Convective Downmixing of Plumes in a Coastal Environment

Abstract

This paper describes the results of an atmospheric tracer study in which sulfur hexafluoride (SF6) was used to investigate the transport and dispersion of effluent from a power plant located in a coastal environment. The field study demonstrated that material emitted into an elevated stable layer at night can be transported out over the ocean, fumigated to the surface, and then he returned at ground level by the sea breeze on the next day. At night when cool stable air from the land encounters the warmer ocean convective mixing erodes the stable layer forming an internal boundary layer. When the growing boundary layer encounters an elevated plume the pollutant material, entrained at the top of the mixed layer, can be rapidly transported in ∼20 min to the surface. Various expressions for the characteristic downmixing time (λ = Zi/w*) are developed utilizing the gradient Richardson number, the Monin-Obukhov length and turbulence intensifies. Calculations using these expressions and the field data are compared with similar studies of convective mixing over the land.