An LES Study of the Impacts of Land Surface Heterogeneity on Dispersion in the Convective Boundary Layer

Abstract

A systematic analysis of the impacts of heat patches and topographical features on the dispersion of passive materials in a shear-free convective boundary layer (CBL) was performed. Large eddy simulations and a Lagrangian particle dispersion model were used for that purpose. Over a homogeneous, flat terrain, the dispersion statistics produced by the model are in agreement with convection tank data and other model results. The horizontal pressure gradients created by surface heat flux heterogeneities generate atmospheric circulations, which impede vertical mixing and, as a result, have a remarkable influence on particle dispersion in the CBL. For a near-surface release, the particles are advected horizontally rather than ?lifted-off,? maintaining a high concentration near the ground surface. Particles released at higher elevations reach the ground surface more slowly than when released above a flat, homogeneous domain. In a shear-free CBL, hilly terrain has little impact on lift-off, dimensionless crosswind-integrated concentration, mean particle height, particle spread, and near-ground-level concentration of particles released near the ground surface. This is true even with hills as high as 25% of the height of the CBL. However, it has a noticeable effect on the dispersion statistics of particles released from higher elevations. In particular, the locus of the maximum in crosswind-integrated concentration of particles released from a source located about 25% of the height of the CBL descends to the surface of an even moderate hill noticeably slower than above a flat, homogeneous domain.