Operational performance of the Vaisala CL61 ceilometer for atmospheric profiling

Abstract. The Vaisala CL61 is a new generation elastic backscatter lidar that extends conventional ceilometer capabilities by providing depolarization ratio measurements. Reliable use of these measurements, however, requires thorough evaluation and characterisation of the instrument performance and subsequent corrections applied. This study introduces a methodology for identifying the background signal and suitable liquid cloud layers for assessing the long-term behavior of multiple CL61 instruments deployed across various sites. Results indicate some variability between instruments, with most of these early production units exhibiting a pronounced decrease in laser power over time, accompanied by an increase in background noise. Normally, the instrument scales the internal calibration factor to compensate for changes in laser power and thus provide consistent attenuated backscatter coefficient values from profile to profile over time. However, for the instrument at the Lindenberg site, by performing manual calibration with atmospheric targets it was noted that once the laser power dropped below 40% there was no further compensation in the internal calibration factor.

The instrumental noise and bias, characterized using the termination hood, were found to vary with temperature. A method was developed for correcting for the instrumental bias and for estimating the associated uncertainty. Additionally, an aerosol inversion approach is presented for retrieving the profile of aerosol particle backscatter coefficient, aerosol depolarization ratio, and their corresponding uncertainties. In a case study, the aerosol-inverted and bias-corrected depolarization ratio was found to deviate by up to 0.1 from the original instrument-provided measurement. This demonstrates the importance of accounting for the molecular contribution when qualitatively interpreting aerosol measurements at the CL61 ceilometer operating wavelength of 905 nm. Finally, signal loss in one unit was traced to optical lens fogging, and attributed to insufficient internal heating linked to the instrument's firmware behavior.