A study of rotation in thunderstorms in a weakly- or moderately-sheared environment Kálmán Csirmaz a, ⁎, André Simon a , Georg Pistotnik b , Zoltán Polyánszky a , Michal Neštiak c , Zsolt Nagykovácsi d , Alois Sokol e a Hungarian Meteorological Service, Kitaibel Pál u. 1, H-1024 Budapest, Hungary b European Severe Storms Laboratory, Münchner Straße 20, D - 82234 Weßling, Germany c Slovak Hydrometeorological Institute, Jeséniova 17, 833 15 Bratislava, Slovakia d Hungarian Association of Stormchasers and Storm Damage Surveyors, Babits Mihály u. 1, 2483 Gárdony, Hungary e Comenius University, Faculty of Mathematics, Physics and Informatics, Mlynská dolina, 842 48 Bratislava 4, Slovakia article info abstract Article history: Received 13 January 2012 Received in revised form 28 July 2012 Accepted 19 September 2012 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. © 2012 Elsevier B.V. All rights reserved. Keywords: Thunderstorm Weak to moderate shear NWP Vorticity equation Supercell Mesovortex 1. Introduction Development of rotation in thunderstorms has been studied for decades due to the recognition that such storms are usually long lasting and accompanied by severe weather (Browning, 1964). Storms which exhibit a deep and persistent mesocyclone (the depth of the rotation needs to be at least one third of the thunderstorm's vertical extent and it needs to last for several tenths of minutes) are defined as supercells (Doswell and Burgess, 1993; Moller et al., 1994; Doswell, 1996). Development of supercell thunderstorms is usually expected in an environment with strong vertical wind shear in the lowest 0–6 km layer of the troposphere (Weisman and Klemp, 1982; hereafter WK82; Thompson et al., 2003). However, there are examples of supercell storms observed under rather weak or moderate shears (well below 20 m/s) Atmospheric Research 123 (2013) 93–116 ⁎ Corresponding author at: Hungarian Meteorological Service, Kitaibel Pál u. 1, H-1024, Budapest, Hungary. Tel.: +36 1 3464771; fax: +36 1 3464649. E-mail address: csirmaz.k@met.hu (K. Csirmaz). URL: http://www.met.hu (K. Csirmaz). 0169-8095/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.atmosres.2012.09.017 Contents lists available at SciVerse ScienceDirect Atmospheric Research journal homepage: www.elsevier.com/locate/atmos