Plume dispersion from the MVP field experiment. Analysis of surface concentration and its fluctuations

Surface concentration and its fluctuations from plume dispersion under unstable conditions in a coastal environment are investigated using the model validation program field experimental data. The goal of this study is to better understand plume dispersion under such conditions. Procedures are described to derive the plume surface concentration from moving vehicle measurements. Convective boundary layer scalings are applied and cumulative density functions (CDF) are studied. The results indicate that the relative concentration fluctuation intensity (σ_c/C(y)) decreases with the normalized downwind distance (X) and that it is relatively small at the plume central line and largely increased at the plume edges, consistent with other field and laboratory results. The relation between σ_c/C(y) at the plume centerline (σ_c/C) and X for elevated sources can be described by σ_c/C=a+b/X. The crosswind plume spread (σ_y) is found to satisfy Deardorff and Willis's (J. Appl. Meteorol., 14 (1975) 1451) form of σ_y/h=a₁X/(1+a₂X)^1/2 scaled with convective layer depth h. For elevated sources, the normalized crosswind integrated concentration (C^y) is found to satisfy a relation of C^y=16X^-3/2, with Yaglom's (Izr. Atmos. Oceanic Phys., 8 (1972) 333) scaling rule on the free convective layer being applied. Empirical CDFs based on the gamma and the clipped probability density functions show agreements with the experimental CDFs, with the former being better than the latter when (c-C)/σ_c>0.5. A new clipped-gamma CDF form is proposed based on the analysis of the present data, showing a better agreement. We suggest that a parameter u₀*(12-0.5h/L) ^1/3, with combined efforts of surface friction velocity (u ₀*), Monin-Obukhov stability length (L) and unstable boundary layer height (h), replace the convective velocity scale (w*) under weak convective conditions in a coastal environment.