Lagrangian modeling of dispersion in the stable boundary layer

In this paper, we investigate passive scalar dispersion from point sources in the stable boundary layer (SBL) using a Lagrangian particle model driven by velocity fields from a large-eddy simulation (LES). The approach is essentially the same as that for studying dispersion in the convective boundary layer (Weil et al., 2004) with particles tracked using the LES resolved velocities and stochastic subgrid-scale velocities. The LES was performed for an SBL in which the geostrophic wind was 8 m/s, the SBL height (h) was 200 m, and the stability was classified as weakly stable based on the ratio (1.6) of the boundary layer height to the Monin-Obukhov length.

Dispersion simulations were conducted for source heights ranging from the surface to about 0.75h. For a surface release, the mean plume height and vertical dispersion followed an approximately parabolic dependence on distance over much of the downwind range (4.5 km); the behavior was consistent with the self-similarity of the vertical concentration profile. The mean plume heights for higher releases exhibited some initial source height dependence but asymptoted to the surface layer results farther downwind. For the surface releases, the variation of the surface concentration with distance was consistent with surface layer similarity (SLS) theory. In addition, plumes from all release heights showed a mean lateral deflection due to the significant wind direction shear. These and other aspects of the results will be discussed.