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| Titel |
Investigating fetch effects and local mixing using near-surface radon gradient measurements |
| VerfasserIn |
S. Chambers, A. G. Williams, W. Zahorowski, A. Griffiths, J. Crawford |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250060559
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| Zusammenfassung |
| Weather and climate predictions are crucially reliant upon the fidelity of model
parameterisations that represent the integrated behaviour of key physical processes
responsible for transport and mixing in the atmospheric boundary layer. The distribution of
pollutants with respect to their natural or anthropogenic sources, as well as their
removal through deposition, is also controlled by these processes. However, our
understanding of many aspects of mixing and transport still requires substantial
refinement. In the stably stratified boundary layer vertical mixing processes remain poorly
understood, particularly in very stable conditions when surface inversions can be
extremely shallow and the thermodynamic structure of the lowest 50-100 m very
complex.
Radon gradients provide a direct, unambiguous measure of near-surface atmospheric
mixing. Since they can be observed using robust, economical instruments, long-term,
continuous datasets, that are suitable for characterising a wide range of site-specific
characteristics in a statistically meaningful manner, can be routinely collected. Here a
31-month dataset of hourly radon measurements at 2 and 50 m is used to characterise the
seasonality and diurnal variability of radon concentrations and gradients at a topographically
complex site on Sydney’s urban fringe. Vertical differencing allows separation of remote
(fetch-related) effects on measured radon concentrations from those due to diurnal variations
in the strength and extent of vertical mixing. With the help of back trajectories and
model-derived mixing depths, we were able to characterise the pronounced seasonal
variability in afternoon surface radon concentrations in terms of air mass fetch,
contact time with land, ABL dilution and regional variability of the radon source
function.
Diurnal composites, grouped according to the maximum nocturnal radon gradient
(ΔCmax), reveal strong connections between radon, wind, temperature and mixing depth on
sub-diurnal timescales. Comparison of the bulk Richardson Number (RiB) and the turbulence
kinetic energy (TKE) with the radon-derived bulk diffusivity (KB) helps to elucidate the
relationship between thermal stability, turbulence intensity and the resultant mixing. On
nights with large ΔCmax, KB and TKE levels are low and RiB is well above the “critical”
value. Conversely, when ΔCmax is small, KB and TKE levels are high and RiB is near zero.
For intermediate ΔCmax, however, RiB remains small whereas TKE and KB both indicate
significantly reduced mixing. The relationship between stability and turbulence is therefore
non-linear, with even mildly stable conditions being sufficient to suppress mixing. |
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