Depolarization ratio of polar stratospheric clouds in coastal Antarctica: comparison analysis between ground-based Micro Pulse Lidar and space-borne CALIOP observations

[摘要] Polar stratospheric clouds (PSCs) play an important role in polar ozonedepletion, since they are involved in diverse ozone destruction processes(chlorine activation, denitrification). The degree of that ozone reductionis depending on the type of PSCs, and hence on their occurrence. ThereforePSC characterization, mainly focused on PSC-type discrimination, is widelydemanded. The backscattering (R) and volume linear depolarization (δ^V) ratios are the parameters usually used in lidar measurements for PSCdetection and identification. In this work, an improved version of thestandard NASA/Micro Pulse Lidar (MPL-4), which includes a built-indepolarization detection module, has been used for PSC observations abovethe coastal Antarctic Belgrano II station (Argentina, 77.9° S 34.6° W, 256 m a.s.l.) since 2009. Examination of theMPL-4 δ^V feature as a suitable index for PSC-type discriminationis based on the analysis of the two-channel data, i.e., the parallel (p-) andperpendicular (s-) polarized MPL signals. This study focuses on thecomparison of coincident δ^V-profiles as obtained fromground-based MPL-4 measurements during three Antarctic winters with thosereported from the space-borne lidar CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) aboard the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) satellite inthe same period (83 simultaneous cases are analysed for 2009–2011 australwinter times). Three different approaches are considered for the comparisonanalysis between both lidar profile data sets in order to test the degree ofagreement: the correlation coefficient (CC), as a measure of therelationship between both PSC vertical structures; the mean differencestogether with their root mean square (RMS) values found between data sets;and the percentage differences (BIAS), parameter also used in profilingcomparisons between CALIOP and other ground-based lidar systems. All of themare examined as a function of the CALIPSO ground-track distance from theBelgrano II station. Results represent a relatively good agreement betweenboth ground-based MPL-4 and space-borne CALIOP profiles of the volume lineardepolarization ratio δ^V for PSC events, once the MPL-4depolarization calibration parameters are applied. Discrepancies betweenCALIOP and MPL-4 profiles in vertical layering structure are enhanced from20 km up, likely due to a decrease of the signal-to-noise ratio (SNR) forboth lidar systems at those altitudes. Regarding the results obtained from the mean and the percentage differences found between MPL-4 and CALIOPδ^V profiles, a predominance of negative values is also observed,indicating a generalized underestimation of the MPL-4 depolarization ascompared to that reported by CALIOP. However, absolute differences betweenthose δ^V-profile data sets are no higher than a 10 ± 11%in average. Moreover, the degree of agreement between both lidar δ^V data sets is slightly dependent on the CALIPSO ground-track overpassdistance from the Belgrano II station. That is, small discrepancies arefound when CALIPSO ground-track distance is as close as far from theground-based station. These results would indicate that MPL-4 depolarizationobservations would reflect relatively well the PSC field that CALIOP candetect at relatively large distances from the ground-based station. As aconsequence, PSC properties can be statistically similar, on average, overlarge volumes, and hence the present weak disagreement found between boththe lidar δ^V data sets can be likely dominated by small spatialPSC inhomogeneities along the CALIPSO separation from the station. Thisstatement is based on the fact that Belgrano II is a station located wellinside the stable Antarctic polar vortex, allowing determined thermodynamicconditions leading to a very low variability in the PSC field, and intheir properties

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