## 3 Results and Discussion
### A Severe PL Pair  
The movement of the strong polar lows (PL2 and PL3), discussed in more detail, was influenced by the upper level low with a cold core (Figure 3), providing a high temperature contrast between the sea surface and the atmosphere. This severe weather system was registered as part of the mesocyclonic system at 04:45 UTC on 19 October on the AVHRR satellite image (Figure 4a). Figure 3: The geopotential heights (dm; black dotted lines) and air temperature (\({}^{\circ}\)C, shading) from the ERA5 reanalysis for 500 hPa (left column) and 850 hPa (right column) at 23:00 UTC, 18 October (**a**,**b**), 14:00 UTC (**c**,**d**) and 23:00 UTC (**e**,**f**) of 19 October 2017. Red dotted lines outline areas with a difference of 40 \({}^{\circ}\)K and 45 \({}^{\circ}\)K between the SST and air temperature at the 500 hPa level. The mesocyclonic system was formed in a shallow baroclinic zone at a distance of 200 km from the sea ice edge (Figure 1), under the southern part of the upper-level low--within the area of \(\Delta\)T \(\geq\) 40 \({}^{\circ}\)K (outlined by red dotted line in Figure 3a). As the upper-level low was moving to the south-southwest toward areas with higher SST (Figure 3d,f), \(\Delta\)T increased to values \(\geq\)45 \({}^{\circ}\)K (Figure 3c,e), resulting in rapid evolution and intensification of the PL2 and PL3 pair. At the initial stage of the PL3 development, a lower troposphere in the mesocyclogenesis area is characterized by the high horizontal temperature gradient (4 \({}^{\circ}\)K/100 km) at the 850 hPa level (Figure 3b). At the mature stage, the PL2 and PL3 were the two-center mesocyclonic system with the storm force wind (\(\geq\)25 m/s). In more intense PL3, the wind speed observed within the period 14-20 UTC on October 19, as noted above, was about 30 m/s. Figure 4: PLs evolution on IR images obtained from: the AVHRR at 04:45 UTC (**a**) and VIIRS at 12:05 UTC (**b**), 17:05 UTC (**c**) and 22:05 UTC (**d**) on 19 October 2017. The red dotted lines are the SLP fields by ERA5. The red and black arrows indicate the genesis areas of PL2 and PL3, respectively. Figure 4 illustrates the PL**2** and PL**3** cloud systems evolution on 19 October. At the initial stage (Figure 4a), the PL**2** cloud structure is not yet distinctly formed, but the central part with an almost cloud-free area and relatively high brightness temperature (Tb = of 255-260 K) is detected. Configuration of the cloud spirals around the PL**3** center (black arrows) indicates the development of the mesocyclonic circulation. The spirals comprise cloud clusters that have shallow vertical extent, and the Tb at the cloud tops is approximately 20 K lower than in the center. ASCAT near-surface wind speed at the initial stage of the PL**3** development was 8-11 m/s (not shown). The mesocyclogenesis area can be delineated by the closed isobar of 998 hPa in the ERA5 SLP field (Figure 4a). At 12:05 UTC, in the maturel, the SLP minimum of 990 hPa was about 50 km south of the cloud field, evidencing good agreement between the ERA5 reanalysis and satellite data (Figure 4b). As compared to the initial stage of the PL**3**, the cloud top temperature of about 230 K is 20-25 K lower, indicating rapid development of convection. The analysis of cloud structures on satellite images shows that the two-center mesocyclonic system (formed by 12:05 UTC) further evolved to a multicenter system, like the \"merry-go-round\" [1]. The narrow cloud streets are the signature of cold air advection from the sea ice areas located to the north and northwest. Convective cells covering the vast area indicate the cold air mass expanded over the entire Chukchi Sea. A cloud top temperature of about 250 K evidences that the convective penetration in the cells is lower than in the PL**2** and PL**3**. At 17:05 UTC on 19 October, the PL**3** was still at the mature stage with the SLP minimum in the center of 990 hPa (Figure 4c).