On the convective origin of tropical upper-tropospheric cirrus using 17 years of CALIOP observations and Lagrangian trajectories

Tropical cirrus clouds are commonly divided into those detrained from convection and those formed in situ. However, the relative contribution of these two categories to tropical high-cloud cover—particularly for thin cirrus in the tropical tropopause layer (TTL)—remains poorly constrained. Here, we take advantage of the 17-year CALIOP spaceborne lidar record to revisit the convective origin of tropical cirrus.

We perform systematic diabatic backward Lagrangian calculations starting from CALIOP curtain observations of both cloudy and clear-sky air, using ERA5 reanalysis winds and heating rates. Air parcels are followed for up to three months or until they intersect a convective cloud, defined when parcel temperature drops below the local brightness temperature inferred from geostationary satellite observations. Using a convective-origin criterion based on the evolution of relative humidity along trajectories, we classify cirrus into convective and in situ categories and characterize their climatology across the tropical band. We further investigate their space-time variability, as well as the geographic origin and transport lifetime of convective cirrus.