Polar stratospheric clouds (PSCs) play key roles in the springtime chemical depletion of ozone at high latitudes. PSC particles provide sites for heterogeneous chemical reactions that transform stable chlorine and bromine reservoir species into highly reactive ozone-destructive forms. Furthermore, large nitric acid trihydrate (NAT) PSC particles can irreversibly redistribute odd nitrogen through gravitational sedimentation, which prolongs the ozone depletion process by slowing the reformation of the stable chlorine reservoirs. However, there are still significant gaps in our understanding of PSC processes, particularly concerning the details of NAT particle formation. Spaceborne observations from the CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) lidar on the CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations) satellite are providing a rich new dataset for studying PSCs on unprecedented vortex-wide scales. In this paper, we examine the vertical and spatial distribution of PSCs in the Antarctic and Arctic on vortex-wide scales for entire PSC seasons over the more than nine-year data record

Pitts, Michael C.

Poole, Lamont R.

NASA Technical Reports Server

  9+ YEARS OF CALIOP PSC DATA: AN EVOLVING CLIMATOLOGY 
Michael C. Pitts 1*, Lamont R. Poole2 
1NASA Langley Research Center, Hampton, VA 23861, USA,*Email: Michael.c.pitts@nasa.gov 
2Science Systems and Applications, Inc., Hampton, VA 23666, USA 
ABSTRACT 
Polar stratospheric clouds (PSCs) play key roles in 
the springtime chemical depletion of ozone at high 
latitudes. PSC particles provide sites for 
heterogeneous chemical reactions that transform 
stable chlorine and bromine reservoir species into 
highly reactive ozone-destructive forms. 
Furthermore, large nitric acid trihydrate (NAT) 
PSC particles can irreversibly redistribute odd 
nitrogen through gravitational sedimentation, 
which prolongs the ozone depletion process by 
slowing the reformation of the stable chlorine 
reservoirs. However, there are still significant gaps 
in our understanding of PSC processes, particularly 
concerning the details of NAT particle formation. 
Spaceborne observations from the CALIOP 
(Cloud-Aerosol Lidar with Orthogonal 
Polarization) lidar on the CALIPSO (Cloud-
Aerosol Lidar and Infrared Pathfinder Satellite 
Observations) satellite are providing a rich new 
dataset for studying PSCs on unprecedented 
vortex-wide scales.  In this paper, we examine the 
vertical and spatial distribution of PSCs in the 
Antarctic and Arctic on vortex-wide scales for 
entire PSC seasons over the more than nine-year 
data record. 
 
1. INTRODUCTION 
Polar stratospheric clouds (PSCs) are known to 
play key roles in the springtime chemical depletion 
of stratospheric ozone at high latitudes1.  First of 
all, heterogeneous reactions on the surfaces PSCs 
convert the stable chlorine reservoirs HCl and 
ClONO2 to chlorine radical species that are 
involved in catalytic ozone destruction.  
Heterogeneous reaction rates depend on particle 
surface area and composition, which includes 
liquid binary H2SO4/H2O droplets (background 
stratospheric aerosol); liquid ternary 
HNO3/H2SO4/H2O droplets (STS); solid nitric acid 
trihydrate (NAT) particles; and H2O ice particles.  
PSCs also affect ozone chemistry through the 
removal of HNO3 from the polar stratosphere 
(denitrification) via the formation and 
sedimentation of large NAT PSC particles.  
Denitrification enhances catalytic ozone depletion 
by delaying the reformation of the stable chlorine 
reservoirs.   
In spite of nearly three decades of research, there 
are still significant gaps in our understanding of 
PSC processes, primarily concerning the details of 
how NAT particles form and evolve and to what 
degree chlorine activation occurs on cold 
background aerosol prior to PSC formation. These 
uncertainties significantly limit our ability to 
accurately represent PSC processes in global 
models and call into question our prognostic 
capabilities concerning future ozone loss in a 
changing climate.  This is of particular concern in 
the Arctic, where winter temperatures hover near 
the PSC threshold and, hence, future stratospheric 
cooling could lead to enhanced cloud formation 
and substantially greater ozone losses. 
The PSC observational database has been greatly 
expanded with the advent of the Cloud-Aerosol 
Lidar with Orthogonal Polarization (CALIOP) on 
the Cloud-Aerosol Lidar and Infrared Pathfinder 
Satellite Observations (CALIPSO) satellite.  
Launched in 2006 as part of the NASA A-train 
satellite constellation, CALIPSO offers an ideal 
platform for studying polar processes since its orbit 
provides extensive measurement coverage over the 
polar regions of both hemispheres with an average 
of fourteen orbits per day extending to 82o latitude. 
CALIOP is providing a rich new dataset for 
studying PSCs2,3,4,5,6 on unprecedented vortex-wide 
scales and the 9+ years of CALIOP PSC 
observations are providing considerable new 
insight into the key remaining questions regarding 
PSC particle formation and chemical processes.      
In this paper, we provide a brief overview of the 
methodology for detection and composition 
classification of PSCs in the CALIOP data and then 
examine the 9+ year CALIOP data record to 
quantify the seasonal and interannual variability of 
PSCs in the Antarctic and Arctic.   
https://ntrs.nasa.gov/search.jsp?R=20160006432 2019-08-31T03:14:13+00:00Z
 2. DETECTION AND COMPOSITION 
CLASSIFICATION 
CALIOP is a dual wavelength (532 and 1064 nm), 
polarization sensitive lidar that measures 
backscatter from cloud and aerosol particles in the 
atmosphere7. The CALIOP PSC detection 
algorithm3,4 utilizes a standard threshold approach 
where PSCs are identified through enhancements 
in the 532-nm lidar scattering ratio (R532) or 
perpendicular backscatter.  Detection is performed 
using a successive horizontal averaging procedure 
(5, 15, 45, 135 km) that ensures detection of 
strongly scattering PSCs (e.g., ice) at the finest 
possible spatial resolution (5 km), while also 
enhancing the detection of very tenuous PSCs that 
are found only through additional averaging (up to 
135 km).  The vertical resolution for detection is 
180 m.  The inclusion of the perpendicular 
backscatter channel provides sensitivity to the 
presence of non-spherical particles, which in the 
case of PSCs are attributed to NAT or ice particles.   
PSC composition classification4,5,6 is based on 
comparison of CALIOP 532-nm particle 
depolarization ratio (ratio of perpendicular and 
parallel particle backscatter) and inverse scattering 
ratio (1/R532) with theoretical optical calculations 
for equilibrium STS and plausible non-equilibrium 
mixtures of spherical liquid (binary aerosol or STS) 
and non-spherical (NAT or ice) particles. PSCs 
detected through enhancements in scattering ratio 
without significant enhancement in perpendicular 
backscatter indicates liquid particle growth from 
binary aerosol to STS and are classified as STS.  
PSCs detected through enhancements in 
perpendicular backscatter indicate the presence of 
non-spherical particles and are classified as either 
liquid/NAT mixtures or ice, depending on their 
particle depolarization and inverse scattering ratio 
values.  PSCs are separated into five composition 
classes: two classes of liquid/NAT mixtures 
(Mix1+Mix2 and Mix2-enhanced) which denote 
lower or higher NAT particle number 
density/volumes in the mixtures; STS; H2O ice; 
and mountain wave ice.  
The standard CALIPSO Level 2 PSC data products 
are available through the Langley Atmospheric 
Sciences Data Center (https://eosweb.larc.nasa. 
gov/project/calipso/ calipso_table). 
3. ANTARCTIC PSCs 
The Antarctic winter stratosphere is characterized 
by a large, stable, and cold polar vortex that creates 
ideal conditions for PSCs to exist.  Figure 1 shows 
examples of the areal coverage of PSCs as 
observed by CALIOP for a representative subset of 
four Antarctic winters.  The time series represent 
the total areal extent of PSCs over the Antarctic 
polar region as a function of altitude and day. 
Although there are differences in the absolute 
magnitudes and detailed structure from year to 
year, the general evolution is similar in terms of 
season length and PSC vertical extent, and the 
year-to-year variability is relatively small.  The 
multi-year (2006-2014) mean PSC areal coverage 
 
 
Figure 1. Daily time series of CALIOP PSC areal coverage in the Antarctic as a function of altitude for the 2007, 
2009, 2011, and 2013 seasons.  Units are in millions of square kilometers. 
 
 
 
 
 
 during the Antarctic season is shown in Figure 2. 
The multi-year composite is a fairly representative, 
albeit smooth, rendition of the seasonal evolution 
of Antarctic PSCs and may be useful for simulating 
PSC occurrence in global models.   
Shown in Figure 3 are the multi-year (2006-2014) 
mean time series of the fraction of total PSC area 
covered by each composition as a function of 
altitude and day. STS is the predominant 
composition early in the season and again late in 
the season.  Lower number density NAT mixtures 
(Mix1+Mix2) are the predominant composition at 
the lowest altitudes throughout the season.  Ice 
PSCs occur on relative small scales (never more 
than about 20% of total PSC area) and their 
occurrence is typically episodic in nature.  Higher 
number density Mix2-enhanced NAT mixtures 
make up a large fraction of the PSCs above ~20 km 
during the period from late June until early 
September.   
4. ARCTIC PSCs  
In contrast to the Antarctic, the Arctic polar 
stratospheric vortex is warmer, more unstable, and 
prone to sudden stratospheric warmings which 
disrupt the vortex and raise temperatures above 
PSC existence thresholds.  As a result, PSC 
occurrence in the Arctic is highly variable from 
year to year and significantly lower overall than in 
the Antarctic. The dramatic year-to-year variability 
is illustrated in Figure 4 which shows the areal 
coverage of PSCs in the Arctic for a subset of four 
winters.  For instance, CALIOP observed the 
largest PSC areal coverage to date in January 2010, 
including synoptic areas of ice PSCs which are 
very uncommon in the Arctic, but a sudden 
stratospheric warming ended the PSC season at the 
end of January.  On the other hand, the 2010-11 
Arctic season was characterized by persistent 
periods of PSCs from December through March, 
resulting in record ozone loss.  The 2012-13 winter 
was another short season with PSCs observed by 
CALIOP only in December and early January. 
With such a highly unstable polar vortex, the 
interannual variability in the Arctic is extremely 
large with PSC occurrence and even composition 
varying significantly from year to year.  As a result, 
a multi-year average is not very meaningful in the 
Arctic as each year is unique. 
 
 
Figure 2. Multi-year mean Antarctic PSC areal 
coverage based on CALIOP data from 2006-2014. 
 
 
 
 
 
 
 
Figure 2. Multi-year mean Antarctic PSC areal 
coverage based on CALIOP data from 2006-2014. 
 
 
Figure 3. Time series of multi-year (2006-2014) mean Antarctic PSC areal coverage for each of four composition 
classes: STS, Mix1+Mix2, ice, and Mix2-enhanced. The values have been normalized by the total PSC area to 
show the relative coverage of each composition class as indicated by the color bars. 
 
 
 
 
 
  
5. SUMMARY 
After more than two decades of research it is well 
established that PSCs play critical roles in the 
annual springtime depletion of ozone in the polar 
stratospheres.  However, there are still significant 
gaps in our understanding of the detailed role of 
PSC processes in ozone depletion. CALIOP has 
ushered in a new era in PSC research and is 
providing a wealth of information on PSC 
occurrence and composition on unprecedented 
spatial scales.  From the more than nine year 
CALIOP data record, we can now reasonably 
quantify the seasonal and interannual variability of 
PSC occurrence and composition in both the 
Antarctic and Arctic polar regions.  The Antarctic 
polar vortex is typically stable and cold and, as a 
result, the evolution of PSCs is similar from year to 
year.  A multi-year mean depiction of the Antarctic 
PSC season is fairly representative and may be 
useful in global modeling applications.  In contrast, 
the Arctic polar vortex is relatively warm and 
unstable, resulting in dramatic variability in PSC 
occurrence from year to year.  A multi-year 
depiction of the Artic PSC season is not very 
meaningful as each year is unique.  
 
REFERENCES 
[1] Solomon, S., 1999: Stratospheric ozone 
depletion: A review of concepts and history, Rev. 
Geophys. 37, 275–316. 
  
 
 
[2] Noel, V., Hertzog, A., Chepfer, H., and Winker, 
D., 2008: Polar stratospheric clouds over 
Antarctica from the CALIPSO spaceborne lidar, J. 
Geophys. Res., 113, D02205, doi:10.1029/ 
2007JD008616. 
[3] Pitts, M. C., Thomason, L. W., Poole, L. R., and 
Winker, D. M., 2007: Characterization of Polar 
Stratospheric Clouds with spaceborne lidar: 
CALIPSO and the 2006 Antarctic season, Atmos. 
Chem. Phys., 7, 5207–5228. 
[4] Pitts, M. C., Poole, L. R., and Thomason, L. W., 
2009: CALIPSO polar stratospheric cloud 
observations: second-generation detection 
algorithm and composition discrimination, Atmos. 
Chem. Phys., 9, 7577-7589. 
[5] Pitts, M. C., Poole, L. R., Dörnbrack, A., and 
Thomason, L.W., 2011: The 2009–2010 Arctic 
polar stratospheric cloud season: a CALIPSO 
perspective, Atmos. Chem. Phys., 11, 2161–2177. 
[6] Pitts, M. C., Poole, L., Lambert, A., and 
Thomason, L.W., 2013: An assessment of CALIOP 
polar stratospheric cloud composition 
classification, Atmos. Chem. Phys., 13, 2975-2988. 
 [7] Winker, D. M., Vaughan, M. A., Omar, A. H., 
Hu, Y., Powell, K. A., Liu, Z., Hunt, W. H., and 
Young, S. A., 2009: Overview of the CALIPSO 
Mission and CALIOP Data Processing Algorithms, 
J. Atmos. Ocean. Tech., 26, 2310–2323.  
 
Figure 4. Daily time series of CALIOP PSC areal coverage in the Arctic as a function of altitude for the 2007-08, 
2009-10, 2010-11, and 2012-13 Antarctic seasons.  Units are in millions of square kilometers. 
 
 
 
 
 
 
 
 
 
Figure 1. Daily time series of CALIOP PSC areal coverage as a function of altitude for the 2007, 2009, 2011, and 
2013 Antarctic seasons.  Units are in millions of square kilometers. 


9+ Years of CALIOP PSC Data: An Evolving Climatology

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