Sensitivities of General Circulation and Waves to Horizontal Subgrid-Scale Diffusion in Long-Term Time Integrations of a Dynamical Core for Venus

Super-rotation and waves are sensitive to horizontal subgrid-scale fourth-order diffusion in long-term time integrations of a dynamical core under a Venus-like condition. The rigid-body rotation component of the super-rotation intensifies as the diffusion strengthens, and the fluid dynamic similarity of the nondimensional winds is found among the different diffusions. In the experiments with weak diffusion, the super-rotation is weak in the presence of strong poleward eddy heat flux, enhancing the downward angular momentum transport by the Eliassen–Palm (E–P) flux. In contrast, the strong super-rotation is developed, and both the poleward eddy heat and vertical E–P fluxes are weak in the strong horizontal diffusion experiments. Because the waves with high zonal wave numbers and poleward heat fluxes are strongly dissipated, the vertical E–P fluxes become weak and the contribution of subgrid-scale vertical diffusion becomes large in the momentum budget. As the horizontal diffusion strengthens, the phase velocities of the equatorial waves increase due to the faster background zonal flow (i.e., mean zonal flow). Although strong empirical diffusion produces strong super-rotation, it is still unknown whether it is valid. Thus, we must carefully assess the robustness of the super-rotation in the long-term simulation based on the sensitivity test of horizontal diffusion.