Recommendations for processing atmospheric attenuated backscatter profiles from Vaisala CL31 Ceilometers

Ceilometer lidars are used for cloud base height detection, to probe aerosol layers in the atmosphere (e.g. detection of elevated layers of Saharan dust or volcanic ash) and to examine boundary layer dynamics. Sensor optics and acquisition algorithms can strongly influence the observed attenuated backscatter profiles so interpretation can require corrections to be applied carefully. This study addresses the commonly deployed Vaisala CL31 ceilometers. Attenuated backscatter profiles are studied to evaluate the impact of both hardware generation and firmware version. In response to this work and discussion within the CL31/TOPROF user community (a European COST Action aiming to harmonise remote sensing networks across Europe), Vaisala released new firmware for the CL31 sensors. These are tested against previous versions showing that several artificial features introduced by the data processing have been removed. Correction procedures are developed to account for the range-dependent electronic background signal and to correct for artefacts in data collected with older firmware. Recommendations are made for the processing of attenuated backscatter observed with Vaisala CL31 sensors, including the estimation of noise which is not provided in the standard CL31 output. Taking these aspects into account, attenuated backscatter profiles from Vaisala CL31 ceilometers are considered to provide valuable information for a range of applications including, e.g. atmospheric boundary layer studies, detection of elevated aerosol layers, and model verification.