Illustration of a propagating shock wave in the Bifrost 3D simulation. Gas temperature surrounding the shock wave example is shown at nine time steps between t = 992 s and t = 1120 s of the simulation run. The leftmost column shows horizontal cuts (top-view) at a height of z = 1.5 Mm, and the middle and rightmost columns show vertical cuts with the height versus the x- and y-coordinates, respectively. The blue markers show the formation height of unity optical depth τ = 1.0 for the mm wavelenghts of ALMA band 3 (2.78 − 3.26 mm). The white crosses and the vertical white dashed line at (x, y) = (0, 0) mark the location of the sampled T_b signature in Fig. 12c. At t = 992 s the shock front is visible and the mm-wavelength intensities at τ = 1.0 are coupled with it at (x, y, z) = (0.3, 0.3, 1.2) Mm. The front is propagating mostly upwards and inwards towards (x, y, z) = (0, 0, 1.6) Mm at t = 1025 s. The T_b signature is peaking in magnitude at this location around t = 1050 s, when a cooler post-shock region is visible where the front has passed. The mm-wavelength intensities are tracking the shock front upwards to z ≈ 2.0 Mm around t = 1090 s and thereafter decouple and sample the post-shock region (t = 1120 s). See Eklund et al. (2020) for a more detailed analysis of propagating shock waves in the Bifrost simulation.