Chapter 4.1 Lagrangian particle model simulation of tracer dispersion in stable low wind speed conditions

Recently we have been studying the turbulence and dispersion characteristics during meandering (large horizontal oscillations of the atmosphere) associated to low wind speed conditions. It was found that the autocorrelation functions (AE) of the horizontal wind components, computed for the low wind cases, show an oscillating behaviour with the presence of large negative lobes and that these AE can be very well fitted by an analytical relationship that contains two parameters, one associated to the classical integral time scale and the second to the meandering characteristics. Then, a new system of two coupled Langevin equations for the horizontal wind components, able to simulate the dispersion in low wind speed conditions, was derived. In this paper we briefly review these topics and, in particular, we show the results, obtained with these new coupled Langevin equations, of the simulation of the INEL and Graz low wind tracer experiments. The comparison showed very good results and, in particular, put in evidence the advantage of using this new low wind simulation tool with respect to previous classical models.