need a model!

I'm looking for an advice on an atmospheric model. I need to simulate the dispersion of a gas cloud. The gas mixture could be slightly denser or slightly lighter than air, it is released simultaneously from a large surface area. The terrain is simple, meteo-conditions are known.
Looking for a user-friendly software to simulate this. Commercial versions are ok. Any advice or experience is appreciated.
На кону жизни 2 миллионов человек и судьба задницы моего студента. Если интересно, могу рассказать, что за проблема такая...