Updating Applied Diffusion Models

Abstract

Most diffusion models currently used in air quality applications are substantially out of date with understanding of turbulence and diffusion in the planetary boundary layer. Under a Cooperative Agreement with the Environmental Protection Agency, the American Meteorological Society organized a workshop to help improve the basis of such models, their physics and hopefully their performance. Reviews and recommendations were made on models in three areas: diffusion in the convective boundary layer (CBL), diffusion in the stable boundary layer (SBL), and model uncertainty. Progress has been made in all areas, but it is most significant and ready for application to practical models in the case of the CBL. This has resulted from a clear understanding of the vertical structure and diffusion in the CBL, as demonstrated by laboratory experiments, numerical simulations, and field observations. All of these investigations have shown the importance of the convective scaling parameter: w*, the convective velocity scale and zi, the CBL height. This knowledge and the non-Gaussian nature of vertical diffusion have already been incorporated in some applied models and show much promise. The workshop has made a number of recommendations concerning the use of this information, with perhaps the most important being the use of w*, zi directly in expressions for the dispersion parameters (σy, σz). Understanding of turbulence structure and diffusion in the SBL is less complete and not yet ready for general use in applications. However, some promising new developments include a similarity framework for turbulence structure over ideal terrain and models to predict vertical dispersion in terms of the local structure. Further development and testing of these models are required, with new data sets—laboratory, numerical, and field—being especially beneficial. As for model uncertainty, it is recommended that natural variability estimates ultimately become an integral part of air quality predictions. Some general frameworks for these estimates include the meandering plume and Eulerian similarity models, with the former being of more immediate utility. However, further evaluation of these models is necessary before they can be recommended for applications.