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Article Type:
Research Article

The Vertical Distribution of Radon in Clear and Cloudy Daytime Terrestrial Boundary Layers

Alastair G. Williams Australian Nuclear Science and Technology Organisation, Sydney, New South Wales, Australia

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Wlodek Zahorowski Australian Nuclear Science and Technology Organisation, Sydney, New South Wales, Australia

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Scott Chambers Australian Nuclear Science and Technology Organisation, Sydney, New South Wales, Australia

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Alan Griffiths Australian Nuclear Science and Technology Organisation, Sydney, New South Wales, Australia

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Jörg M. Hacker Airborne Research Australia, Flinders University of South Australia, Adelaide, South Australia, Australia

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Adrian Element Australian Nuclear Science and Technology Organisation, Sydney, New South Wales, Australia

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Sylvester Werczynski Australian Nuclear Science and Technology Organisation, Sydney, New South Wales, Australia

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Print Publication:
01 Jan 2011
DOI:
https://doi.org/10.1175/2010JAS3576.1
Page(s):
155–174
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Abstract

Radon (222Rn) is a powerful natural tracer of mixing and exchange processes in the atmospheric boundary layer. The authors present and discuss the main features of a unique dataset of 50 high-resolution vertical radon profiles up to 3500 m above ground level, obtained in clear and cloudy daytime terrestrial boundary layers over an inland rural site in Australia using an instrumented motorized research glider. It is demonstrated that boundary layer radon profiles frequently exhibit a complex layered structure as a result of mixing and exchange processes of varying strengths and extents working in clear and cloudy conditions within the context of the diurnal cycle and the synoptic meteorology. Normalized aircraft radon measurements are presented, revealing the characteristic structure and variability of three major classes of daytime boundary layer: 1) dry convective boundary layers, 2) mixed layers topped with residual layers, and 3) convective boundary layers topped with coupled nonprecipitating clouds. Robust and unambiguous signatures of important atmospheric processes in the boundary layer are identifiable in the radon profiles, including “top-down” mixing associated with entrainment in clear-sky cases and strongly enhanced venting and subcloud-layer mixing when substantial active cumulus are present. In poorly mixed conditions, radon gradients in the daytime atmospheric surface layer significantly exceed those predicted by Monin–Obukhov similarity theory. In two case studies, it is demonstrated for the first time that a sequence of vertical radon profiles measured over the course of a single day can consistently reproduce major structural features of the evolving boundary layer.

Corresponding author address: Alastair Williams, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia. Email: alastair.williams@ansto.gov.au

Abstract

Radon (222Rn) is a powerful natural tracer of mixing and exchange processes in the atmospheric boundary layer. The authors present and discuss the main features of a unique dataset of 50 high-resolution vertical radon profiles up to 3500 m above ground level, obtained in clear and cloudy daytime terrestrial boundary layers over an inland rural site in Australia using an instrumented motorized research glider. It is demonstrated that boundary layer radon profiles frequently exhibit a complex layered structure as a result of mixing and exchange processes of varying strengths and extents working in clear and cloudy conditions within the context of the diurnal cycle and the synoptic meteorology. Normalized aircraft radon measurements are presented, revealing the characteristic structure and variability of three major classes of daytime boundary layer: 1) dry convective boundary layers, 2) mixed layers topped with residual layers, and 3) convective boundary layers topped with coupled nonprecipitating clouds. Robust and unambiguous signatures of important atmospheric processes in the boundary layer are identifiable in the radon profiles, including “top-down” mixing associated with entrainment in clear-sky cases and strongly enhanced venting and subcloud-layer mixing when substantial active cumulus are present. In poorly mixed conditions, radon gradients in the daytime atmospheric surface layer significantly exceed those predicted by Monin–Obukhov similarity theory. In two case studies, it is demonstrated for the first time that a sequence of vertical radon profiles measured over the course of a single day can consistently reproduce major structural features of the evolving boundary layer.

Corresponding author address: Alastair Williams, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia. Email: alastair.williams@ansto.gov.au

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