Structure of the gas in the two-dimensional slab under study. This is a close-up view on the region X ≃ 0 in which the incoming flows of WNM collide to form cold structures, and only regions in which are shown. The colour image shows the gas density n_H in logarithmic scale, while contours show the gas temperature T_MHD at 20 K (higher densities), 30 K and 50 K (lower densities). The two-dimensional projection of the velocity field (V_X,V_Y) is also shown as yellow arrows, the length of which indicate the velocity modulus at that location (at the centre of each arrow). The A and B extremities of the observational one-dimensional strip are indicated for reference, and the dashed line marks an example location for the extracted density profiles on which the PDR code is run (Fig. 5). The grey areas are outside of the domain used for PDR computations.