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

The impact of climate change suggests that the environments favorable for severe thunderstorms development, in particular supercells will become more frequent across Europe.

Bulgaria is a country with a high frequency of thunderstorms during the warm season. Every year, the development of supercells is observed over its territory, and storm splitting process is not rare. This phenomenon can pose difficulties for the forecasters. For this reason, correct radar analysis and estimation of wind shear are of great importance to the forecasting and the nowcasting.

A case of a splitting supercell that developed on the 29-th of May 2022 is analysed. Radar data from S-band Doppler radars were used. 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 flow on the upper and near-ground level converged (from SW, following the circulation of the trough and from SE, following the movement of the Mediterranean cyclone respectively). This leads to strengthening of the vertical wind shear. This synoptic setup was favorable for severe convection development. Data for instability indices and wind hodograph were additionally used.

In the afternoon an isolated convective cell first appeared at 15:45 EET in the layer between 5th (-9.4°C) and 10th km (-42.4°C) with maximum radar reflectivity Zmax = 29 dBZ. The cell grew rapidly and after 20 minutes Zmax was 60 dBZ and the storm had 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. 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, and TBSS are presented. After splitting, the left-moving (LM) and the right-moving (RM) cells kept high values of radar characteristics, but RM had shorter life time and its intensity was lower.  Both cells produced multiple convective hazards along their paths. The hazards associated with the RM were severe hail (hailstone sizes greater than 2 cm in diameter), strong wind gusts, heavy rain and intense lighting activity. The storm severity and the presence of large hail stones were confirmed by maximum radar reflectivity, VIL (Vertical integrated liquid) and VILD (VIL density).