Unraveling the genesis of von Kármán vortices behind storm supercell by numerical simulations

The impact of severe atmospheric events on human society and activities is becoming a primary issue, considering that the actual trend of climate change is expected to increase the frequency and intensity of such events. Despite exceptional advancements that have been done in the Numerical Weather Prediction field in the last decades, there is still a lack of knowledge on features of atmospheric phenomena at the lower and less energetic scales. The present contribution focuses on the genesis mechanism and evolution of periodic secondary vortices, of the von Karman wake type, that arise downwind of thunderstorm supercells. These structures develop at intermediate altitudes, are often less energetic and have a shorter lifetime than the principal supercell. Therefore, they are hardly captured by meteorological observations, but they can play an important role in transporting kinematic and thermal qualities, also anticipating the impact of the supercell itself. 

To address this topic, a real case has been studied and numerically reproduced: the thunderstorm supercell generated on 5th September 2015 over the Gulf of Naples (Italy), which was of exceptional intensity for the Mediterranean area. The analysis of multi-platform data (including data from ERA5, ground and space-borne radar, and local measurements) enabled the identification of the secondary vortices of interest and to derive the geometric, thermal and kinematic characteristics of the system. A simplified model of the supercell was then designed and used to set up a Large-Eddy Simulation, based on computational fluid dynamics techniques, to directly solve the physics of most large and energetic scales of motion. 

The numerical experiment reproduced the fundamental structure of the supercell and its well-known features, including interactions with the tropopause (e.g. overshooting top, anvil, hydraulic jump, gravity waves). The wake of secondary vortices downwind of the main body of the supercell was reproduced and analysed, and the mechanism of generation of these turbulent structures was described.  

To the best of the authors' knowledge, this is the first contribution integrating direct observations and Large-Eddy Simulation numerical simulations to analyse secondary vortex trails behind storm supercells.