On the limits of CALIOP for constraining modelled free‐tropospheric aerosol

The space‐borne Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument provides valuable information on the vertical distribution of global aerosol, and is often used to evaluate vertical aerosol distributions in General Circulation Models (GCMs). Here we show, however, that the detection limit of the CALIOP retrievals mean background aerosol is not detected, leading to substantially skewed statistics that moreover differ significantly by product. In the CALIOP Level 2 product this missing low‐backscatter aerosol results in the retrieved aerosol distribution significantly over‐representing aerosol backscatter and extinction in the mid‐ and upper‐troposphere if taken to be representative of the undetected aerosol. The CALIOP Level 3 product assumes no aerosol where none is detected which then leads to an under‐estimation in the aerosol extinction profile in the upper troposphere. Using the ECHAM‐HAM GCM, we estimate that CALIOP nighttime (daytime) retrievals miss 41% (44%) of aerosol mass in the atmosphere