Towards an automatic lidar cirrus cloud retrieval for climate studies

In the present study, a methodology to calculate lidar ratios for distinct cirrus clouds has been implemented for a site located in the Southern Hemisphere. The cirrus cloud lidar data processing has been developed to consider a large cloud variability with the final aim of cirrus cloud monitoring through a robust retrieval process. Among the many features lidar systems can extract for cirrus detection, we highlight: cloud geometrical information and extinction-to-backscatter ratio (also called lidar ratio - LR). LR's can, in general, provide important information on cirrus cloud microphysics due to the presence of ice crystals and their properties such as shape, size, composition and orientation of particles and their effect on LR values. Conditions for LR calculations and their resulting uncertainty have been improved as their analysis requires identifying cirrus cloud stationary periods through the use of a specific statistical approach well-established in the literature and employed here with good results, allowing for the study of specific cases with multi-layer cirrus cloud occurrence. The results from the measurements taken in the region of the Metropolitan City of Sao Paulo - MSP have been used to implement and test the methodology developed herein. In addition to the geometrical parameters extracted, improved values of LR's were calculated and showed significantly different values for the different layers inspected, varying between 19 ± 01 sr and 74 ± 13 sr. This large value interval allowed us to indirectly verify the presence of different ice crystal sizes and shapes and those associated with different air mass sources for the cirrus cloud formation.