Insights into long-term Atmospheric Profiling with the Vaisala CL61 Ceilometer

The Vaisala CL61 is increasingly deployed in both research infrastructures, such as ACTRIS, and operational meteorological networks for applications including aviation and air-quality forecasting. As a new generation elastic backscatter lidar, it extends conventional ceilometer capabilities by providing depolarization ratio measurement. While this measurement is highly valuable, especially for unattended, autonomous operation, its use in network applications requires careful characterization.

We developed a methodology for identifying background signals and suitable liquid cloud layers to evaluate the long-term performance of multiple CL61 instruments across different sites. Results show some variability between instruments, with several of these early production units exhibiting a pronounced decrease in laser power over time, accompanied by increased background noise. Although internal calibration normally compensates for laser power degradation, external atmospheric calibration at the Lindenberg site revealed that this compensation becomes insufficient when laser power falls below 40%.

Termination hood measurements were used to characterize instrument noise and bias profiles. Both were found to exhibit temperature dependence and to deviate from zero in the near range, below approximately 2 km but extending up to 5 km for one instrument. A method for bias correction, along with an estimation of the associated uncertainty, is presented. In addition, an aerosol inversion approach is also introduced for retrieving the profile of aerosol particle backscatter coefficient, aerosol depolarization ratio, and their corresponding uncertainties. A case study demonstrates that bias-corrected, aerosol-inverted depolarization ratio can differ by up to 0.1 from the original instrument values, emphasizing the importance of accounting for instrumental bias and, in particular, molecular contributions at the CL61 operating wavelength of 905 nm.

Lastly, we observed signal loss in one instrument and found that it was due to optical lens fogging caused by insufficient internal heating linked to firmware behaviour. It is particularly important to identify and exclude such periods to ensure the reliability of the measurement.