Measuring cloud optical depth with a balloonborne microlidar operated from the stratosphere

The Balloonborne Cloud Observing micrOLidar (BeCOOL) has been developed to be operated onboard a stratospheric balloon in order to monitor the atmosphere below 20 km, more particularly thin ice clouds. This backscatter lidar operating at 802 nm was designed to maintain a high level of performance while keeping its mass below 6 kg and limiting its power consumption to 4 W on average. Several balloons embarking BeCOOL instruments have been launched from the Tropics (Seychelles Islands, −4.68° S +55.45° E) during the Stratéole2 campaign organized by the French Space Agency (Centre National d'Etudes Spatiales, CNES) in autumn and winter 2021–2022. The microlidar system, its operational performances, and the data processing to estimate optical properties are described. BeCOOL is able to measure optical depth of upper level thin ice clouds down to 2×10-5. It is possible to constrain the lidar ratio when the cloud optical depth is larger than 3×10-2. In this case, the optical depth relative uncertainty is less than 5 %.