## 4 Conclusions  
This work examines a series of polar lows (PLs) identified over the Chukchi and Beaufort Seas in the second half of October 2017 under anomalously low sea ice extent conditions. We combined multi-sensor satellite active and passive measurements to study the life-cycle and tracks of the PLs, the vertical and horizontal structure of cloud systems and wind conditions at various stages of its evolution. A significant part of this work is devoted to assessing the capabilities of ERA5, MERRA-2 and NCEP-CFSv2 reanalysis data sets in the PL detecting and tracking, as well as representing their intensity. This study is motivated by the fact that deriving PL climatology from multi-satellite measurements is challenging, as it would require a long record of regular, continuous and homogeneous satellite observations. With the advance of high-resolution reanalyzes, it will become possible to better reproduce the observed PLs (including severe events) and analyze their climatology. In October 2017, the predominantly ice-free Pacific Arctic (PA) region exhibited a highest frequency of favorable conditions for PLs since 1979. The sea ice concentration map as of 19 October 2017 showed a significant northward retreat of the sea ice edge to its median position in September and October for 1981-2010. The high SST along the northwest coast of Alaska also contributed to the mesoscale cyclogenesis. Throughout October 2017, on the visible and IR MODIS, VIIRS and AVHRR images, varying intensity mesoscale cyclones were detected over the Chukchi-Beaufort seas and the adjacent Arctic Ocean area. From 18 to 22 October, a series of mesocyclones were observed in this region, four of which, with wind speeds \(\geq\) 15 m/s, were classified as PLs. On 19 October 2017, the most intensive PL pair originated near the marginal ice zone north of the 73 rd parallel during a marine-cold air outbreak from the central Arctic. Both PLs moved southward as a mesocyclonic system called the \"merry-go-round\" under the influence of the upper level low with a cold core. Multi-sensor satellite measurements indicated that, in the mature stage, the strongest PL was characterized by high winds (W) of close to hurricane force over 30 m/s, which is extremely rare for the Chukchi and Beaufort Seas. It is possible that, over the coming decades, such mesoscale processes will be observed more frequently in the PA region because of the predicted northward shift in the cold-air outbreak activity as the sea ice extent decreases [44]. A three-dimensional structure of the strongest PL (marked as PL3) was also obtained from collocated measurements of AMSR2 (GCOM-W1) and CPR (CloudSat) sensors constituting an \"A-Train\" satellite constellation. In the mature stage, the vertical extent of the PL3 exceeded 4 km, which is close to the typical cloud top altitude (5 km) observed for 82 PL cases in the Nordic seas [48]. Additionally, we detected a cloud band north of the PL3 eye with high CPR radar reflectivity but low retrieved cloud liquid water con tent. The predominance of ice crystals with specific microphysics properties may cause these discrepancies. We compared tracks of four PLs detected from the cloud structures on satellite images and obtained from the reanalysis data sets applying two methods--based on sea level pressure and relative vorticity at 850 hPa level. All three reanalyses identified only PLs with a scale of over 220 km (PL1 and PL3). ERA5 showed the best correspondence with satellite observations in detecting the PLs center with track errors of 50-60 km. Moreover, ERA5 shows almost the same results for both PL tracking methods. The CFSv2 exhibits slightly worse results for the vorticity-based tracking algorithm (compared to the ERA5 data set), but in the SLP fields, this operational analysis represents only the most intense PL3.