Mean ⟨3D⟩_z entropy (black solid line) vs. depth, and 1D models for different mixing length parameters, α_MLT = 1.5 − 2.5 (blue lines), for the solar model (left panel) and a metal-poor dwarf with [Fe/H] − 2.0,T_eff = 4500 K, and log g = 4.5 (right panel). We indicate the constant entropy value of the deep adiabatic convection zone, s_bot, in both figures by the horizontal dotted line. In the deeper layers, we extended the 1D models (dashed lines) with the aid of the entropy gradient from the ⟨3D⟩ models. The calibration of the mixing length parameter α_MLT for the solar model is illustrated by the smaller insets, which depict the relative differences between the 1D and 3D models (δs = s_1D/s_3D − 1) for s_bot (solid) and the entropy jump Δs (dashed). For the solar model the two approaches result in α_MLT = 1.98 and 2.09.