Characterizing London’s Urban Boundary Layer Based on Ceilometer Observations

Long-term statistics of atmospheric boundary layer (ABL) dynamics are required for the interpretation of seasonal variations and trends in surface air quality observations and for the evaluation of chemistry transport models. Using aerosols as tracers, attenuated backscatter recorded by e.g. automatic lidars and ceilometers (ALC) can portray the various layers in the ABL. Especially in cities, where aerosol loading is often high, even sensors with a lower signal-to-noise ratio (SNR) can be used successfully to track the mixed layer height (Z_ML). As ALC operate automatically and require only little maintenance, they are most suitable to continuously monitor profiles of attenuated backscatter with high temporal and vertical resolution.

A new algorithm is presented to Characterise the Atmospheric Boundary layer based on ALC Measurements (CABAM). Using profiles of attenuated backscatter and cloud base height reported by a Vaisala CL31 ceilometer, CABAM tracks Z_ML and additional significant aerosol layers that might contribute to the ABL, e.g. in form of the nocturnal residual layer. Further, the algorithm includes a first-order ABL classification scheme that facilitates the analysis and interpretation of mixed layer results, and a novel rainfall flag based on the ALC attenuated backscatter. Both mixed layer height and classification scheme show good performance when compared to independent reference measurements. As the CL31 has the advantage of reaching complete overlap already at 70 m range, it is possible to detect Z_ML even during stable night-time conditions. detect the mixed layer height from ALC measurements, with a specific focus on the near-range.

CABAM is successfully applied to characterise the urban boundary layer over central London, UK. Diurnal variations in Z_ML are found to depend on season but also change with ABL class. Links between turbulent fluxes of sensible and latent heat with ABL characteristics as well as the impact of Z_ML on pollution concentrations observed in the dense city centre are being explored.