Comparison of Two LIDAR Types During the ClearfLo Project in London

The interaction between surface heat fluxes, mixing height (MH) and local air quality play a particularly important role for human health conditions in the urban environment. These aspects, amongst others, were addressed by the NERC funded ClearfLo project undertaken in London, UK. To obtain information about the depth of the boundary layer and variations in aerosol profile concentrations two types of LiDAR were used: Vaisala CL31 (Finland) and HALO Doppler Lidar (UK). Results from the two LiDAR instruments are compared, including the raw backscatter profiles (vertical gradients and signal-to-noise ratio) and the derived cloud base height and MH. The effect of data processing (vertical and temporal smoothing) on derived MH is evaluated. Cloud base height and MH are compared between instrument types and between various locations across London. Finally, the aerosol layer dynamics (characterised by backscatter profiles) are put into context by using the turbulence derived MH (from vertical velocity variance observed with the Doppler LiDAR). Using these data an assessment is made of the spatial and temporal variations in boundary layer height across London. Consideration is given to the implications of the strength and weaknesses of the observation techniques (e.g. performance under clear and cloudy conditions).