The diurnal structure of the atmospheric boundary layer has an important impact on the dispersion of chemical compounds. The main characteristic of the ABL is its turbulent nature that drives scalar transport with a broad range of spatial and temporal scales. Turbulent eddy motions transport and mix primary and secondary pollutants throughout the ABL. Large-scale turbulent eddy motions (e.g., thermals and subsidence motions) characterize the daytime convective boundary layer while the nocturnal boundary has significantly smaller eddies.

The full diurnal ABL cycle consists of the three flow regimes, i.e. stable, neutral and unstable, two transition states after sunrise and after sunset. The main limitation in simulating the diurnal ABL cycle resides in the difficulty of resolving both the small scales that characterize the nocturnal boundary layer and the large ones of the day time case with the same sub-grid scale model. A successful LES depends on the ability to accurately simulate the dynamics that are not explicitly resolved. The increasing atmospheric stability conditions from day to nighttime flow enhances the difficulty to perform successful simulations.

References:

• S. Basu, F. Porté-Agel, E. Foufoula-Georgiou, J.-F. Vinuesa, and M. Pahlow, 2006: Revisiting the local scaling hypothesis in stably stratified atmospheric boundary layer turbulence: an integration of field and laboratory measurements with large-eddy simulations, Boundary-Layer Meteorology, 119, 473-500.
• J.-F. Vinuesa, S. Basu, and S. Galmarini, 2007: Diurnal evolution of 222Rn and its progeny in the atmospheric boundary layer, Atmospheric Chemistry and Physics, 7, 5003-5019.
• S. Basu, J.-F. Vinuesa, and A. Swift, 2008: Dynamic LES Modeling of a Diurnal Cycle, Journal of Applied Meteorology and Climatology, 47, 1156–1174.