Boundary-layer dynamics over London, UK, as observed using Doppler lidar

Barlow, J.F.; Dunbar, T.M.; Nemitz, E.G.; Wood, C.R.; Gallagher, M.W.; Davies, F.; O'Connor, E.; Harrison, R.M.. 2010 Boundary-layer dynamics over London, UK, as observed using Doppler lidar. Atmospheric Chemistry and Physics Discussions, 10. 19901-19938. 10.5194/acpd-10-19901-2010

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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 5 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 190m on the BT telecommunications tower. During calm, nocturnal 10 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 ca. 300 to 800 m, increasing to ca. 200 to 850m under clear skies. The aerosol layer height was sometimes significantly different to the mixing height, particularly at night under clear skies. For con15 vective 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 20 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 ca. 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 25 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

Item Type: Publication - Article
Digital Object Identifier (DOI): 10.5194/acpd-10-19901-2010
Programmes: CEH Topics & Objectives 2009 - 2012 > Biogeochemistry > BGC Topic 1 - Monitoring and Interpretation of Biogeochemical and Climate Changes
CEH Sections: Billett (to November 2013)
ISSN: 1680-7367
Additional Information. Not used in RCUK Gateway to Research.: Atmospheric Chemistry and Physics Discussions is an Open Access journal - to access fulll text, please click on the OFFICIAL URL link
NORA Subject Terms: Atmospheric Sciences
Date made live: 16 Dec 2010 11:44

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