Dust-to-gas density ratio at t = 400 T₀ from models with different vertical prescriptions of the α parameter. Panel a : model in which α = 10⁻³. Panel b : model in which α decreases from 10⁻³ in the midplane to 10⁻⁴ in the atmosphere. Panel c: model in which α increases from 10⁻⁴ in the midplane to 10⁻³ in the atmosphere. Panel d: model in which α = 10⁻⁴. Last panel: Corresponding vertical profiles of α, given by Eqs. (30)-(33). Dashed curves in panels b and c denote contours for one gas scale height. The shaded regions indicate where the transition of α occurs and are bounded by aspect ratios evaluated at the inner and outer radial boundaries.