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| Titel |
Boundary layer dynamics over London, UK, as observed using Doppler lidar during REPARTEE-II |
| VerfasserIn |
J. F. Barlow, T. M. Dunbar, E. G. Nemitz, C. R. Wood, M. W. Gallagher, F. Davies, E. O'Connor, R. M. Harrison |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 11, no. 5 ; Nr. 11, no. 5 (2011-03-09), S.2111-2125 |
| Datensatznummer |
250009450
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| Publikation (Nr.) |
 copernicus.org/acp-11-2111-2011.pdf |
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| Zusammenfassung |
| Urban boundary layers (UBLs) can be highly complex due to the heterogeneous
roughness and heating of the surface, particularly at night. Due to a
general lack of observations, it is not clear whether canonical models of
boundary layer mixing are appropriate in modelling air quality in urban
areas. This paper reports Doppler lidar observations of turbulence profiles
in the centre of London, UK, as part of the second REPARTEE campaign in
autumn 2007. Lidar-measured standard deviation of vertical velocity averaged
over 30 min intervals generally compared well with in situ sonic
anemometer measurements at 190 m on the BT telecommunications Tower. During
calm, nocturnal periods, the lidar underestimated turbulent mixing due
mainly to limited sampling rate. Mixing height derived from the turbulence,
and aerosol layer height from the backscatter profiles, showed similar
diurnal cycles ranging from c. 300 to 800 m, increasing to c. 200 to 850 m
under clear skies. The aerosol layer height was sometimes significantly
different to the mixing height, particularly at night under clear skies. For
convective and neutral cases, the scaled turbulence profiles resembled
canonical results; this was less clear for the stable case. Lidar
observations clearly showed enhanced mixing beneath stratocumulus clouds
reaching down on occasion to approximately half daytime boundary layer
depth. On one occasion the nocturnal turbulent structure was consistent with
a nocturnal jet, suggesting a stable layer. Given the general agreement
between observations and canonical turbulence profiles, mixing timescales
were calculated for passive scalars released at street level to reach the BT
Tower using existing models of turbulent mixing. It was estimated to take c.
10 min to diffuse up to 190 m, rising to between 20 and 50 min at
night, depending on stability. Determination of mixing timescales is
important when comparing to physico-chemical processes acting on pollutant
species measured simultaneously at both the ground and at the BT Tower
during the campaign. From the 3 week autumnal data-set there is evidence for
occasional stable layers in central London, effectively decoupling surface
emissions from air aloft. |
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