## 3 Results
### Sea-ice Concentration Evaluation
#### 3.2.4 Merging with Good-Quality MODIS Sea-Ice Concentration  
An example to demonstrate the benefit of including good-quality MODIS SIC is presented in Figure 11, which shows a scene recorded in the Beaufort Sea on 22 May 2020. The scene comprises one large open lead, several narrower leads and large, contiguous ice floes of 100% SIC. Some of the narrower leads are completely overfrozen, some are partly overfrozen and some are completely open. It is hard to determine one scene-wide threshold which classifies all pixels with open water as open and all overfrozen leads, even those with frazil or very thin ice, as overfrozen. Some misclassification is likely to occur, so that the thin-ice SIC probably also includes small amounts of pixels which are classified as ice, but are actually open water. Our approach of choosing one thin-ice threshold per scene, not one global thin-ice threshold for all scenes, reduces the risk of this misclassification. The MODIS SIC resolve the leads as reduced SIC, but undershoot the reference SIC of the larger ice floes. The AMSR2 SIC consistently show 100% SIC over the large floes. The overfrozen yet still recognisable leads are not visible in the AMSR2 SIC, except for a coarsely resolved large open lead in the middle of the scene. This is not a failure of the algorithm. Rather, the coarse resolution is because of the frequency and the insensitivity towards overfrozen leads is because the polarisation difference which the algorithm uses to derive SIC approaches the AMSR2 ice tie-point rapidly once the freezing started [4]. Furthermore, the nonlinear nature of the AMSR2 algorithm reduces variability near 100%. This can lead to an overestimation of SIC between 90% and 100% and may explain why we do not see traces of narrow leads in the AMSR2 SIC data. The merged SIC resolves both the narrow, overfrozen leads and the large, open lead as reduced SIC. At the same time, tuning them to preserve the AMSR2 mean causes them to show close to 100% SIC over the large ice floes where the MODIS SIC are around 90%. A small open-water area in the southeastern corner of the scene is partly masked in the MODIS SIC, but retrieved by the AMSR2 and the merged SIC. The mean of the merged SIC is 98.7%, while the means of the thin- and thick-ice Sentinel-2 SIC are 95.9% and 90.2%, respectively. The MODIS SIC is 88.7%, which means an underestimation compared to both the thin- and the thick-ice Sentinel-2 SIC. The AMSR2 SIC have the smallest RMSD (12.0%) compared to the thin-ice SIC; the RMSDs of the merged SIC (13.5%) and the MODIS SIC (15.6%) are higher. All RMSDs increase when considering thin ice as water (thick-ice SIC). The MODIS RMSD is now 16.3%, while the merged and AMSR2 RMSDs are at 19.9% and 19.8%, respectively. The open-water extent of the merged SIC (771 km\({}^{2}\)) is close to that of the thin-ice Sentinel-2 SIC (878 km\({}^{2}\)). The open-water extent of AMSR2 is much smaller (182 km\({}^{2}\)), while that of MODIS is much higher (2760 km\({}^{2}\)). This value is close to the thick-ice Sentinel-2 open-water extent (2313 km\({}^{2}\)). The area where measurements of all three sensors are available is 11,350 km\({}^{2}\). Table 3 gives an overview over all parameters. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Merged** & **MODIS** & **AMSR2** & **S2 (Thin)** & **S2 (Thick)** \\ \hline \(\mu\) & 0.944 & 0.813 & 0.943 & 0.981 & 0.930 \\ \(\Delta_{thin}\) & 0.036 & 0.167 & 0.038 & N/A & N/A \\ \(\Delta_{thick}\) & \(-\)0.014 & 0.117 & \(-\)0.013 & N/A & N/A \\ \(RMSD_{thin}\) & 0.093 & 0.235 & 0.069 & N/A & N/A \\ \(RMSD_{thick}\) & 0.110 & 0.212 & 0.092 & N/A & N/A \\ \(OWE\) [km\({}^{2}\)] & 818 & 4508 & 292 & 208 & 1045 \\ \hline \hline \end{tabular}
\end{table}