Sporadic Turbulence and Dispersion in the Stable Boundary Layer

Abstract

It is commonly assumed that the levels of turbulence and dispersion in the stable planetary boundary layer (PBL) are low. This assumption strongly affects the modeling of air quality and plume dispersion in the nocturnal atmospheric surface layer. However, beginning with the early observations by Durst (1933), Gifford (1952), and Lyons et al. (1964) and latter by, for example, Bean et al. (1973), Kondo et al. (1978), Mahrt (1985), and Gossard et al. (1985) it is now clear that sporadic outbreaks of turbulence and enhanced vertical diffusion are common in the stable PBL. These effects have not been accounted for in conventional PBL plume diffusion theory and, because of this, model predictions of surface deposition may be underestimated. For long-term air quality considerations, the assumption that vertical transfer is small in the stable PBL is generally acceptable; however, in treating the problem of short-term exposure to hazardous materials consideration of the non-stationary aspects of the PBL is crucial. For example, during a breakdown event pollutants released in the upper levels of the PBL can be brought downward to the surface layer with relatively little dilution. Harrison et al. (1978) and Winkler (1980) show that sudden increases in ozone concentration at the ground surface are correlated with disturbances in the stable PBL as indicated by sodar traces. They relate these concentration increases to turbulent transport from higher levels where ozone concentrations are higher than at the ground surface.