Doppler lidar observations of urban boundary layer dynamics in London, UK

Urban boundary layers can exhibit complex structure and dynamics in response to the heterogeneous surface and characteristic surface energy balance. The best known example is the urban heat island due to sustained positive sensible heat fluxes some hours after sunset. However, few measurements have been made of the turbulence structure to ascertain whether it conforms to results established using similarity theory scaling. This study presents the first Doppler lidar observations of vertical velocity variance in central London, alongside flux measurements made on the 190m high BT Tower. Measurements took place in October/November 2007 as part of the REPARTEE campaign to determine physical and chemical processes governing urban aerosol particle variability. In situ validation of the lidar-determined standard deviation of vertical velocity showed excellent agreement, apart from isolated overnight periods where sampling rate was too slow to capture all turbulent scales. Use of backscatter and velocity variance to determine aerosol layers and mixing height showed an apparent lag in the vertical growth of aerosol layers behind the turbulent mixing. Convective, stable and neutral variance profiles were observed which conformed reasonably well to existing expressions. The mixing timescales from surface up to the BT Tower were calculated to be typically 10 minutes, increasing to between 20 and 50 minutes overnight. The lidar dataset shows a very rich set of processes determining particulate transport and growth throughout the boundary layer, particularly in the more complex nocturnal boundary layer. Comparison with in situ particulate size distributions and fluxes will also be presented, and conclusions drawn concerning local and long range transport of urban particulate matter.