•Defined local Froude number (Frl) to asses jet characteristics during calculations.•A model to dynamically prescribe turbulent Schmidt and Prandtl numbers is proposed.•Model identifies regions with possible occurrence of Kelvin-Helmholtz instability.•Improved results in case with occurrence of Kelvin-Helmholtz instability. To theoretically investigate atmosphere mixing in an enclosure, three experiments of interaction of a vertical air jet with a helium-rich layer performed in SPARC, PANDA and MiniPanda experimental facilities, were simulated using the URANS approach. For the purposes of this work, firstly, a local Froude number is introduced, which can be directly used in computational fluid dynamics calculations to specify the regions with possible occurrence of Kelvin-Helmholtz instability. Namely, it is shown that in these regions, turbulent diffusion is underpredicted and turbulence model fails to replicate some phenomena. Secondly, a model for dynamically prescribe turbulent Schmidt and Prandtl numbers is proposed. The proposed model improves the results in isothermal case with possible occurrence of Kelvin-Helmholtz instability, while its effects are negligible in cases with higher temperature jets with lower local Froude number values, where the conditions for the Kelvin-Helmholtz instability are not established.