| 05 Jan 2026
Measuring cloud optical depth with a balloonborne microlidar operated from the stratosphere
Abstract. 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 lidar system 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 STRATEOLE-2 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 10 %.
Review: Measuring cloud optical depth with a balloonborne microlidar operated from the stratosphere
The article describes the technical and data processing aspects of the balloon-borne lidar BeCOOL. A technique for deriving optical depth of upper tropospheric clouds is also shown. Overall the article is well written, the technique is well described and mature, the overlap calibration is reasonable, the transmission retrieval using forward and backwards integration is correct, the overlap correction and intentional misalignment is novel, the signal normalization using ECMWF is reasonable, and the two approaches for constrained vs. unconstrained optical depth retrievals make sense. I have no major comments or concerns for the article.
Minor points and comments:
The following points can be addressed or ignored by the authors as they choose. They are only suggestions, minor corrections or comments.
1) The Introduction could use a short paragraph about the importance of measuring clouds in the UTLS. Not critical, but it would provide some motivation for flying lidars on balloons.
2) Line 49: "BECOOL" --> "BeCOOL"
3) Figure 1. Please ensure all acronyms are explained in the caption.
4) Figure 2. We have started to 3D print our entire detector assemblies using carbon fibre. It is lighter, very rigid, and has nearly no thermal expansion. We have shown the technique to Gordien Strato and Jacques Porteneuve. It might be worth considering for future versions of BeCOOL
5) Line 129. Are you referring to the Junge Layer?
6) Adding a short paragraph to the conclusions addressing the significance of improved optical depth measurements would tie the article together nicely. Match with the points raised in Introduction paragraph.