A study of rotation in thunderstorms in a weakly- or moderately-sheared environment

This study investigates two cases of thunderstorms with rotating characteristics, which occurred in Hungary and formed in an environment of relatively low or moderate wind shear (well below 20 m/s) in the lowest 6 km layer of the troposphere. For the selected cases, the properties of the thunderstorms and their environment were examined both from observational and modeling perspectives. The observed storms showed both multicellular and supercellular features (e.g. multiple, fast developing maxima of radar reflectivity but also presence of Bounded Weak Echo Reflectivity or couplets of Doppler radar velocity extremes). Cloud-base rotation was observed by the Hungarian storm-chasers. Cloud-resolving and real-data numerical simulations with the WRF model produced generation of meso-γ-scale vortices in both mid- and low-tropospheric levels connected to convective storms. The simulated low-level vortices exhibited quasi-permanent behavior and their intensity seemed to be comparable to supercell mesocyclones. Vorticity equation terms were analyzed on the model fields in order to explain the origin of the rotation and its relation to the environmental wind shear and wind profile. The results indicated that the more transient midlevel vortices were generated via the tilting mechanism, whereas the evolution of the quasi-persistent low-level vortices was initiated by a relatively small tilting along a gust front, then they subsequently rapidly intensified by the stretching of the vertical vorticity. The storms analyzed in the model field exhibited a hybrid behavior, since the structure and evolution of the vorticity field resembled supercellular mesocyclones and mesovortices as well.

► We investigated two cases of rotating thunderstorms in weakly sheared environment.
► The storms were characterized by permanent low-level vortices.
► The low-level vortices were initiated by a small amount of tilting.
► The longevity of vortices was due to strong stretching effects along a boundary.
► Gust fronts played the main role in generating low-level permanent vortices.