Pollutant dispersion in stable atmospheric conditions is still a phenomenon that is highly difficult to model or to reproduce in an experimental wind tunnel. However, such conditions are of major interest in the field of risk assessment because they are generally conservative and the low level of turbulence induces the most important distance for toxic impact. Using LES approach appears relevant and promising to overcome difficulties related to stable conditions modelling. The objective of this paper is to present the preliminary results obtained in terms of wind flow modelling with the open source CFD code FDS, from NIST, which is based on Large Eddy Simulation approach. It is essential to determine the best parameterization of this type of code for atmospheric gas dispersion modelling. Starting from atmospheric flow conditions that were observed during INERIS experimental campaign of ammonia release, the process related to satisfy this requirement will be presented.