The Behavior of Passive and Buoyant Plumes in a Convective Boundary Layer, as Simulated with a Large-Eddy Model

With a large-eddy model the large-scale flow structure of the convective boundary layer is simulated in a box of (5 × 5 × 2) km. The calculation is run till the turbulence has reached a quasi-steady state. At that time we introduce a line-source of contaminants and the calculation is continued with an additional equation for the concentration. We consider both passive and buoyant sources. The latter are simulated by increasing the temperature of the line-source with respect to the ambient temperature. We present data for two dimensionless release heights: zs/h = 0.15 and zs/h = 0.48. For a passive source our results agree well with the results of Willis and Deardorff and the results of the CONDORS experiments. With respect to a buoyant source we found that the influence of buoyancy on the plume parameters can be described in terms of the dimensionless buoyancy parameter F*; this conclusion is based on runs for F* = 0.01 and F* = 0.02. The simulations for buoyant plumes are compared with the laboratory experiments of Willis and Deardorff and with the field experiments of Carras and Williams. Only with the latter data did we obtain reasonable agreement. In this comparison we paid special attention to a correction for the initial momentum in the field data and to a correction for the initial dimensions of the line-source in the simulation data. The large-eddy results allow us to distinguish between plume motion caused by convective turbulence and that caused by the plume buoyancy. We found that the plume motion caused by buoyancy does not obey Briggs' 2/3 law, but is more in agreement with a plume rise formula proposed by Nieuwstadt and de Valk which is based on the assumption that the plume grows due to large scale convective turbulence.