Turbulent flows in the
Atmospheric Boundary Layer:
Atmospheric Boundary Layer:
dispersion, chemical transformation and surface effects
Jordi Vilà-Guerau de Arellano
Jean-François Vinuesa
Alessandro Dosio
Meteorology and Air Quality Group Wageningen University
The distribution and evolution of inert species (dispersion) and reactants (transformation) in the atmospheric boundary layer are studied by means of large-eddy simulation model (LES). To study the main structure and characteristics of both flows, it is essential to describe the most relevant spatial and temporal scales. LES is applied to simulate the dispersion of a tracer released at different heights and to analyze the transport (flux) and the variability (co-variance) of reactive species. The LES model used has been developed in collaboration with the Institute of Marine and Atmospheric Research (IMAU, Utrecht University) and the KNMI. With the knowledge obtained from the LES results, parameterizations of the dispersion and chemical transformation processes are derived. These representations will be implemented in the future in large atmospheric models.
The dispersion of a pollutant in the atmospheric boundary layer is influenced by the characteristics of the atmospheric boundary layer. In particular, in an atmospheric boundary layer driven by convection, pollutants released close to the ground have a tendency to move upwards transported by the updraft thermals whereas the pollutants released at higher heights are carried downwards by the subsidence motions. The evolution of three different scalars emitted at different heights studied in detail. In order to obtain reliable statistics, several similar experiments are carried out and ensemble averages of the plume dispersion are calculated to estimate the dispersion parameters. Figure 1 shows the vertical cross section of the concentration distribution of a pollutant release close to ground and in the middle of the boundary layer. It is clear to observe the influence of the boundary layer characteristics on the release's height of the plume. The combined effect of buoyancy and shear on dispersion is studied as well by means of prescribing different values of the geostrophic wind and surface heat flux. The influence of the wind on the vertical and horizontal plume's spreading and the ground concentrations are also analyzed.
The turbulent mixing and ultraviolet radiation are the driving forces of atmospheric chemistry. For reactants with a short chemical lifetime or with a lifetime at the same order of magnitude as the turnover time of the atmospheric boundary layer (ABL), the turbulent mixing limits their chemical transformations and, and in consequence, it has an influence on the spatial distribution of reacting scalars. The main goal of our research is to study this influence, and in particular to determine the effect of turbulence on the reactant fluxes and their co-variances. The turbulent reacting flow is characterized by a bottom-up diffusive species injected at the surface and a top-down reactant that is detrained at the top of the CBL. These species can react following two chemical pathways: a second order reaction and a cycle. Figure 2 shows an instantaneous cross section of the vertical velocity field (positive values are related with upward motions) and the reaction zones for a second-order chemical reaction. It is clear to observe that the atmospheric boundary layer is far from being a perfect reaction vessel. The maximum reaction zone is close to the surface and at the core of the thermal motions where turbulence is more active. The result of this limitation of the chemical reactivity of turbulence is a slow down in the transformation of the reactants.
Figure 1. Vertical cross section of the pollutant concentration release at ground level (upper panel)
and in the bulk of the atmospheric boundary layer (lower panel).
Figure 2. Vertical cross section of the vertical velocity (w) and reaction zone for a second order chemical reaction.
References
Molemaker J. and Vilà-Guerau de Arellano (1998) Control of chemical reactions by convective turbulence in the boundary layer. Journal of the Atmospheric Sciences 55, 568-579.
Vilà-Guerau de Arellano J. and Cuijpers J.W.M (2000) The chemistry of a dry cloud: the effects of radiation and turbulence. Journal of the Atmospheric Sciences 57, 1573-1584.