Turbulence fields for atmospheric dispersion models in horizontally non-homogeneous conditions

When atmospheric pollutant dispersion is simulated over complex terrain, the turbulence input parameters are often prescribed according to standard parameterisation based on surface layer quantities. These last are available from literature. In the past, several different parameterisations for the first few moments of the turbulence velocity statistic and for the Lagrangian time scale have been developed and tested in different stability conditions. The main shortcomings for using these parameterisations are their inadequateness in predicting the turbulence field in horizontally non-homogeneous boundary layer (like for example in complex terrain or in urban heat island) and the essentially local nature of the prescribed turbulence. The purpose of this work is to suggest proper methods for predicting turbulence field for dispersion model over complex terrain and, more generally, in horizontally non-homogeneous conditions. The modelling system RMS (RAMS-MIRS-SPRAY) is applied to the present study. We introduced new non-local turbulence closures in the RAMS circulation model, which provides both mean wind and turbulence fields. Then, through the interface code (MIRS), the input quantities needed to the Lagrangian stochastic dispersion model (SPRAY) are computed. The model system is validated against a tracer experiment performed in a wind tunnel over a schematic two-dimensional valley.