New Developments in Disrersion Experiments for the Convective Boundary Layer

Summary and Conclusions

Results from experiments on buoyant plume dispersion in a laboratory convection tank were presented with emphasis on highly-buoyant plumes (F_*≳0.1), which have posed a major problem to dispersion modeling. For the mean concentration field, some results showed trends similar to those found by Willis and Deardorff (1987), but our results went beyond theirs.

Our data showed the following. 1)The CWIC fields including the surface values exhibited systematic dependencies on X and F_*. Far downstream (X≥3), the plumes were characterized by a vertically well-mixed distribution below z _i with a C^y maximum aloft, in contrast to the nonuniform profiles obtained earlier by Willis and Deardorff over the same X range. 2)The lateral and vertical dispersion parameters including the rms meander, relative dispersion, and total dispersion followed systematic trends with X and F_*. These data were the first laboratory measurements to show a buoyancy-enhanced σ _y , with σ _y /z _i αF ^1/3_* X^2/3 for the two most buoyant cases F_*=0.2,0.4), and a qualitative agreement of σ _y with field observations. 3)Measurements of the concentration fluctuation intensity σ _c /C along the plume centerline showed that close to the source the near-surface σ _c /C increased systematically with buoyancy due to the greater concentration intermittency. For F_*≤0.2, the surface σ _c /C decreased steadily with distance and attained a nearly universal distribution with X for Xã1.5; this was attributed to the strong vertical mixing in the mixed layer.