11A.5
Lagrangian particle modeling of dispersion in planetary boundary layers with and without a plant canopy
Jeffrey C. Weil, CIRES/Univ. of Colorado, Boulder, CO; and E. G. Patton and P. P. Sullivan
We investigate passive scalar dispersion from point sources in neutral (NBL)
and stable (SBL) boundary layers using a Lagrangian particle model (LPM)
driven by velocity fields from large-eddy simulations (LESs). The approach
is the same as that adopted earlier for dispersion in convective and stable
boundary layers (e.g., Weil et al., 2004) with particles tracked using
velocities from the LES resolved fields and a stochastic subgrid-scale model.
In the LES, the geostrophic wind was either 5 m/s or 8 m/s, and the boundary
layer height was either 615 m (NBL) or 200 m (SBL). A plant canopy and its
mechanical drag were modeled using the approach of Patton et al. (2003).
Dispersion simulations were conducted for source heights ranging from the surface
to the upper part of the boundary layer. As a test of the LPM-LES approach,
we compared the surface concentrations from a surface release in the no-canopy
case with field data from the Prairie Grass experiments and found good agreement.
For sources within and above a canopy, the mean plume height increased more
rapidly with downstream distance than in the no-canopy case. This was caused
by the positive vertical gradient of the vertical velocity variance over a
greater height range near the surface (i.e., the roughness sublayer) than in
the no-canopy case. The positive gradient led to a greater mean upward
"drift" velocity of the particles. In addition, plumes from all release
heights showed a mean lateral deflection due to the wind direction shear,
which was stronger in the no-canopy case. These and other aspects of the
results will be discussed.
Session 11A, LAND-SURFACE-PBL COUPLING—III
Wednesday, 11 June 2008, 1:30 PM-3:00 PM, Aula Magna Vänster
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