Time evolution of the radial profiles of the actual and radiative temperature gradients ∇ and ∇_rad, respectively, in the 2.5D simulation with b = 10⁴ and N_r = 1024. The gradients are normalised using the adiabatic temperature gradient ∇_ad. Simulation time is given as a fraction of the thermal timescale τ_th on the colour bar. Each curve, except for that showing the initial condition, corresponds to a time average over 0.05τ_th. The inset zooms in onto the convective-penetration layer with dots marking the location of the steepest gradient in ∇/∇_ad.