Spaceborne observations of polar stratospheric clouds (PSCs) with the Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite provide a comprehensive picture of the occurrence of Arctic and Antarctic PSCs as well as their microphysical properties. However, advances in understanding PSC microphysics also require measurements with ground-based instruments, which are often superior to CALIOP in terms of, for example, time resolution, measured parameters, and signal-to-noise ratio. This advantage is balanced by the location of ground-based PSC observations and their dependence on tropospheric cloudiness. CALIPSO observations during the boreal winters from December 2006 to February 2018 and the austral winters 2012 and 2015 are used to assess the effect of tropospheric cloudiness and other measurement-inhibiting factors on the representativeness of ground-based PSC observations with lidar in the Arctic and Antarctic, respectively. Information on tropospheric and stratospheric clouds from the CALIPSO Cloud Profile product (05kmCPro version 4.10) and the CALIPSO polar stratospheric cloud mask version 2, respectively, is combined on a profile-by-profile basis to identify conditions under which a ground-based lidar is likely to perform useful measurements for the analysis of PSC occurrence. It is found that the location of a ground-based measurement together with the related tropospheric cloudiness can have a profound impact on the derived PSC statistics and that these findings are rarely in agreement with polewide results from CALIOP observations. Considering the current polar research infrastructure, it is concluded that the most suitable sites for the expansion of capabilities for ground-based lidar observations of PSCs are Summit and Villum in the Arctic and Mawson, Troll, and Vostok in the Antarctic.
The existence of polar stratospheric clouds (PSCs) is of critical importance for stratospheric ozone depletion during polar winter. They provide the surface for heterogeneous reactions which transform stable chlorine and bromine species into their highly reactive ozone-destroying states
Since the early 1990s, airborne and ground-based lidar remote-sensing observations of PSC optical properties have been used to classify PSCs into different types according to their size, shape, and chemical composition
Ground-based lidar observations of PSCs are generally performed at the mercy of tropospheric clouds. Since its launch in June 2006, the Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) aboard the
Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite
Traditionally, two approaches are used to match ground-based lidar measurements to spaceborne observations. Either statistics from a time series of ground-based measurements are compared to those obtained from averaging spaceborne observations for a specific grid box around the ground station or individual ground-based observations are matched to the data of the closest CALIPSO approach
Locations of research stations in
Overview of the location of Arctic and Antarctic research stations. Station abbreviations in the first and fifth columns are used to mark the corresponding sites in Figs.
Figure
Information on tropospheric clouds is taken from the CALIPSO level 2 version 4.10 Cloud Profile product (05kmCPro.v4.10), which provides information on the vertical extent of different cloud types as well as profiles of the optical properties of clouds with a resolution of 5 km along the CALIPSO ground track and 30 m height bins below 8.2 km height (60 m height bins between 8.2 and 20.2 km height). The extracted parameters are time, latitude, longitude, and the cloud type as provided in the Vertical Feature Mask product.
Features that are identified as clouds in the CALIPSO retrieval are further classified into eight cloud types
Number of considered CALIPSO profiles with PSC observations for different tropospheric cloudiness in the Arctic (December 2006 to February 2018) and the Antarctic (winters of 2012 and 2015). The sum of cloud-free conditions and profiles with transparent clouds makes up the view of a ground-based lidar.
The CALIOP version 2 PSC detection and composition classification algorithm (CALIPSO polar stratospheric cloud mask v2) separates stratospheric cloud features into STS, NAT mixture, ice, NAT enhanced, and wave ice. The polar stratospheric cloud mask product has an along-track resolution of 5 km, identical to the tropospheric CALIPSO products, and a vertical resolution of 180 m. The new CALIPSO polar stratospheric cloud mask corrects known deficiencies in previous versions
While all boreal winters from December 2006 to February 2018 are considered in the analysis of Arctic PSCs, only the austral winters of 2012 and 2015 are included in the analysis of Antarctic PSCs. However, the generally higher occurrence rate of Antarctic PSCs means that a larger number of individual PSC profiles was observed during the 2 Antarctic winters compared to the 12 considered Arctic winters (see Table
Because of CALIPSO's top-down viewing geometry, profiles start with the uppermost height bin (bin 1) down to the lowermost height bin (bin 583). Profiles in the CALIPSO polar stratospheric cloud mask v2 product extend down to 8.2 km. They can therefore contain contributions of upper-tropospheric cirrus, as visualised in Figs. 13 and 20 of
Information on cloud type from the Vertical Feature Mask in the 05kmCPro.v4.10 Cloud Profile product is used to sum up the number of height bins with different tropospheric cloudiness for each CALIPSO profile. This information is used to identify cloud-free conditions (a total of zero counts for each of the eight cloud types) and situations with only transparent tropospheric clouds that would still enable meaningful PSC observations with a ground-based lidar, i.e. altocumulus (transparent), cirrus (transparent), or a combination of the two. In addition, all sky refers to the use of all profiles independent of tropospheric cloudiness.
The CALIPSO polar stratospheric cloud mask v2 is processed analogous to the Vertical Feature Mask for tropospheric clouds by accumulating the number of height bins with different PSC composition for each CALIPSO profile. PSCs that extend over just one height bin are excluded from the analysis. A CALIPSO profile is referred to as containing a certain PSC composition (e.g. STS-containing or ice-containing profiles) if the respective component is identified in at least one of the PSC height bins.
Normalised number of CALIPSO profiles with PSCs detected over the Arctic (
To enable a combined analysis of cloudiness in the polar troposphere and stratosphere, the data extracted from the 05kmCPro.v4.10 and polar stratospheric cloud mask v2 products are temporally matched and reduced to only those profiles with detected PSCs. The data set is then filtered according to the occurrence of (i) tropospheric clouds and (ii) PSCs with different composition. The filtered data are gridded into cells of 1.25
Normalised occurrence rate of CALIPSO height bins that contain
Occurrence rate of STS (green), NAT mixtures (yellow), NAT enhanced (red), ice (blue), and wave ice (dark blue) for the entire Arctic as well as for the Arctic ground stations listed in Table
The matched observations of tropospheric and stratospheric clouds allow for a direct comparison of individual PSC profiles as well as long-term PSC statistics as seen from ground and space independent of the considered instruments. Specifically, the same profile can be evaluated from two perspectives, i.e. from space as well as from the point of view of a ground-based instrument. In that context, the latter perspective translates to a CALIPSO-synchronous measurement protocol at a ground station. True PSC statistics unaffected by tropospheric cloudiness, i.e. during all-sky conditions, at a certain location can only be obtained with a spaceborne lidar. In contrast, filtering with respect to tropospheric cloudiness is applied to emulate the likely conditions for meaningful ground-based PSC measurements in the CALIPSO data set. Specifically, we assume that a ground-based lidar would only provide meaningful results during conditions with no clouds or only transparent clouds that would not already attenuate the laser beam before it can reach PSC altitudes. This is referred to as the ground-based view of the CALIPSO data set. It provides sampling that is dependent on the CALIPSO return rate and must not be confused with actual ground-based measurements that can provide localised PSC observations in the time range from hours to weeks.
We subsequently separate the ground-based view of the CALIPSO data set into two scenarios for which (i) all cases of the ground-based view are considered and (ii) one-third of the profiles of the ground-based view was randomly selected. The first scenario corresponds either to a continuously operating lidar or a manually operated system that is active during every single CALIPSO overpass with possible downtime in between without any interference by tropospheric clouds or measurement-inhibiting factors. The second scenario also refers to CALIPSO-synchronous measurements with the caveat that interfering factors reduce the number of measured lidar profiles to one-third of what would ideally be possible. This latter scenario is much more realistic as (i) most ground-based lidar instruments are operated manually and on campaign basis; (ii) the decision to start a measurement, i.e. the assessment of tropospheric cloudiness, is made subjectively by the operator; and (iii) infrastructural challenges (e.g. system downtime, logistical problems, and lack of personnel) affect the operation of a ground-based lidar at a remote location and under harsh conditions.
To assess the representativeness of ground-based PSC measurements, PSC statistics are obtained for boxes of 2
The absolute number of observed PSC profiles (normalised to a maximum count of 2478) and the PSC occurrence rate (the ratio of observed CALIPSO PSC profiles versus all CALIPSO profiles) are shown in Fig.
Same as Fig.
The occurrence rate of PSCs with different chemical compositions in the Arctic for all-sky conditions is shown in Fig.
Figure
Same as Fig.
Same as Fig.
The localised view for 15 ground stations in the Arctic reveals the impact of tropospheric cloudiness on the statistics on PSC microphysical properties as expected from Fig.
Apart from the different effect of tropospheric cloudiness, Fig.
Figure
The maps of the occurrence rates of different PSC compositions in the Antarctic during all-sky conditions in Fig.
The statistics of Antarctic PSC microphysical properties are shown in Fig.
Number of CALIPSO PSC profiles in the
Figure
There is a rich literature on airborne and ground-based PSC measurements going back to the 1980s. The thus collected time series have been used to obtain statistics of microphysical properties of PSCs in the Arctic and Antarctic. While the impact of using different PSC classifications schemes has been assessed in the past
The combination of the occurrence rate of PSCs and of suitable conditions for ground-based PSC observations allows us to assess the suitability of a ground station for long-term lidar measurements of PSCs. This suitability is related solely to atmospheric conditions and does not consider challenges with respect to logistics, personnel, or training. According to this definition, measurements at more suitable sites will require less measurement effort to obtain a data set that can be used to infer statistically significant PSC data. This knowledge is important as ground-based lidars are generally more advanced than spaceborne instruments and allow researchers to independently retrieve backscatter and extinction coefficients as well as the particle linear depolarisation ratio at multiple wavelengths and at a better signal-to-noise ratio. Their measurements are therefore invaluable for a better understanding of processes related to PSC formation and persistence.
Of the established PSC observatories only Concordia, Eureka, McMurdo, and Ny Ålesund are found to fall into a category that provides a good balance between PSC occurrence and tropospheric cloudiness. Dumont d'Urville is at the lower end of available PSC observations, while Esrange, Sodankylä, and Syowa all show only about 1000 CALIPSO PSC profiles during conditions for ground-based measurements. The occurrence rate of PSCs in the Arctic is much lower than in the Antarctic. Hence, the assessment presented here is particularly important for Arctic sites. Considering only atmospheric conditions, it is found that Villum, Summit, Zackenberg, Thule, and Alert would be the best choices for establishing new PSC observatories with state-of-the-art lidar instruments in the Arctic. In the Antarctic, this is the case for Vostok, Troll, Mawson, Jang Bogo, Belgrano II, and Neumayer III.
The strong dependence of PSC formation on temperature suggests a crucial role of processes that enhance local cooling
CALIPSO Cloud Profile data were obtained from the ICARE Data and Services Center (
MT and PA conceived the study, developed the methodology, and analysed the data. CALIPSO polar stratospheric cloud mask v2 data were provided by MCP. All authors contributed to the discussion of the data and the preparation of the article.
The authors declare that they have no conflict of interest.
We thank the CALIPSO Science Team for providing CALIPSO data for tropospheric clouds.
This work was supported by the Franco-German Fellowship Programme on Climate, Energy, and Earth System Research (Make Our Planet Great Again – German Research Initiative, MOPGA-GRI, grant number 57429422) of the German Academic Exchange Service (DAAD), funded by the German Ministry of Education and Research.
This paper was edited by Farahnaz Khosrawi and reviewed by Vincent Noel and one anonymous referee.