Polarization Lidar at Summit, Greenland, for the Detection of Cloud Phase and Particle Orientation

Abstract

Accurate measurements of cloud properties are necessary to document the full range of cloud conditions and characteristics. The Cloud, Aerosol Polarization and Backscatter Lidar (CAPABL) has been developed to address this need by measuring depolarization, particle orientation, and the backscatter of clouds and aerosols. The lidar is located at Summit, Greenland (72.68N, 38.58W; 3200 m MSL), as part of the Integrated Characterization of Energy, Clouds, Atmospheric State, and Precipitation at Summit Project and NOAA's Earth System Research Laboratory's Global Monitoring Division's lidar network. Here, the instrument is described with particular emphasis placed upon the implementation of new polarization methods developed to measure particle orientation and improve the overall accuracy of lidar depolarization measurements. Initial results from the lidar are also shown to demonstrate the ability of the lidar to observe cloud properties.

Depolarization calibration and measurements using the CANDAC Rayleigh–Mie–Raman lidar at Eureka, Canada

“…Gimmestad (2008) provides motivation for moving away from all δ descriptions, pointing out that d is consistent with the rest of optical physics and scattering theory. Expressing depolarization as d has since been adopted in such publications as Thayer (2009, 2012) and Neely III et al (2013). Results in the present paper will be provided in terms of both d and δ so that readers working under either paradigm can readily make use of the figures and calculations.…”

“…• with respect to both polarization analyzers (Spinhirne et al, 1982;McGill et al, 2002;Alvarez et al, 2006;David et al, 2012;Wang et al, 2009;Freudenthaler et al, 2009;Neely III et al, 2013;Bravo-Aranda et al, 2013;Freudenthaler, 2016). This type of calibration is not sensitive to polarization effects in the transmitter optics or to any optics upstream of the halfwaveplate.…”

“…This typically means omitting at least the telescope and sometimes more optics. A notable exception is Neely III et al (2013), which has a waveplate optic in the roof window.…”

“…In an alternate version of the half-waveplate calibration, this waveplate may be placed in the transmitter to control the orientation of the plane of polarization of the laser light transmitted to the sky (Liu and Wang, 2013;Neely III et al, 2013;Freudenthaler, 2016;Bu et al, 2017). Eloranta and Piironen (1994) use a Pockels cell in their laser to the same effect.…”

“…Other calibrations consist of adjusting compensation optics (typically by adjusting their rotation angle) which will remain in the lidar during regular measurements. Some of these are the same optics used for the calculations of k. At present, the lidar of Neely III et al (2013) seems most capable of a whole-system calibration. Their calibration optics exist in multiple places within the transmitter and multiple places within the receiver.…”