The seasonal variation of the urban convective boundary layer: results from the ACTUAL project for London

The depth of the boundary layer over urban areas has a great impact on pollutant concentrations due to mixing and hence dilution. The evolution of the convective boundary layer depends heavily on the characteristics of the underlying surface and presents differences over areas with land-use differences. In particular, thermal and mechanical (surface shear) controls of the convective boundary layer will differ over urban and rural areas, due to differences in evapotranspiration properties, thermal inertia, anthropogenic heat release, as well as aerodynamic roughness characteristics.Observations throughout the depth of the urban boundary layer have been lacking, impeding systematic study of urban boundary layer structure and evolution and whether it differs from classical boundary layer behaviour. The aim of the present study was to examine the seasonal variation of the major phases (morning expansion, midday developed and afternoon decay) of the diurnal cycle of the convectiveboundary layer over urban and rural areas, using more than 1 year of measurements taken in the frame of the ACTUAL (Advanced Climate Technology Urban Atmospheric Laboratory) project.Fluxes and meteorological conditions were monitored in central London, UK, using two identical eddy covariance systems with weather stations, operating at 190m and 18m agl. A Doppler lidar system operating in two modes (continuous stare mode and Doppler Beam Swinging mode)observed the vertical profiles of turbulence and wind speed, and was used to derive mixing heights. For the same period an eddy covariance system was in operation and a Doppler lidar observed the vertical turbulence structure of the rural boundary layer at the Chilbolton Observatory (100 km south-west of London). The seasonal variability of the daytime mixing height observed over the urban area is compared with the seasonal variability over the rural area. Differences observed in the daytime mixing heights and turbulence structure over London and Chilbolton as a response to the different surface characteristics will be discussed.