Abstract
A brief description of the Water vapor, Cloud and Aerosol Lidar (WACAL) system is provided. To calibrate the volume linear depolarization ratio, the concept of “ Δ 90 ° -calibration” is applied in this study. This effective and accurate calibration method is adjusted according to the design of WACAL. Error calculations and analysis of the gain ratio, calibrated volume linear depolarization ratio and particle linear depolarization ratio are provided as well. In this method, the influences of the gain ratio, the rotation angle of the plane of polarization and the polarizing beam splitter are discussed in depth. Two groups of measurements with half wave plate (HWP) at angles of (0 ° , 45 ° ) and (22.5 ° , −22.5 ° ) are operated to calibrate the volume linear depolarization ratio. Then, the particle linear depolarization ratios measured by WACAL and CALIOP (the Cloud-Aerosol Lidar with Orthogonal Polarization) during the simultaneous observations were compared. Good agreements are found. The calibration method was applied in the third Tibetan Plateau Experiment of Atmospheric Sciences (TIPEX III) in 2013 and 2014 in China. Vertical profiles of the particle depolarization ratio of clouds and aerosol in the Tibetan Plateau were measured with WACAL in Litang (30.03° N, 100.28° E, 3949 m above sea level (a.s.l.)) in 2013 and Naqu (31.48° N, 92.06° E, 4508 m a.s.l.) in 2014. Then an analysis on the polarizing properties of the aerosol, clouds and cirrus over the Tibetan Plateau is provided. The particle depolarization ratio of cirrus clouds varies from 0.36 to 0.52, with a mean value of 0.44 ± 0.04. Cirrus clouds occurred between 5.2 and 12 km above ground level (a.g.l.). The cloud thickness ranges from 0.12 to 2.55 km with a mean thickness of 1.22 ± 0.70 km. It is found that the particle depolarization ratio of cirrus clouds become larger as the height increases. However, the increase rate of the particle depolarization ratio becomes smaller as the height increases.
Highlights
As a significant branch in the field of lidar, polarization lidar plays an important role in the detection of spatial and temporal distribution of aerosols and clouds
Sassen and Benson (2001) studied the midlatitude cirrus clouds by using 694 nm polarization lidar and they found the depolarization ratios of cirrus display a steady increase with height [9]
The volume linear depolarization ratio is defined as the ratio of the cross-polarized lidar return signal (P⊥) to the parallel-polarized backscatter signal (P ) [28]
Summary
As a significant branch in the field of lidar, polarization lidar plays an important role in the detection of spatial and temporal distribution of aerosols and clouds It is a well-established technique to measure the profile of the physical properties of clouds [1,2] and aerosol [3,4,5]. Lidar ratio, Ångström exponent and other intensive optical properties of particles, the cloud phase, dust, smoke, urban, maritime and other types of aerosol are classified [22,23,24,25]. CALIOP provides several optical properties of aerosol and clouds such as backscatter coefficient, particle linear depolarization ratio and color ratio. Algorithms based on integrated attenuated backscatter and depolarization ratio have been developed to classify aerosol and cloud [26]
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