COMPARISON OF CLOSURE SCHEMES USED TO SPECIFY THE VELOCITY PDF IN LAGRANGIAN STOCHASTIC DISPERSION MODELS FOR CONVECTIVE CONDITIONS

Lagrangian stochastic dispersion models make use of the probability de nsity function (PDF) of the Eulerian vertical turbulent velocities. Fo r convective conditions, the PDF is often assumed to have a bi-Gaussia n form. Using new laboratory measurements of velocity PDFs in the conv ective boundary layer (CBL), we propose a new closure for constructing this bi-Gaussian PDF and compare results with three other closure sch emes in current use. Of the three existing closures, two utilize the s econd and third moments of the vertical velocity as inputs, while the third one also incorporates the fourth moment. The new closure is defi ned with the desirable property that it collapses to a simple Gaussian in the limit or zero skewness. The value of an adjustable parameter i n this closure scheme is selected using laboratory data for the third and fourth velocity moments. We determine the parameters in the PDF ex pression obtained using the four closures, and compare them with those derived by fitting velocity PDF data from the convection tank experim ents. Significant differences are found between the values of the PDF parameters From the various closures and the water tank data. The perf ormance of the closure schemes is compared by using a Lagrangian stoch astic model to compute ground-level crosswind-integrated concentration s from particles released at four source heights. It is shown that the differences between the concentration estimates obtained using variou s closures increase as the source height increases. Using, as the benc hmark, the dispersion results calculated from the Lagrangian stochasti c model incorporating the laboratory velocity data without any closure , we recommend our new closure scheme. The results highlight the impor tance of turbulence observations in the CBL for accurate dispersion mo delling.