Turbulent dispersion in cloud-topped boundary layers 1Department of Multi-Scale Physics, Delft University of Technology, Delft, The Netherlands
2Meteorology and Air Quality Section, Wageningen University, The Netherlands
*current affiliation: Royal Netherlands Meteorological Institute, De Bilt, The Netherlands
Received: 22 Sep 2008 – Published in Atmos. Chem. Phys. Discuss.: 18 Nov 2008 – Published: 18 Feb 2009Abstract. Compared to dry boundary layers, dispersion in cloud-topped boundary layers
has received 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 atmospheric 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 cloud boundary layer
as well as the stratocumulus situation can be well understood by borrowing
concepts from previous studies of dispersion in the dry convective boundary
layer. A general result is that the presence of clouds enhances mixing and
dispersion – a notion that is not always reflected well in traditional
parameterization models, in which clouds usually suppress dispersion by
diminishing solar irradiance.
The dispersion characteristics of a cumulus cloud layer turn out to be
markedly different from the other three cases and the results can not be
explained by only considering the well-known top-hat velocity distribution.
To understand the surprising characteristics in the shallow cumulus layer,
this case has been examined in more detail by 1) determining the velocity
distribution conditioned on the distance to the nearest cloud and 2)
accounting for the wavelike behaviour associated with the stratified dry
Citation: Verzijlbergh, R. A., Jonker, H. J. J., Heus, T., and Vilà-Guerau de Arellano, J.: Turbulent dispersion in cloud-topped boundary layers, Atmos. Chem. Phys., 9, 1289-1302, doi:10.5194/acp-9-1289-2009, 2009.