Planetary-size dependence of zonal jets: Effects of horizontal diffusion in an idealized Earth-like general circulation model

The planetary-size dependence of atmospheric circulation and the effects of subgrid-scale horizontal diffusion are elucidated in this study using an idealized general circulation model. An equatorial super-rotation is formed when a planetary radius is equal to or smaller than 1/3 that of the Earth (r∗ ​≤ ​1/3, where the planetary-size parameter r∗ is normalized by the Earth's radius). The mid-latitude jet becomes stronger and shifts poleward as r∗ decreases to 1/8. The empirical Smagorinsky horizontal diffusion does not significantly affect mid-latitude jets in large-planet experiments of r∗ ≥ 1/8. In contrast, in small-planet experiments with r∗ ​= ​1/20, equatorial super-rotation is predominant when applying the Smagorinsky diffusion, whereas high-latitude jets are predominant in the absence of such a diffusion. Therefore, the possibility of two different circulation patterns must be considered: an equatorial jet pattern and a high-latitude jet pattern. In the small-planet experiment with the Smagorinsky diffusion, zonal jets are developed around poles where the Reynolds number is low for zonal circulation. In such a case, subgrid-scale dissipations significantly affect the high-latitude jet during spin-up. After this process, the developed high-latitude jets collapse, and the equatorial super-rotation becomes predominant. Because the wind deviating from the rigid-body rotation is sensitive to the subgrid-scale diffusion parameter at high latitudes, the validity of the parameterized horizontal diffusion must be carefully assessed when investigating the planetary-size dependence of the general circulation.