Optical properties of cirrus clouds at a mid-latitude EARLINET station
Giannakaki, E. , Balis, D. , Amiridis, V. , Kazadzis, S.
[abstract] Measurements performed with a backscatter and Raman lidar at Thessaloniki, Greece were used to characterize cirrus clouds and aerosol layers by determining their optical properties. This is achieved through the application of different post-processing algorithms. We retrieved the cirrus clouds optical properties by using three independent mathematical methods. In the first method, an iterative procedure was used based on the criterion that forward and backward integration coincide to the desired degree of accuracy. In the second method, the optical depth of the cirrus cloud can be determined by comparing the backscattering signals just bellow and above the cloud if the lidar signals are correctly represent the scattering medium. The third method, the well known Raman method, is applicable to night time measurements and is capable for determining the vertical profile of lidar ratio. The results are considerably influenced by multiple scattering effects, that not taken into account and this leads to a significant underestimation of calculated cirrus optical depths and lidar ratios. To estimate and correct this effect we have applied a radiative transfer model that calculates the multiple scattering contributions for each cirrus case analyzed. We have compared the resulting optical depths and lidar ratios and found a good agreement between these methods. The comparison has been performed to the effective values of optical depth and lidar ratio.
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