Dispersion in Cloudy Boundary Layers

In contrast with dispersion in dry boundary layers, dispersion in cloud topped boundary layers is a topic that has received much less attention. In this LES based numerical study we investigate the dispersion of a passive tracer in the form of Lagrangian particles for four kinds of atmosperic boundary layers: 1) a dry convective boundary layer (for reference), 2) a 'smoke' cloud boundary layer in which the turbulence is driven by radiative cooling, 3) a stratocumulus topped boundary layer and 4) a shallow cumulus topped boundary layer.

We show that the dispersion characteristics of the smoke topped boundary layer as well as the stratocmulus situation can be well understood by borrowing concepts from the dry convective boundary layer. In general the presence of clouds tends to significantly enhance mixing and dispersion - a notion that is not always reflected well in traditional parameterization models, in which clouds usually suppress mixing by diminishing solar irradiance.

The dispersion characteristics of a cumulus cloud layer however turn out to be markedly different from the other three cases and the results can by no means be explained by only considering the well-known top-hat velocity distribution. To understand the surprising behaviour in the shallow cumulus layer, this case has been examined in more detail by determining velocity distributions both with respect to height and distance to cloud. The relevance of these results with regard to chemical processes between pollutants and atmospheric compounds is discussed.