Analysis of splitting supercell storm based on the Doppler weather radar data

Bulgaria is a country with a high frequency of thunderstorms during the warm season. Extreme convection, leading to torrential rain and large hail, and more specifically a supercell development was discussed in this case study. Radar data from S-band Doppler radars were used for the analysis. The full volume scan had a maximum range of 300 km and was completed in 4 minutes. All radar products shown throughout the study were generated by the Interactive Radar Information System (IRIS) Analysis of Vaisala.

On the 29-th of May 2022 over the territory of Bulgaria there was a 500-hpa trough with the center of the upper level low over the Scandinavian Peninsula. On the surface pressure chart there was a sequence of cold fronts. The upper level flow and the one near the ground had opposite directions – northern and predominantly southern respectively. This leads to strengthening of the vertical wind shear. This synoptic setup was favorable for severe convection development. Data for temperature stratification of the atmosphere, instability indices, wind hodograph, etc. were additionally used.

In the afternoon an isolated convective cell was registered in Southern Bulgaria, which had maximum radar reflectivity Zmax=23 dBZ at height of 6.8 km (-23°C) at 15:47 EET. The cell grew rapidly and after 20 minutes Zmax was 60 dBZ and the storm acquired characteristics of a supercell. A mesocyclonic vortex was visible on the Doppler radar. The cell acquired a V-shaped structure, and later on it split into two cells, both of which retained high radar reflectivity. The cells passed over the Stara planina mountain and the left-moving cell kept high values of radar characteristics for a long time. The right-moving cell also maintained high values of radar reflectivity, but had shorter life time and its intensity was lower.  Both cells produced multiple convective hazards along their paths. The hazards associated with the left-moving cell were severe hail (hailstone sizes greater than 2 cm in diameter), strong wind gusts, heavy rain and intense lighting activity.

Based on the radar data the presence of the typical supercellular radar signatures, such as weak echo region (WER), bounded weak echo region (BWER), hook echo, V-notch and TBSS are presented. The storm severity and the presence of large hail stones were confirmed by maximum radar reflectivity, VIL (Vertical integrated liquid) and VILD (VIL density).