TwoPopPy simulation based on R2 quantities, but with the temperature scaled by ξ = 5. The line color denotes the time in the simulation according to the color bar, with a darker color indicating a later time. Left: radial profile of the vertically integrated dust-to-gas ratio Σ_dust/Σ_gas. Right: radial profile of the pebble Stokes number St₁, i.e., the Stokes number of the large grains in the two-population approximation. The dashed lines indicate the value expected from the two population model for this setup, f_fSt_frag. The separate dashed green line at St₁ = 10⁻² indicates the minimal Stokes number required for strong clumping via the streaming instability.