Atmospheric flow decoupling and its effects on urban plume dispersion

The Air Resources Laboratory, Field Research Division of NOAA (NOAA/ARLFRD) conducted a set of field experiments in Oklahoma City during July, 2003 designed to measure dispersion of pollutant plumes in an urban setting. SF6 tracer gas was released in the downtown area and measured at an array of fixed, time-integrated bag samplers and fixed continuous real time analyzers deployed downwind of the release site. One of the more prominent observations made during this study was the phenomenon of nocturnal flow decoupling. Flow decoupling was expressed as alterations to the wind speed profile as well as by slower plume travel speeds and longer plume decay times, diminished effectiveness of plume dilution and vertical mixing, changes in the character of plume concentration fluctuations, and reduced penetration of clean air aloft into the canopy. The primary determinant for the onset of decoupling was the change in thermal regime (day versus night). An understanding of flow decoupling is essential for reliably anticipating possible plume dispersion scenarios.