Spinup of macroturbulent circulation

The animation shows the spinup of a macroturbulent circulation from an axisymmetric circulation. The axisymmetric circulation is that shown in Figure 3a of the review; the statistically steady state of the macroturbulent circulation is that shown in Figure 3d. Contour intervals are 10 K for potential temperature; 4 m s^-1 for zonal wind; 20 x 10⁹ kg s^-1 for stream function; and 0.1Ω a² for angular momentum (as in Figure 1a and c). (The animation was produced by Chris Walker.)

[Spinup Animation (QuickTime)]

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In the macroturbulent simulation, there is no subgrid-scale diffusion of heat or momentum above the planetary boundary layer. Vertical subgrid-scale diffusion of heat and momentum is necessary for stability in the axisymmetric simulations. Because the subgrid-scale diffusion is turned off when the three-dimensional perturbation is added to the axisymmetric circulation, the Hadley circulation first weakens before it strengthens when large-scale eddies form.