After sunset, the ground begins to cool, in turn cooling the air immediately above it. In many places, this process often creates a 10- to 100-meter-thick blanket of cold, calm air known as the stable atmospheric boundary layer (SABL).
Within the SABL, winds are weak. Cool air sinks, preventing the vertical mixing that usually disperses pollutants into the atmosphere during the day. Without strong wind, small-scale “submeso” air movements—e.g., cold air sliding down a gentle slope—dictate airflow. Sporadic submeso motions cause intermittent gravity waves, which allow cool air to escape the SABL, carrying pollutants with it.
Predicting intermittent turbulence in the SABL is important for understanding nighttime pollutant dispersion. But, so far, computer models of the SABL are unable to realistically generate submeso motions. Even state-of-the-art models cannot capture seemingly random turbulence in the SABL.
To set the stage for future insights, Sun et al. have now compiled and reviewed the latest knowledge on one potential mechanism for SABL turbulence: wave-turbulence interaction.
Waves in the air are a type of submeso motion that may lead to turbulence. In turn, turbulence can affect waves. Together, they may impact intermittent turbulence events that vertically transport pollutants from the SABL.
The authors delve into current theory, high-precision observations, and the latest numerical models of wave-turbulence interaction. The analysis is based on published literature and discussions from a recent workshop organized by the National Center for Atmospheric Research.
Key research strategies could speed progress. For example, the scientists propose a field experiment that specifically focuses on wave-turbulence interaction in the SABL. Field observations could help make theoretical models more realistic, and the authors also call for wider collaboration to collect data in more locations.
Such advancements would improve predictions of pollutant dispersion and formation of fog and potentially damaging frost. Such advancements could eventually have geophysical implications far beyond the SABL, such as similar intermittent turbulent events in the upper atmosphere, clouds, the ocean, and even the solar wind—the stream of charged plasma particles that flows from the Sun. (Reviews of Geophysics, doi:10.1002/2015RG000487, 2015)
—Sarah Stanley, Freelance Writer
Citation: Stanley, S. (2016), What drives pollutant dispersion at night?, Eos, 97,doi:10.1029/2016EO045287. Published on 8 February 2016.