Balloon-borne lidar observations of tropical cirrus clouds and comparison with CALIOP
Résumé
Key component of the climate system, tropical cirrus clouds modulate both the Earth’s radiative
budget and the amount of water vapor transported to the stratosphere. The range of optical depth of
such clouds pans several orders of magnitude, from thick opaque cirrus detrained from deep
convection to ultra thin ones just below the stratosphere. Although sub-visible cirrus clouds have
been largely documented from CALIOP space-borne lidar, its detection lower limit is an obstacle to
accurately quantify the tropical cirrus coverage. The microlidar BeCOOL (Balloon-borne Cirrus and
convective overshOOT Lidar) has been designed to tackle this issue. Between October 2021 and
January 2022, in the framework of Strateole-2 project, three BeCOOL microlidars have been flown
onboard super-pressure balloons in the lower tropical stratosphere (~20 km). Case studies of
collocated observations with CALIOP, along with statistical comparisons, highlight the very good
agreement between the two lidars and the enhanced sensitivity of BeCOOL to ultra thin clouds. This
higher sensitivity is achieved thanks to the low speed of the balloons and the small distance to the
observed clouds, which allow to integrate the observations over a longer time. BeCOOL’s
observations reveal the structure and the significant coverage of optically ultra thin clouds undetected
from space.