large fluctuations in concentration over short time periods, e.g., a
few seconds or minutes. This is due to the random nature of the ABL
turbulence field, which leads to especially large concentration
fluctuations in the convective boundary layer (CBL). Such fluctuations
are caused by the meandering of a small "instantaneous" plume by the
large CBL eddies. The root-mean-square (rms) concentrations are often
as large as or larger than the mean value at short distances from a
source and are important for health effects due to routine pollutants
and toxic substances.
In this paper, we investigate rms fluctuations and peak concentrations
from sources in the convective and stable boundary layers using a new
Lagrangian two-particle dispersion model (L2PDM) driven by large-eddy
simulations (LESs) of the ABL. In this approach, we track the motion
of two particles that start from a small source, spread due to
inertial-subrange turbulence, and result in relative dispersion about
the local plume centerline. We extend Thomson's (1990) L2PDM for
homogeneous turbulence to the more complex ABL flows by linking the
model with LES. As in "one-particle" models, the total particle
velocity is divided into "resolved" and "subfilter-scale" (SFS)
components. However, in the L2PDM the SFS velocity of each particle
depends on the postion of both particles due to their correlation,
which is explicitly included. The correlation of the resolved
velocities is included in the LES resolved field.
The L2PDM-LES was applied first to an instantaneous puff in the CBL and
demonstrated to give the correct variation of relative dispersion with
time. We also found that the L2PDM mean concentration agreed with results
from the convection tank experiments of Deardorff and Willis (1976, 1978,
1981) and Hibberd (2000). The modeled rms concentrations showed good
agreement with Hibberd's data for a range of source heights. In addition,
we have determined the variation of the rms fluctuations and cumulative
distribution of concentration with the averaging time from about 40s to
1/2 hr. These results and others, e.g., peak concentration versus averaging
time, will be presented and discussed.
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