Symmetry breaking in supercell thunderstorms*

At its base, a supercell thunderstorm hosts twin vortices that spin about their vertical axes with the same circulation but with opposite directions of spin. As the twin vortices are advected vertically, one of them, which is spinning in the cyclonic direction, becomes dominant, while the other withers away. This symmetry breaking, which is called "preferential enhancement," must be caused by asymmetric factors affecting the dynamics of vortices. It is widely believed that this symmetry breaking stems from the steering of the ambient wind (the turning of the wind-shear vector with height), wherein the Coriolis force from Earth's rotation is thought not to directly affect the vortex dynamics. Here, using state-of-the-art, non-hydrostatic, three-dimensional simulations, we show that, even in the absence wind steering, Coriolis force can lead to symmetry breaking, where the asymmetry is proportional to the Coriolis force. Our analysis highlights the significant but hitherto ignored role of Coriolis force in the dynamics of supercell thunderstorms.