## 3 Results
### Sea-ice Concentration Evaluation
#### 3.2.3 Merging with Poor-Quality MODIS Sea-Ice Concentration  
In Figure 9, we present SIC maps for the scene with the largest discrepancy between the merged and the MODIS SIC. It is located in the Kara Sea and was acquired on 18 May 2019. The gap in the southwestern corner is not caused by a cloud, but by high MODIS ice tie-points (see Figure 10b,c). The edge of the gap exactly follows the 270 K contour, above which no MODIS SIC are retrieved currently (see Section 2.4). The poor quality of the MODIS SIC all over the scene shows that this threshold which we chose initially is too conservative in this case. Special attention is thus paid in Section 3.3 to derive a threshold above which no MODIS SIC retrieval is executed. The performance of the MODIS SIC retrieval is further deteriorated by an unscreened cloud in the eastern part of the scene (Figure 10a), so that this scene shows both the susceptibility of the MODIS SIC retrieval towards high surface temperatures and the susceptibility towards unscreened clouds. Figure 8: (**a**) Time-series of the merged and MODIS SIC corresponding to the Sentinel-2 time-series from Figure 7. The horizontal dashed lines show the mean of the respective dataset. (**b**) Difference between the merged and MODIS SIC and the thin-ice Sentinel-2 SIC from Figure 7. Positive differences mean that the Sentinel-2 SIC is higher. The horizontal dashed lines show the mean difference for the respective dataset. (**c**) Same as panel (**b**), but for the thick-ice Sentinel-2 SIC. The x-axis shows the date, the vertical lines mark the days with Sentinel-2 scenes. Figure 9: Sentinel-2 scene in the Kara Sea on 18 May 2019 and the corresponding SIC maps. **(a)** Sentinel-2 RGB image. Sentinel-2 SIC with thin ice considered as ice **(b)** and water **(c)**. **(d–f)** The merged SIC and their difference to the thin- and thick ice Sentinel-2 SIC. Panels **(g–i,j-l)** show the same for MODIS and AMSR2 SIC, respectively. The acquisition times for the single scenes in UTC are given in brackets in the titles of the maps in the left row. In the part without MODIS data, the actual resolution of the merged dataset goes down to 5 km. The almost vertical stripes in the AMSR2 and merged SIC are due to rapid fluctuations in the radiometer gain between the two scans of the 89 GHz channels that cannot be corrected for currently. The part of the scene where MODIS data are available is resolved more finely, but the structures do not resemble those of the Sentinel-2 image. The finer resolution patterns of the merged SIC resemble the reference SIC, however the single floes are too small and the time difference is too large (114 min) to recognise distinct floes of the reference SIC scene in the merged SIC. The influence of the large difference between the MODIS SIC and the Sentinel-2 SIC on the merged SIC is mitigated as we tune the merged SIC to preserve the AMSR2 mean. We conclude that in this scene including the poor-quality MODIS SIC does not significantly deteriorate the quality of the merged SIC, but we also cannot conclude that it enhances the quality. All datasets underestimate the thin-ice SIC; the merged and AMSR2 SIC by 3.6% and 3.8%, respectively; and the MODIS SIC by 16.7%. The difference compared to the thick-ice SIC is smaller for all datasets (\(-1.4\%\), \(11.7\%\) and \(-1.3\%\) for the merged/MODIS/AMSR2 SIC, respectively). The percentages are absolute values, i.e., not relative to the other percentage value. The small differences do not necessarily indicate a more precise result, but are the consequence of positive and negative differences cancelling out for the merged and AMSR2 SIC. This is underlined by their RSMD. It is smaller when computed towards the thin-ice SIC (0.093 for the merged SIC and 0.069 for the AMSR2 SIC) than when computed towards to the thick-ice SIC (0.110 for the merged SIC and 0.092 for the AMSR2 SIC). For the MODIS SIC, the RMSD is lower for the thick-ice SIC.