## 4 Discussion
Weaker than usual amplitudes of planetary-scale Rossby waves in the September-October period helped maintain a strong vortex and led to below-average Antarctic ozone columns in late Australian spring and early summer. [42]. Since ice PSCs are easy to detect, have a large backscatter ratio, and are also easily classified for the same reason, the probability that both lidars detect them is larger than, for instance, the STSs of NAT mixtures, having an average backscatter ratio and small depolarization. Thus, the presence of many ice PSCs enhances the agreement between the observations of both lidars. PSC formation is strongly related to the local temperature. In Figure 11, the areas with temperatures below T\({}_{NAT}\)-3K are shown in green and indicate where and when PSCs are likely to be formed based on temperature only. The red areas correspond to temperatures below the frost temperature of ice and indicate where ice PSCs are expected, but we must bear in mind that NAT and STSs might also be observed at these temperatures. The local temperatures were obtained by merging radio sonde data recorded at Concordia Station, with a vertical resolution of about 5 m, with NCEP data with a vertical resolution of about 2 km. Note that the radio sondes are released once per day on a weather balloon from Concordia Station. These weather balloons explode at a certain altitude, which varies from about 17 km during winter up to more than 30 km during summer. The merged temperatures thus mainly rely on radio sonde measurements and use NCEP data for the upper part of the lidar measurements. Subsequently, the merged temperature is gridded on a 180 m vertical scale to conform with the lidar data. The formation temperatures were calculated from local pressure, water vapor, and nitric acid mixing ratios obtained from MLS data with a vertical resolution of about 1 km and were also gridded on a 180 m vertical scale. MLS data have a footprint of 5 \(\times\) 200 km (cross-track and along-track, respectively). Figure 10: The figure shows the PSC area of the southern hemisphere in 2021 at a potential temperature of 450 K. The red line shows the PSC area in the SH 2021, the black lines correspond with the minimum and maximum values recorded in the previous 10 years. The yellow line stands for the mean value in the preceding 10 years  
In Figures 8 and 11, we can observe from the reporting of the PSC observations and temperature fields, respectively, during 2021 that PSCs are seldom observed when the local temperature exceeds T\({}_{NAT}\)-3K. For example, during the first 10 days of June, temperatures below 14-16 km are too high to allow the formation of PSCs. Also, very few PSCs can be observed above 26 km, where the temperature is too high. The red contours, indicating T\({}_{ice}\) enclose favorable conditions for ice PSC formation, and it can be observed that ice PSCs are observed below T\({}_{ice}\), and it is clear that T < T\({}_{ice}\) is a necessary condition but not sufficient condition for ice PSC formation. With respect to previous years, the 2021 polar vortex was rather cold, as can be seen in Figure 11, where large areas of temperatures below T\({}_{ice}\) can be observed. Until the middle of June, temperatures are too high for ice PSCs to form, and we mainly see NAT mixtures, where NAT is formed by heterogeneous nucleation on background aerosol or other condensation nuclei. In July and August, ice PSCs are often observed in correspondence with temperatures below 185 K. The formation of ice PSCs leads to dehydration, which can also be observed in MLS measurements of the water vapor mixing ratio (see Figure 12). At temperatures below T\({}_{ice}\), the coexistence of ice, NAT mixtures, and STSs can be observed. The observation of STSs in September by the ground-based lidar is somewhat surprising considering the temperature maps. PSCs are seldom observed by CALIOP in September above Concordia Station [8], but this may be due to a minor sensitivity with respect to the ground-based lidar.