Abstract
Abstract. The unambiguous retrieval of cloud phase from polarimetric lidar observations is dependent on the assumption that only cloud scattering processes affect polarization measurements. A systematic bias of the traditional lidar depolarization ratio can occur due to a lidar system's inability to accurately measure the entire backscattered signal dynamic range, and these biases are not always identifiable in traditional polarimetric lidar systems. This results in a misidentification of liquid water in clouds as ice, which has broad implications on evaluating surface energy budgets. The Clouds Aerosol Polarization and Backscatter Lidar at Summit, Greenland employs multiple planes of linear polarization, and photon counting and analog detection schemes, to self evaluate, correct, and optimize signal combinations to improve cloud classification. Using novel measurements of diattenuation that are sensitive to both horizontally oriented ice crystals and counting system nonlinear effects, unambiguous measurements are possible by over constraining polarization measurements. This overdetermined capability for cloud-phase determination allows for system errors to be identified and quantified in terms of their impact on cloud properties. It is shown that lidar system dynamic range effects can cause errors in cloud-phase fractional occurrence estimates on the order of 30 % causing errors in attribution of cloud radiative effects on the order of 10–30 %. This paper presents a method to identify and remove lidar system effects from atmospheric polarization measurements and uses co-located sensors at Summit to evaluate this method. Enhanced measurements are achieved in this work with non-orthogonal polarization retrievals as well as analog and photon counting detection facilitating a more complete attribution of radiative effects linked to cloud properties.
Highlights
Changing Arctic conditions lead to many changes in regional surface energy and mass budgets, which have a profound impact on humans outside the region (Curry et al, 1996; Hansen et al, 2011)
By assuming the more general form of the backscattering phase matrix, Eq (A3), which allows for horizontal orientation of scatterers as opposed to only random orientation, and observing scatterers in an off-zenith direction, no ambiguity arises in the interpretation of depolarization measurements as seen for example by Thomas et al (1990) or Winker et al (2009) where low depolarization, typically associated with liquid, from ice is observed from organized specular reflections off horizontally oriented ice crystals (HOICs)
This work will not perform the gluing procedure presented for several reasons: first it is impractical to calculate gluing coefficients for Clouds Aerosol Polarization and Backscatter Lidar (CAPABL) by atmospheric calibration as access to the CAPABL system is limited to once or twice a year, second it is not clear how to combine analog and photon counting signals at a single height to adequately account for error introduced by temporal variation of gluing coefficients, third it is not clear how the range correlation of signals required for the Klett inversion method is affected by the thresholds of the gluing procedure, and combining the data at the product level, and not the raw data level, is already required to combine orthogonal and non-orthogonal retrievals
Summary
Changing Arctic conditions lead to many changes in regional surface energy and mass budgets, which have a profound impact on humans outside the region (Curry et al, 1996; Hansen et al, 2011). Though liquid-only and mixed-phase clouds can be found up to heights of approximately 6 km above mean sea level (a.m.s.l.) in the Arctic, they have been found by many to be predominately low-lying with high optical thickness (Curry et al, 1996; Intrieri et al, 2002; Turner, 2005; Shupe et al, 2006; de Boer et al, 2009; Shupe, 2011; Shupe et al, 2013) Such characteristics make these clouds hard to measure accurately from both the ground and space.
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