Centrifugal-buoyancy-driven vortex motion and symmetry breaking in rotating Rayleigh-Bénard convection

We report measurements of the horizontal motion of columnar vortices in rapidly rotating Rayleigh-Bénard convection at a fluid height z=H/4 of a cylindrical cell. With the Froude number Fr<0.05, the vortices are advected laterally in a random manner that reveals the character of Brownian motion. When Fr>0.05, the centrifugal buoyancy starts to influence the vortical dynamics, leading to prominent radial vortex motions: uprising (hotter) cyclones are attracted towards the rotating axis while downwelling (cooler) anticyclones are accelerated outwards. The ensemble statistics of the vortex trajectories is interpreted by a set of Langevin equations which incorporates a parabolic potential associated with the centrifugal force. Strong centrifugal buoyancy creates a noticeable radial thermal gradient in the background fluid with warmer fluid accumulated near the cell center. For a certain parameter range this effect breaks the symmetry of the vorticity in the sense that anticyclones become stronger than cyclones in the inner region. In consequence we observe abnormal cyclonic motion when both cyclones and anticyclones are moving outwards as they form clusters of regular vortex grid.