Environmental Engineering Reference
In-Depth Information
where the superscripts denote contravariant components of the velocity vector and
diffusivity tensor. Since the grid is uniform in the computational space, the same
numerical solution algorithms used in CMAQ can be used directly.
Fig.
1.
Comparison of PM
2.5
concentrations (μg m
−3
) at Fort Benning, GA during a prescribed
burn on April 9, 2008: (a) standard CMAQ with 1.33 km grid resolution, (b) adaptive CMAQ
with dynamically adapting mesh. Boxes shown are cutouts of the Fort Benning area from the
model domain
The adaptive grid provides significant improvement of the PB plume simulation
over the standard CMAQ (
Fig. 1).
3. Sub-grid Scale Plume Model
The dynamics of smoke plumes from PB are often far more complex than the
dynamics of plumes from industrial stacks. Daysmoke, a dynamical-stochastic
plume model, was developed specifically to simulate smoke from PB in a manner
consistent with how the burns are engineered by land managers. It is an extension
of a plume model developed to simulate deposition of ash from sugar cane fires
[3]. Daysmoke employs a dynamic plume-rise algorithm and a large eddy para-
meterization to describe the plume boundaries; a stochastic turbulence model diffuses
the particles through the boundaries of the plume. Its predictions of smoke con-
centrations generally agreed with ground-level measurements at several locations
beneath the plumes (
Fig. 2
) and were also confirmed with lidar observations.
The major task ahead is the development of a suitable coupling technique that
can inject Daysmoke particles into the CMAQ grid cells without significant loss of
accuracy. For this, a Fourier analysis technique is being explored. First, the smoke
particle concentrations predicted by Daysmoke are represented as spectra of waves
with different frequencies. Then, the waves whose frequencies cannot be sup-
ported by the adaptive CMAQ grid are identified. If the amplitudes of those waves
are negligible, then the plume is handed over to CMAQ; otherwise the plume is
