## 3 Results
#### Ground-Based and CALIOP PSC Observations in 2021  
A large part of the ground-based lidar observations at Concordia Station are synchronized with overpasses of the CALIPSO satellite, with an air-borne lidar (CALIOP) on board. CALIPSO was launched in April 2006 as a component of the A-train satellite constellation [36; 37]. With an orbit inclination of 98.2\({}^{\circ}\), it provides extensive daily measurement coverage over the polar regions of both hemispheres at up to 82\({}^{\circ}\) in latitude. The primary instrument of CALIPSO is CALIOP. CALIOP has extensively been used for observing PSCs [20; 38; 39; 40]. CALIPSO performs 14 to 15 orbits per day. On average, two of those have a footprint within 300 km of Concordia Station, although very few orbits have a footprint of less than 10 km. A quasi-synchronous observation is obtained by starting the ground-based observation 16 min before the overpass, with a duration of 32 min. Considering the geometries and time difference between the two observations, it seldom occurs that both lidars observe the same airmass at the same time. But, we showed in a previous work [8] that these quasi-coincident observations often had a good agreement for what concerns the detection of PSCs and, albeit with a lesser frequency, they also produced similar classifications. The comparison of observations made by different instruments, in terms of detection and classification, requires that similar algorithms and thresholds be used. [PERSON] and [PERSON] [41] reported a study of different classification schemes for PSCs and showed that measurements made by a ground-based lidar but elaborated with different criteria often produced a different outcome. The comparison between ground-based and satellite-borne lidars is even more difficult because the observation geometries and the frequency of data acquisition are very different. For instance, ground-based lidars have to deal with bad weather conditions or the presence of optically thick cirrus clouds, which attenuate the laser beam, while satellite-borne lidar does not have this kind of problem. On the other hand, the satellite-borne lidar observes from a large distance, with an orbit being about 800 km from the surface, while the ground-based lidar is much closer to the observed PSCs. Ground-based lidar always observes the same space, and during polar winter, real clear sky measurements seldom occur. Satellite-borne lidar during each orbit observes both PSCs and clear sky profiles. This is important for calibration purposes and may introduce major calibration errors for ground-based systems. Figure 8 shows 264 PSC profiles observed by ground-based lidar and 262 CALIOP profiles. The figure was obtained as follows: the ground-based data were gridded on a 180 m grid, since the CALIOP data have a vertical resolution of 180 m. The color codes identify the different PSC classes as follows: NAT mixtures, STSs, ice, and enhanced NAT mixtures are displayed in green, orange, blue, and red, respectively. From the global CALIOP data, all overpasses within a latitude-longitude range defined by the coordinates 73.1 degS < lat < 77.1 degS and 116.33 degE < lon < 130.33 degE centered on Concordia Station were selected. Then, for each overpass, the profile with the nearest footprint with respect to Concordia Station was used to extract the PSC classification as provided in the PSC special product data set to be compared with the quasi-coincident ground-based profile, as can be observed in Figure 8. The average distance of the CALIOP footprints was 146 km, while the largest distance was 300 km. Instead of taking the nearest profile, other approaches were considered but rejected, based on a previous study of a great number of CALIOP tracks (see [8]). For instance, one might average the optical parameters of several CALIOP profiles during an overpass, but this would invariably produce average values for these parameters, leading to a predominant classification of NAT mixtures at the cost of STS and ice.