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

Abstract

The effects of surface inhomogeneity of sensible heat flux on fumigation have been investigated by large-eddy simulation (LES) in the present study. The surface inhomogeneity consists of regularly aligned squares of park area surrounded by built-up area, the two types of surface having different values of sensible heat flux. The dynamics of such a CBL, named the urban CBL (UCBL), and the effects on dispersion of plumes initially placed inside the UCBL have been examined by Cai (Q. J. Royal Meteor. Soc. 125 (1999) 1427) and Cai (Atmos. Env. 34 (2000) 61) respectively. The present study delivers the following major findings. (i) The results of mean plume height and ground-level concentration (GLC) over the two landuse types for a fumigation case are opposite to those for an “in-UCBL” dispersion case studied in Cai (Atmos. Env. 34 (2000) 61). In other words, for a fumigation case mean plume height is lower and GLC is higher over the built-up area (in comparison with those over the park area), whereas for an “in-UCBL” dispersion case mean plume height is higher and GLC is lower over the built-up area. (ii) In general, the two quantities have larger fluctuations over the built-up area than those over the park area. The above characteristics in (i) and (ii) are the consequence of stronger turbulence over the built-up area than that over the park area. (iii) The length scale of surface patchy pattern, d, is the most effective surface parameter that affects the contrast of several variables (mean plume height, GLC, and dispersal parameters) between the two landuse types. (iv) For the same d, the case with a larger value of skewness of surface pattern has a larger contrast of these variables between two landuse types during the early phase of fumigation.