Operational performance of the Vaisala CL61 ceilometer for atmospheric profiling

The Vaisala CL61 is a new generation elastic backscatter lidar that extends the capabilities of conventional automatic low-power lidars and ceilometers by providing depolarization ratio measurements. Reliable use of these measurements, however, requires thorough evaluation and characterization of the instrument performance and subsequent corrections applied. In this study, performance of multiple CL61 instruments across different sites over 3-year period has been assessed. Results indicate some differences 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 likely due to weaker return signals. 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 is noted that once the laser power dropped below 40 % there is no further compensation in the internal calibration factor. The instrumental background noise and bias, characterized using the termination hood, are found to vary with temperature. A method has been developed for correcting the instrumental bias and for estimating the associated uncertainty. Additionally, an approach to estimate the uncertainty of volume and particle backscatter and linear depolarization ratio is presented. In a case study representing low aerosol load conditions in Finland, correcting for the instrumental bias changes the volume linear depolarization ratio by up to 0.005 relative to the instrument-provided values. On the other hand, the difference between volume linear depolarization ratio and particle linear depolarization ratio reached up to 0.1. These findings demonstrate that the CL61 firmware effectively removes the majority of the mean background bias, whereas accounting for the molecular contribution is important for quantitative interpretations of aerosol measurements at CL61's wavelength of 910.55 nm. Finally, signal loss in one unit has been traced to fogging of the inside surface of the window, and attributed to insufficient internal heating linked to the instrument's firmware behavior.