During the last three decades, a good understanding of atmospheric diffusion has been achieved in the convective boundary layer as a result of numerical, laboratory, and field investigations. All previous studies, however, have focused on day-time, clear-sky convection over land, and consequently either eliminated or simply ignored convection due to the presence of low clouds at the top of
the boundary layer. Low, stratocumuli clouds, especially in nocturnal conditions, are able to generate a rather strong convection, and therefore can have an important impact on the structure of atmospheric turbulence and diffusion. In the daytime convective boundary layer (CBL), dispersion depends on the location of the source, due to a positive vertical velocity skewness. On the other hand, the skewness in the cloud-topped boundary layer (CTBL) is generally negative. Consequently, dispersion in the CTBL is also expected to be dependent on the location of the source, but in a "reverse" way. In order to evaluate this notion, we performed a preliminary test based on a large-eddy simulation (LES) model with a bulk parameterization of clouds and Lagrangian particle model. Dispersion computations include simulations of passive tracer released from point sources at various heights above the ground.