Description: [1]  Assessing the accuracy of the Aerosol-Above-Cloud (AAC) properties derived by CALIOP (the Cloud-Aerosol LIdar with Orthogonal Polarization) is challenged by the shortage of accurate global validation measurements. We have used measurements of aerosol vertical profiles from the NASA Langley airborne High Spectral Resolution Lidar (HSRL-1) in 86 CALIOP-coincident flights to evaluate CALIOP AAC detection, classification and retrieval. Our study shows that CALIOP detects ~23% of the HSRL-detected AAC (i.e. CALIOP records at least one valid aerosol extinction coefficient above the cloud with coincident HSRL AAC). According to our CALIOP-HSRL dataset (mostly over Eastern, Central and South Central United States), the majority of AAC aerosol optical depth (AOD) values are 〈 0.1 at 532 nm. Our analyses show that the standard CALIOP retrieval algorithm substantially underestimates the occurrence frequency of AAC over North America when optical depths are less than ~0.02. Those aerosols with low AOD values can still have a consequent radiative forcing effect depending on the underlying cloud cover and overlying aerosol absorption properties. We find essentially no correlation between CALIOP and HSRL AAC AOD (R 2  = 0.27, N = 151), with a majority of the CALIOP AOD retrievals (68%) outside the ±40% envelope of the CALIOP = HSRL line during both nighttime and daytime operations. We show that the CALIOP underestimation of AAC is mostly due to tenuous aerosol layers with backscatter less than the CALIOP detection threshold. The application of an alternate CALIOP AAC retrieval method (Depolarization Ratio or DR) to our dataset yields very few coincident cases. However, when available, the three different observations of AAC AOD (CALIOP standard, CALIOP alternate and HSRL) show reasonable agreement. We stress the need for more extensive suborbital CALIOP validation campaigns to acquire a process-level understanding of AAC implications (e.g. AAC-radiation interaction) and further evaluate CALIOP's AAC detection and retrieval capability, especially over the ocean and in different parts of the world where AAC are more frequently observed and show higher values of aerosol optical depth.