Atmospheric boundary-layer characteristics from ceilometer measurements. Part 2: Application to London's urban boundary layer

Long-term measurements of mixed layer height (Z_ML) are possible with advances in detecting Z_ML based on Automatic Lidars and Ceilometers (ALC) observations. Six years of ALC measurements in central London are analysed using the CABAM (“Characterising the Atmospheric Boundary layer (ABL) based on ALC Measurements”) algorithm which provides Z_ML and an ABL classification by cloud cover and type. The boundary-layer dynamics are shown to respond to day-length, cloud cover and cloud type. Seasonal median daily maxima range from 707 m (stratiform clouds) to 1704 m (days with convective boundary-layer clouds following a clear night). A common approach to ABL classification and clear definition of key Z_ML-indicators can facilitate inter-city comparison. A simple parametrisation based on empirical coefficients derived from the London measurements is proposed to generalise the description of diurnal and seasonal variations in Z_ML, including cloud conditions. This has the potential to aid improved understanding of the complex relations between surface air quality and boundary-layer dynamics.