This increase in convective activity occurs nearly 20 minutes before the hail occurrence. The radar reflectivity pattern depicts a similar trend, with increase in reflectivity (to more than 65 dBZ) in the upper levels of the storm before hail occurrence and decrease in reflectivity after that, denoting the reduction of hail amounts within the storm. The second storm formed north of the first storm and caused high winds and small hail in Ribeirão das Neves at nearly 1840 UTC. Both radar and overshooting top location indicated a storm motion to the left of the mean wind, which is associated with the presence of a rotating updraft. Brightness temperatures in the overshooting top decreased from ~240 to ~210 K in nearly 10 minutes, which indicates a very intense updraft. Such a rapid decrease in cloud top brightness temperature would be impossible to monitor without the rapid scan data. The analysis of these cases evidence the critical role the rapid scan can play on nowcasting of severe storms. There are clear advantages of using 1-minute data against the usual 15-minute data for rapidly-evolving severe storms, mainly when calculating trends in cloud-top characteristics such as brightness temperature and differences between channels.