Extensive coverage of ultrathin tropical tropopause layer cirrus clouds revealed by balloon-borne lidar observations

Lesigne, Thomas; Ravetta, François; Podglajen, Aurélien; Mariage, Vincent; Pelon, Jacques

Tropical tropopause layer (TTL) clouds have a significant impact on the Earth's radiative budget and regulate the amount of water vapor entering the stratosphere. Estimating the total coverage of tropical cirrus clouds is challenging, since the range of their optical depth spans several orders of magnitude, from thick opaque cirrus detrained from convection to sub-visible clouds just below the stratosphere. During the Strateole-2 observation campaign, three microlidars were flown on board stratospheric superpressure balloons from October 2021 to late January 2022, slowly drifting only a few kilometers above the TTL. These measurements have unprecedented sensitivity to thin cirrus and provide a fine-scale description of cloudy structures both in time and in space. Case studies of collocated observations with the spaceborne Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) show very good agreement between the instruments and highlight the Balloon-borne Cirrus and convective overshOOt Lidar's (BeCOOL) higher detection sensitivity. Indeed, the microlidar is able to detect optically very thin clouds (optical depth inline-formula M1inlinescrollmathml italic τ < normal 2 × normal 10 - normal 3 60pt14ptsvg-formulamathimga0c9a08306df74bff5a82e7afe8e1086 acp-24-5935-2024-ie00001.svg60pt14ptacp-24-5935-2024-ie00001.png ) that are undetected by CALIOP. Statistics on cloud occurrence show that TTL cirrus appear in about 50 % of the microlidar profiles and have a mean geometrical depth of 1 km. Ultrathin TTL cirrus (inline-formula M2inlinescrollmathml italic τ < normal 2 × normal 10 - normal 3 60pt14ptsvg-formulamathimgeb3910891512e2b39cb2804634f14b9d acp-24-5935-2024-ie00002.svg60pt14ptacp-24-5935-2024-ie00002.png ) have a significant coverage (23 % of the profiles), and their mean geometrical depth is 0.5 km.

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Lesigne, Thomas / Ravetta, François / Podglajen, Aurélien / et al: Extensive coverage of ultrathin tropical tropopause layer cirrus clouds revealed by balloon-borne lidar observations. 2024. Copernicus Publications.

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