LACRIT: an automated lidar framework for detecting cirrus and contrails and retrieving their radiative properties

Abstract. The accurate retrieval of radiative properties for cirrus and contrail-induced clouds remains a significant challenge, particularly in the optically thin and subvisible regimes where molecular scattering dominates and signal-to-noise constraints are critical. We present a layer-based retrieval framework for Cloud Optical Depth (COD) and Lidar Ratio (LR) utilizing complementary ground-based observations from a co-located elastic–depolarization lidar, microwave radiometer, and all-sky camera deployed in Toulouse, France. Here, COD is retrieved using a transmittance method grounded in robust log-median statistics applied to molecular reference windows. Subsequently, a modified version of the particular-integration formulation is employed to retrieve LR, ensuring consistent altitude indexing between normalized signal ratios and the physical molecular profile. The close agreement observed between transmittance-based and particular-integration CODs, with relative differences lower than 4 %, underscores the internal consistency and numerical stability of the retrieval method across a broad range of optical depths, including subvisible cloud systems with COD lower than 0.05.

When applied to representative cirrus and contrail cases, the framework reveals significant variability in retrieved LR for similar COD, highlighting the sensitivity of LR to factors such as cloud altitude which is linked with thermodynamic conditions and microphysical state of the cloud ice crystals. Nearly-fresh contrails display lower LR and depolarization around 23 % which is consistent with expected ice-particle evolution. This proposed framework offers a solid foundation for long-term monitoring of cirrus and contrail radiative properties, as well as for evaluating model parameterizations of aviation-induced cloudiness.