Comparison of continuous detection of mixing layer heights by ceilometer with radiosonde observations

The mixing layer height (MLH) is an important factor which influences exchange processes of ground level emissions. The continuous knowledge of MLH is supporting the understanding of processes directing air quality. If the MLH is located near to the ground, which occurs mainly during winter and night-time, air pollution can be high due to a strongly limited air mass dilution. Ceilometers CL31 (backscatter profiles at 910 nm) were applied to detect the MLH in Augsburg since 2006. Radiosonde data cannot be used alternatively because they do not provide sufficient information. The Vaisala ceilometers LD40 and CL31 are operated which are eye-safe commercial lidar systems. Special software for these ceilometers provides routine retrievals of lower atmosphere layering from vertical profiles (vertical gradient) of laser backscatter density data. The performance of the ceilometers is sufficient to detect convective layer depths exceeding 2000 m and nocturnal stable layers down to 50 m. The radiosonde data from the station Oberschleissheim near Munich (about 50 km away from Augsburg city) are also used for MLH determination. A summer and a winter episode of MLH measurement results are investigated. The profile behaviour of relative humidity (strong decrease) and virtual potential temperature (inversion) of the radiosonde agree mostly well with the MLH indication from ceilometer laser backscatter density gradients. The remote sensing by ceilometers can fill the temporal gap of information between the two radiosonde profiles per day so that the daily course of the MLH is available.