Overall results of our numerical simulations for different combinations of the disk structure and F_p/_g. Top panels: given disk structures (the black lines; see Table 1). Middle panels: resultant midplane pebble-to-gas ratio of the spatial density, ρ_p/ρ_g. Bottom panels: resultant planetesimal surface density formed by the ND mode and SI (Eq. (8)). The blue, green, and red lines indicate time evolution. By t ≃ 1 × 10⁵ yr, the system reaches a steady-state in ρ_p/ρ_g and then planetesimals keep forming in a steady width. The dashed gray lines in the top panels represent analytically derived critical α_mid/α_acc for a given F_p/g (Eq. (11)), indicating that disks of smaller α_mid/α_acc than this critical value develop the ND mode. The gray lines in the middle panels represent the critical ρ_p/ρ_g above which the ND mode occurs (i.e., ρ_p/ρ_g = 1). Here, α_acc = 10⁻², τ_s = 0.1, M_g = 10⁻⁸ M_⊙ yr⁻¹, and k_SI = 10⁻³ are used.