Mixing and venting in clear and cloudy boundary layers using airborne radon measurements

Radon-222 is a radioactive noble gas emitted from terrestrial surfaces at a rate that can be assumed to be uniform and steady on diurnal timescales. As its continental flux is more than two orders of magnitude higher than its oceanic flux, radon is recognised as an excellent tracer for continental air and is widely used in evaluations of transport models and estimations of regional fluxes of climatically sensitive gases. Within the atmospheric boundary layer (ABL), radon can be used to construct quantitative measures of the degree of mixing and exchange with the surface and the free atmosphere. Radon is therefore a useful tool in the effort to reduce systematic errors in the quantitative representation of diurnal and seasonal cycles in weather and climate prediction models. ANSTO develops and applies state-of-the-art systems for the measurement of radon concentrations in air, and deploys them on meteorological towers and aircraft for characterisation of vertical mixing in the lower atmosphere. Together with supporting meteorological and turbulence measurements, these novel systems are yielding new insights into mixing and exchange processes across the atmospheric surface layer, the nocturnal stable layer and the convective mixed layer, and are beginning to be used in the evaluation of boundary layer mixing schemes in weather and climate models. We present vertical profiles of radon and turbulence statistics in convective daytime boundary layers over rural inland Australia measured during a number of winter and summer field campaigns using samplers mounted on a motorized glider operated by Airborne Research Australia. Conditions ranged from clear skies to moderately developed fair-weather cumulus and stratocumulus. As can be seen in the figure, radon displays a strong gradient within the (sub-cloud) mixed layer despite significant turbulent mixing in all cases. This distinctly “unmixed” shape in the radon profiles is a result of the characteristically “bottom-up top-down” nature of turbulent mixing in the convective boundary layer. Due to its 3.8-day half-life, radon concentrations in the free atmosphere are constrained to be 1-3 orders of magnitude lower than near-surface values. This ensures that a large gradient is maintained between the ABL and the air high above, leading to significant entrainment of radon across the ABL top even when the entrainment mass flux remains moderate. This process is further enhanced in the presence of active boundary layer clouds. In the cloudy cases on the right hand side of the figure (non-precipitating cumulus on 19 & 22 May 2006; stratocumulus on 24 May 2006), radon concentrations remain high throughout the cloud layer. Given that the aircraft flew mainly in the spaces between clouds, the radon data thus indicates the extent to which air is being detrained out of the clouds in what is effectively an enhanced boundary layer venting process.