Zonal circulation faster than the planetary rotation (i.e., superrotation) has been observed in planetary atmospheres (Imamura et al., 2020; Read & Lebonnois, 2018). On Venus and Titan, fast zonal flows of >100 m s −1 and meridional circulation are driven by diabatic heating in aerosol and cloud layers in the middle atmosphere (Flasar & Achterberg, 2009; Read & Lebonnois, 2018; Sanchez-Lavega et al., 2017), which corresponds to the stratosphere and mesosphere. In the general circulation models (GCMs) of such slowly rotating and small-sized planets, superrotation is driven by meridional circulation with the help of equatorward eddy momentum flux. This is known as the Gierasch-Rossow-Williams concept (Gierasch, 1975; Rossow & Williams, 1979). In Gierasch (1975), the diffusion processes are simply assumed as a hypothetical parametrization of angular momentum transport in the superrotation. In Rossow and Williams (1979),