Cyclic supercell Storm in Bulgaria observed on 13 June 2024: thermodynamic conditions, evolution and structure

In 2024, several supercell thunderstorms were registered in Bulgaria between April and June. A long-lived isolated supercell developed on 13 June 2024. This supercell persisted for over 7 hours, traveling more than 350 km and producing hail larger than 2 cm in diameter, which caused significant damage.

A thermodynamic analysis was conducted for estimation of the severity of the thunderstorm development. The obtained high values of CAPE, strong 0–6 km wind shear, and a typical curved hodograph shape indicated the potential of development of supercells. 

To investigate the supercell’s evolution, radar data (updated every 4 minutes) and high-resolution visible (HRV) satellite imagery (at 5-minute intervals) were analyzed. Key radar parameters included maximum reflectivity (Zmax), the Vertically Integrated Liquid above the freezing level (dVIL), and VIL Density (VILD), defined as the ratio of VIL to the height of 15 dBZ contour. 

Four cycles of intensification were distinguished during the mature severe stage of the supercell development. Most of the typical radar signatures of powerful convective cells, such as V-shaped radio echo, comma shape, Bounded Weak Echo Regions (BWER), Three-Body Scatter Spikes (TBSS), and Side lobe, were registered.

During the evolution of the supercell:
-    V-shaped radar echo was registered several times, followed by a significant deviation in the storm motion of up to 60–65 degrees from the mean wind flow.
-    There is a pronounced relationship between the storm's cyclic behavior and variations in dVIL and VILD.
-    The intensification of the cycles was associated with the new formation of feeder cells and their subsequent merging with the supercell.
-    Maximum radar reflectivity exceeded 65 dBZ for more than 2 hours, with peaks reaching above 70 dBZ during the severe stage of the supercell.