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

M. Schenkels, Ministry of Infrastructure and Water Management, Royal Netherlands Meteorological Institute (KNMI)
De Bilt : KNMI

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.

31 p.
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(Technical report = Technisch rapport ; TR-366)
With a bibliogr.