Cirrus clouds in the tropical tropopause layer observed at Koror, Palau (7.34°N, 134.47°E)

A Compact Cloud and Aerosol LIDAR (ComCAL) is operated in Koror, Palau (7.34°N, 134.47°E) since 2018. Palau is located in the Pacific warm pool, which plays an important role in global stratosphere-troposphere exchange in the upper troposphere and the lower stratosphere (UTLS). ComCAL is operated during nighttime, carried out observations of atmospheric profiles of aerosols and clouds, and the lidar profile extends from 8 km to 30 km. Cirrus clouds were detected with very high occurrence in the upper troposphere (above 12 km). The subvisible clouds (with an optical thickness of less than 0.3) often occur in the higher region of the tropical tropopause layer (TTL) above about 16 km which is close to the cold point. The transport of air in this layer with thin cirrus and subvisible clouds was investigated by the TRACZILLA Lagrangian model, a variation of FLEXPART. The back-trajectory analysis gives insight into the origins of cirrus clouds in the TTL whether it is related to the convection or the in situ uplifting of the air masses.