Toroidal and poloidal energy in rotating Rayleigh-Benard convection

We consider rotating Rayleigh-Benard convection of a fluid with a Prandtl number of Pr = 0.8 in a cylindrical cell with an aspect ratio Gamma = 1/2. Direct numerical simulations (DNS) were performed for the Rayleigh number range 10(5) <= Ra <= 10(9) and the inverse Rossby number range 0 <= 1/R0 <= 20. We propose a method to capture regime transitions based on the decomposition of the velocity field into toroidal and poloidal parts. We identify four different regimes. First, a buoyancy-dominated regime occurring while the toroidal energy etor is not affected by rotation and remains equal to that in the non-rotating case, e(tor)(0). Second, a rotation-influenced regime, starting at rotation rates where e(tor) > e(tor)(0) and ending at a critical inverse Rossby number 1/R0(cr) that is determined by the balance of the toroidal and poloidal energy, e(tor) = e(pol). Third, a rotation-dominated regime, where the toroidal energy e(tor) is larger than both e(pol) and e(tor)(0). Fourth, a geostrophic regime for high rotation rates where the toroidal energy drops below the value for non-rotating convection.