Raman-Mie Lidar measurements of low and optically thin cloud

摘要

In this paper, we implement and compare two complementary methods for the measurement of low cloud optical depth with a Raman-Mie lidar over the metropolitan area of New York City. The first approach, based on the method of S. Young, determines the cloud optical depth by regressing the elastic signal against a molecular reference signal above and below the cloud. Due to high aerosol loading below and above the low cloud, correction for aerosol influence was necessary and achieved with the combined Raman-elastic returns. The second approach uses N_2-Raman signal to derive cloud extinction profiles and then integrate them to determine optical depth. We find excellent agreements between these two retrievals for cloud optical depths as large as 1.5. Extinction-to-backscatter ratio within the low cloud is obtained and is shown to be consistent to values calculated from liquid water cloud model. The varied lidar ratios at cloud edge may imply the changes of cloud droplet size providing clues to the CCN seeding process. Furthermore, multiple-scattering effects on retrieving cloud optical depths are estimated by using an empirical model and specific lidar parameters.