Determining the optical thickness of cirrus clouds using a Raman aerosol lidar
G.P. Kokhanenko, Yu.S. Balin, I.E. Penner, S.V. Nasonov, V.A. Shishko
V.E. Zuev Institute of Atmospheric Optics of Siberian Branch of the Russian Academy of Science, Tomsk, Russia
Keywords: lidar, aerosol, crystalline clouds, optical depth, elastic and Raman scattering
Abstract
Cirrus clouds cover a significant part of the Earth's surface and have a significant impact on the radiation balance and climate. To calculate the solar radiation transmittance of clouds, it is necessary to control the optical thickness of clouds. Lidar sensing is an optimal method for this. The paper compares two lidar methods for determining the optical thickness τ of cirrus clouds, using elastic or Raman scattering signals. Both techniques are based on recording signals from the clear atmosphere down to and above a сloud, but the use of elastic scattering is associated with errors caused by the possible presence of sub-cloud or above-cloud aerosol layers. Different methods for processing experimental data were compared, and the error in determining the optical thickness was assessed. It is shown that deviations in retrieving the thickness from elastic scattering signals can attain ~ 0.5 for clouds with the optical thickness τ > 2. At t ≤ 1, the correlation coefficient between the data of the two methods is 0.94, which enables the use of elastic scattering signal measurements, available even during daytime, to estimate the optical thickness of cirrus clouds. The results of the work can be used in the development of lidar systems for sensing upper-level clouds.
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