Relevance of mixed layer scaling for daytime dispersion based on raps and other field programs

Abstract

A brief review and assessment of field measurement programs that provide data for mixed layer diffusion research is presented. The majority of programs emphasize either the meteorological aspects of the mixed layer or plume characterization. Few programs are available that provide the complimentary blend of plume and appropriate meteorological measurements needed to adequately validate mixed layer diffusion theory. Three major U.S. EPA (Environmental Protection Agency) field programs that provide databases for model development and validation of mixed layer diffusion processes are described and discussed in more detail. The Regional Air Pollution Study (RAPS) focused on measurements of surface and mixed layer turbulent transport processes in the urban environment. The Tennessee Plume Study (TPS) obtained a database with coincident measurement of boundary layer turbulent structure and plume dispersion for a large coal-fired power plant in nonuniform terrain over the diurnal cycle. The North East Regional Oxidant Study (NEROS) obtained data on transport and dispersion of regional air mass along with supporting documentation on the spatial variations of mixed layer depths, vertical turbulent transport processes, cloud fluxes, energy budget and synoptic conditions. A design feature common throughout these experimental programs, but primarily in the RAPS and TPS, was the provision to study significant land-use scale variations and processes which influence the diffusion process. Current similarity predictions of the relevant turbulent parameters are assessed in this context. Additionally, the role of convective clouds rooted within the mixed layer in pollution dispersion as a consequence of mixed layer processes is briefly described.