Skewnesses, S_w and S_θ, and the TOM, $\overline{w^3}$, in a 3D RHD simulation of convection in the upper envelope of a DA white dwarf. Top: Comparison of S_w (left) and S_θ (right) for the closure models discussed in Fig. 1 with input data taken from the 3D RHD simulation. The colour coding is the same as for Fig. 1 and 2. Bottom: Test of these closure models for the TOM $\overline{w^3}$ (left) alongside a zoom into the top half of the simulation domain (right). See a detailed discussion in the main text.