Analysis of a stratified store thermocline entrainment process is done by a side-by-side comparison of experiments and a direct numerical simulation. The experimental and numerical analyses are shown to be complementary; where the experiment is a true realisation of the thermocline entrainment process, the numerical simulation provides the temporal and spatial resolution for detailed analysis. The agreement between both is good. It appears that during collision of the buckling jet with the thermocline inhomogeneous penetration and back flow occur. At the point where the jet flows back into the bottom layer, upper-layer fluid is dragged into the bottom layer. After detachment, the dragged-down fluid filament becomes unstable due to overturning motions, resulting in Kelvin–Helmholtz-like waves. Subsequently the fluid filament is completely mixed with bottom-layer fluid by the action of stretching- and-folding stimulated diffusion. From a comparison between 2D- and 3D-simulation results it appeared that for store optimisation 2D-numerical simulations provide sufficient accuracy.