Retrograde rotation of the large-scale circulation in turbulent rotating Rayleigh-Benard convection at large Rossby numbers up to 200*

We examine the azimuthal rotation of the large-scale circulation (LSC) for turbulent Rayleigh-Benard convection in the present of week rotations about a vertical axis at angular velocities $1.0{\times}10^{-3}{\le}{\Omega}{\le}0.1$(rad/s). Over the entire Rossby-number range $1{\le}Ro{\le}200$ studied, linear retrograde rotations of the LSC circulating plane are observed. With increasing $Ro({\sim}{1/\Omega})$ the retrograde rotating velocity ${\langle}-\dot{\theta}{\rangle}$ decreases monotonically, but the ratio ${\gamma}={\langle}-\dot{\theta}{\rangle}/{\Omega}$ experiences a transition at $Ro^{\star}{\approx}80$ above which ${\gamma}$ increases sharply. We discuss the $Ro$-dependence of ${\gamma}$ for $Ro>Ro^{\star}$ and show that a maximum ratio ${\gamma_{max}}=0.36$ is observed at $Ro=200$, more than twice larger than other results reported before in a lower-Ro regime [1]. The experimental findings may shed new light to interpret the low precession rate under weak Coriolis force within the framework of the LSC models [2].\\[4pt] [1] J. E. Hart, S. Kittelman, and D. R. Ohlsen, Phys. Fluids 14, 955 (2002); J.-Q. Zhong and G. Ahlers, J. Fluid. Mech. 665, 300 (2010).\\[0pt] [2] E. Brown and G. Ahlers, Phys. Fluids 18, 125108 (2006).