Fumigation and dispersion of pollutants into a growing CBL

Fumigation of pollutants into an growing convective boundary layer: a large eddy simulation

The fumigation phenomenon involves the entrainment of pollutants in the inversion layer into a growing convective boundary layer (CBL). The plume is brought downwards by convective eddies and consequently causes high ground level concentrations (GLCs). Although many models, most of which are Gaussian type or Lagrangian particle models, have been developed for predicting fumigation, it is still not well understood yet how important the convective thermals are in contributing to fumigation of the elevated pollutants. This study uses large-eddy simulation (LES) to investigate the fumigation of a passive plume in the inversion into the CBL.

Configuration of the LES is similar to that in Cai (2000), but it adopts a 64´ 64´ 99 mesh with a resolution of 60m´ 60m´ (2-30)m. Close to the ground, the vertical spacing is about 2 m and stretched with height. Near the entrainment zone, the vertical spacing is 2 m and it gradually increases in both upward and downward directions. Initial height of the CBL is 500 m. Instantaneous line sources in the stable layer above the CBL are specified, with their cross section as a Gaussian distribution (half width of 12 m). The initial heights of the line sources are 580 m, 640 m, 700 m, and 760 m . For a growing CBL capped by a strong inversion and a stably stratified layer aloft, relevant dimensional parameters to affect fumigation include the following: the Richardson number, Ri=gD q Z_i/q w_*², which has been considered as the controlling parameter of entrainment processes at a given time in the strong inversion, and the convective Froude number, Fr=w_*/(NZ_i), which reflects the "rigidity" of the stable layer above the inversion base. The LES results are assessed based on these two parameters. The turnover time scale of convective eddies, t_*=Z_i/w_* , is used to scale the modelling time. For fumigation problem, initial elevation and size of the plume, z_l and s _z0, are important and both can be scaled by Z_i.

Validation of the LES is made for fumigation through comparison with experimental data, which include those from the water tank experiments by Deardorff and Willis (1980) and the saline tank experiments by Hibberd and Luhar (1996). The quantities assessed against experiments include: dimensionless crosswind-integrated concentrations (evaluated by contours on X-Z plane, GLCs vs. X, and maximum GLC vs. w_e/w_*), duration of a fumigation episode, and lateral spread of the plume.

References

Cai, X.-M., 2000: Dispersion of a passive plume in an idealised urban convective boundary layer: a large-eddy simulation, Atmos. Env., 34, 61-72.

Deardorff, J.W. and Willis, G.E., 1982: Ground-level concentrations due to fumigation into an entraining mixed layer. Atmos. Env., 16, 1159-1170.

Hibberd, M.F. and A.K. Luhar, 1996: A laboratory study and improved PDF model of fumigation into a growing convective boundary layer, Atmos. Env., 30, 3633-3649.