Aerosol Optical Depth and Cloud Parameters from Ascension Island retrieved with a UV-depolarisation Lidar : An outlook on the validation

Aerosol-Cloud Interactions (ACIs) are one of the least understood climate feedbacks. Ground-based remote sensing has a large potential to study these interactions. In this study a UV-depolarisation lidar has been set up on Ascension Island, a remote island in the southeast Atlantic Ocean, for one month in summer 2016 and one month in summer 2017, to study the aerosol, cloud microphysical properties, and their interaction. In clear-sky (cloud-free) periods, the backscattered signal is used to calculate the Aerosol Optical Depth (AOD). The AOD is a measure of the attenuation of the lidar beam due to suspended particles, which can be estimated in clear-sky, by integration of the extinction profile over the column and accounting for the Rayleigh (molecular) attenuation. A UV-depolarisation lidar measures not only the backscattered signal but also the depolarisation of the returned signal. Depolarisation of the lidar beam can occur due to multiple-scattering inside liquid water clouds. The multiple-scattering inside the cloud near the cloud-base was simulated using a Monte Carlo (MC) model inside an idealised semi-adiabatic liquid water cloud. Using lookup tables generated by the MC model, the cloud microphysical properties such as the cloud droplet number concentration and the effective radius were derived from the depolarisation ratio observed by the lidar. The aim of the study was to validate these retrieval methods. The AOD product was compared to AERONET data and data from hand-held sun-photometers. The multiplescattering (MS) based inversion method for the retrieval of the cloud droplet effective radius was compared to a method using radar observations on the island from the US ARM mobile facility.

The lidar AOD retrievals show similar values in 2016 as the AERONET data. In 2017 the AOD lidar retrievals do not correlate well with the AERONET, Calitoo and Microtops data. Misalignment has caused difficulties in the AOD retrieval in 2017. The effective radius in clouds retrieved from the lidar data could not be compared to the methods with radar data in 2016, because the radar was operating for only 9 days of our measurement period. In 2017 the lidar retrieved effective radius agrees within the error bands with the radar retrieved effective radius. These results show the UV-depolarisation lidar to be a potentially useful instrument to study the AOD and the cloud droplet effective radius. With more research focusing on the validation of the methods to employ the lidar, the instrument could prove valuable for Aerosol-Cloud Interaction monitoring.

Keywords: UV-depolarisation lidar, aerosol optical depth, multiple-scattering, cloud effective radius, AERONET, Calitoo, Microtop, Cloud radar, Ascension Island