Concentration Fluctuations and Variability at Local and Regional Scales: Use of a Lagrangian Two-Particle Dispersion Model Coupled with LES Fields Jeffrey Weil 1 , Peter Sullivan 1 , Edward Patton 1 , and Andrezj Wyszogrodski 2 1 National Center for Atmospheric Research, Boulder, CO, USA 80307 and 2 Polish Bureau of Meteorology, Warsaw, Poland Abstract A Lagrangian two-particle dispersion model (L2PDM) driven by ve- locity fields from large-eddy simulations (LESs) is used to compute the mean and fluctuating concentrations in a highly convective boundary layer. The model re- sults agree with data from two convection tank experiments. Keywords Concentration fluctuations, Lagrangian two-particle modeling, LES 1. Introduction A striking feature of atmospheric dispersion is its large variability. This is espe- cially true in the convective boundary layer (CBL) where the plume from an ele- vated source meanders due to the large convective eddies, producing high fluctua- tions in surface concentrations. The concentration peaks are caused by intermittent transport of plume segments to the surface by CBL downdrafts, where the plume spread about the local centerline is due to small-scale turbulence (Gifford, 1959). The concentration fluctuations are characterized by their root-mean-square (rms) value σ c or the fluctuation intensity σ c /C, where C is the ensemble-mean concen- tration. Measurements show that surface σ c /C can be large ranging from 1 to 10 for short averaging times (< 5 min) and downstream distances (< 5 km). In this work, we extend Thomson’s (1990) Lagrangian two-particle dispersion model (L2PDM) for concentration fluctuations in homogeneous turbulence to the inhomogeneous conditions of the CBL by coupling it with velocity fields from large-eddy simulations (LES) (e.g., Moeng and Sulllivan, 1994). Thomson’s mod- el handles the two-particle motion due to the ``unresolved” or LES subfilter-scale (SFS) velocities, whereas the LES ``resolved” velocities address particle dis- placements due to the larger-scale motion.