Polar stratospheric cloud climatology based on CALIPSO spaceborne lidar measurements from 2006 to 2017
Authors: Pitts Michael C. , Poole Lamont R. , Gonzalez Ryan 
Published: August 2018

Abstract. The Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on the CALIPSO
(Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations)
satellite has been observing polar stratospheric clouds (PSCs) from mid-June
2006 until the present. The spaceborne lidar profiles PSCs with unprecedented
spatial (5 km horizontal×180 m vertical)
resolution and its dual-polarization capability enables classification of
PSCs according to composition. Nearly coincident Aura Microwave Limb Sounder
(MLS) measurements of the primary PSC condensables (HNO3 and
H2O) provide additional constraints on particle composition. A new
CALIOP version 2 (v2) PSC detection and composition classification algorithm
has been implemented that corrects known deficiencies in previous algorithms
and includes additional refinements to improve composition discrimination.
Major v2 enhancements include dynamic adjustment of composition boundaries to
account for effects of denitrification and dehydration, explicit use of
measurement uncertainties, addition of composition confidence indices, and
retrieval of particulate backscatter, which enables simplified estimates of
particulate surface area density (SAD) and volume density (VD). The over 11
years of CALIOP PSC observations in each v2 composition class conform to
their expected thermodynamic existence regimes, which is consistent with
previous analyses of data from 2006 to 2011 and underscores the robustness of
the v2 composition discrimination approach. The v2 algorithm has been applied to the CALIOP dataset to produce a PSC
reference data record spanning the 2006–2017 time period, which is the
foundation for a new comprehensive, high-resolution climatology of PSC
occurrence and composition for both the Antarctic and Arctic. Time series of
daily-averaged, vortex-wide PSC areal coverage versus altitude illustrate
that Antarctic PSC seasons are similar from year to year, with about 25 %
relative standard deviation in Antarctic PSC spatial volume at the peak of
the season in July and August. Multi-year average, monthly zonal mean cross
sections depict the climatological patterns of Antarctic PSC occurrence in
latitude–altitude and also equivalent-latitude–potential-temperature
coordinate systems, with the latter system better capturing the
microphysical processes controlling PSC existence. Polar maps of the
multi-year mean geographical patterns in PSC occurrence frequency show a
climatological maximum between longitudes 90∘ W and
0∘, which is the preferential region for forcing by orography and
upper tropospheric anticyclones. The climatological mean distributions of
particulate SAD and VD also show maxima in this region due to the large
enhancements from the frequent ice clouds. Stronger wave activity in the Northern Hemisphere leads to a more disturbed
Arctic polar vortex, whose evolution and lifetime vary significantly from
year to year. Accordingly, Arctic PSC areal coverage is distinct from year
to year with no “typical” year, and the relative standard deviation in
Arctic PSC spatial volume is >100 % throughout most of the
season. When PSCs are present in the Arctic, they most likely occur between
longitudes 60∘ W and 90∘ E, which is consistent with the
preferential location of the Arctic vortex. Comparisons of CALIOP v2 and Michelson Interferometer for Passive
Atmospheric Sounding (MIPAS) Antarctic PSC observations show excellent
correspondence in the overall spatial and temporal evolution, as well as for
different PSC composition classes. Climatological patterns of CALIOP v2 PSC
occurrence frequency in the vicinity of McMurdo Station, Antarctica, and
Ny-Ålesund, Spitsbergen, are similar in nature to those derived from
local ground-based lidar measurements. To investigate the possibility of
longer-term trends, appropriately subsampled and averaged CALIOP v2 PSC
observations from 2006 to 2017 were compared with PSC data during the 1978–1989
period obtained by the spaceborne solar occultation instrument SAM II
(Stratospheric Aerosol Measurement II). There was good consistency between
the two instruments in column Antarctic PSC occurrence frequency, suggesting
that there has been no long-term trend. There was less overall consistency
between the Arctic records, but it is very likely due to the high degree of
interannual variability in PSCs rather than a long-term trend.