Impacts of depolarization calibration methods on cloud phase interpretation at Eureka during 2013 and 2014 CRL lidar measurement campaigns

Abstract:

The radiative behaviour of clouds is dependent on cloud particle phase. Water droplets can exist in temperatures well below 0° C for extended periods. Lidar depolarization measurements allow liquid and solid states to be differentiated in individual clouds at high spatial-temporal resolution. The 2012, 2013 and 2014 Canadian Arctic ACE Validation Campaigns in Eureka, Nunavut, Canada (80°N, 86°W) provided an opportunity to make extensive depolarization measurements using the CANDAC Rayleigh-Mie-Raman Lidar (CRL) in the troposphere.

To date, most calibration methods in the literature are applicable to lidars which do not have non-ideal polarizing optics upstream of the polarizing analyzers in the receiver. We demonstrate a more complete matrix algebra calibration of the CRL to take the extra optics from six upstream lidar channels into account.

Differences in depolarization parameter from 2013 and 2014 measurements show the advantage of the more extensive calibration for this lidar compared to the simpler traditional approach. The largest differences are found for depolarization parameter values around d = 0.50 (corresponding to δ = 0.33). Depolarization ratio values of δ = 0.2 to 0.3 are generally taken to be the cutoff between interpretations of ice (higher δ) or water (lower δ), and many CRL measurements lie in this particularly diagnostic range. An uncertainty analysis becomes important when extending the depolarization parameters to interpretation of the clouds in the atmosphere above Eureka.