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Abstract

The geostrophic turbulence in rapidly rotating thermal convection exhibits characteristics shared by many highly turbulent geophysical and astrophysical flows. In this regime, the convective length and velocity scales and heat flux are all diffusion-free, i.e. independent of the viscosity and thermal diffusivity. Our direct numerical simulations (DNS) of rotating Rayleigh–Bénard convection in domains with no-slip top and bottom and periodic lateral boundary conditions for a fluid with the Prandtl number Pr = 1 and extreme buoyancy and rotation parameters (the Rayleigh number up to Ra = 3 × 1013 and the Ekman number down to Ek = 5 × 10−9 ) indeed demonstrate all these diffusion-free scaling relations, in particular, that the dimensionless convective heat transport scales with the supercriticality parameter ˜Ra ≡ Ra Ek4/3 as Nu − 1 ∝ ˜Ra3/2 , where Nu is the Nusselt number. We further derive and verify in the DNS that with the decreasing ˜Ra, the geostrophic turbulence regime undergoes a transition into another geostrophic regime, the convective heat transport in this regime is characterized by a very steep ˜Ra-dependence, Nu − 1 ∝ ˜Ra3 .