Monday, 10 January 2000: 11:15 AM

J. C. Weil, CIRES/Univ. of Colorado, Boulder, CO; and P. P. Sullivan and C. H. Moeng

Results are presented from Lagrangian statistical modeling
of the mean crosswind-integrated concentration (CWIC)
field due to a scalar point source in the convective
boundary layer (CBL). In this approach, one follows
``passive particles" in a turbulent flow given the
time-dependent Eulerian velocity fields, which are generated
by large-eddy simulation (LES). The CWIC is found from a
``one-particle" Lagrangian model using the computed probability
density function of particle position, i.e., from a large
ensemble of particle trajectories. The LESs covered a 5 km x
5 km x 2 km domain and were generated for highly-, moderately-,
and weakly-convective CBLs corresponding to the stability index
h/|L|=110, 16, and 5, respectively; here, h is the CBL height
and L is the Monin-Obukhov length. In all cases, h and the
surface heat flux were approximately the same but the surface
shear or friction velocity differed as a result of different mean winds.

For the most unstable case (h/|L|=110), the
modeled CWIC fields agreed well with the Willis and
Deardorff laboratory experiments and reproduced the
descent and ascent of plume centerlines from elevated and
surface sources, respectively. For h/|L|=16 and 5, the
modeled CWIC fields were qualitatively similar to those
above, but the dispersion was noticeably reduced---the rate
of ascent and descent of plume centerlines and the particle
dispersion were all smaller. The slower dispersion was
due to the greater surface shear and turbulence dissipation
rate in the CBL surface layer and immediately above it,
which led to smaller turbulence time and length scales.
These results are important because there are currently
no experiments, simulations, or observations showing the
variation in the plume dispersive properties over a broad
range of h/|L|.

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