Vertical cross-sections along the x-axis at y = 0.577 × 10⁹ m through the same model snapshot as in Fig. 1, displaying the following physical quantities: a) gas temperature, b) logarithmic gas pressure, c) logarithmic gas density, d) the difference between the local logarithmic optical depth and its horizontal average (log τ_Ross(x,z) − < log τ_Ross>_x,y), e) absolute velocity multiplied by the sign of the vertical velocity component, and f) the Mach number. In panels e and d, streamlines for the flow field in the view plane and a dot-dashed contour for v = 0 km s^-1, which thus divide upflows (v> 0) from downflows (v< 0), are plotted. The height of optical depth unity is represented by the black solid line around z = 0 m, which thus marks the boundary between photosphere and convection zone.