# Validation of Copernicus Sea Level Altimetry Products in the Baltic Sea and Estonian Lakes
## 3 Results
### SLA\({}_{l3}\) and SLA\({}_{nemo}\) Comparison with In Situ SLA\({}_{tg}\) Measurements  
The CMES \(SLA_{l3}\) along-track altimetry product comparison with \(SLA_{tg}\) measurements yielded accurate results with \(r\) = 0.83 for all stations in weighted sum (Table 6, Figure 11a). The RMSE for all stations together was 0.18 m, ME = 0.12 m and the STD between \(SLA_{TG}\) and \(SLA_{l3}\) was 0.14 m. These results were similar to previous findings for the area even though the RMSE was larger [36]. The correlation between \(SLA_{TG}\) and \(SLA_{NEMO}\) was higher (\(r\) = 0.91, Figure 11b, Table 6). However, the RMSE and ME are much higher at 0.27 m and 0.26 m, respectively, which indicated a bias in the model fields. STD between \(SLA_{TG}\) and \(SLA_{NEMO}\) was 0.10 m. The comparison with Narva-Joesuu station (tide gauge number 3 in Figure 2, Table 6) shows the worst performance regarding its low correlation and high RMSE (0.35 m) and ME (0.30 m). A closer look at the data shows that approximately 30% of the data were collected in winter, which may be one of the reasons why the satellite results were underestimated. In addition, the exact measurement point in Narva-Joesuu is at the mouth of Narva River which may hinder the quality of comparison results, as the dynamics of the river itself are not recorded in the satellite results within a 10 km radius. Figure 10: Comparison of Sentinel-3 (\(SSH_{SA}\)) and buoy (\(SSH_{buoy}\)) sea surface heights on (**a**) S3A pass 186 and (**b**) S3A pass 397 in the Gulf of Riga. The red line is the linear regression between the two datasets. In Figure 12, a comparison of the \(\mathit{SLA}_{\mathit{buay}}\) data from the Gulf of Riga open sea station (green circle in Figure 2) with \(\mathit{SLA}_{\mathit{L3}}\) data and \(\mathrm{SLA}_{\mathrm{NEMO}}\) data is shown. Surprisingly, the along-track SLA accuracy gained little to no improvement compared with the results of coastal measurements, whereas NEMO reanalysis model outcomes improved significantly in the open sea. This suggests that the NEMO reanalysis fields represent sea-level variations in the open sea with high accuracy, while in the coastal region where the sea level is influenced by various processes and factors (shallow areas, coastal effects, waves, etc. ), the model reanalysis data loses some of its accuracy. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Station** & **Station Name** & \begin{tabular}{c} **Common** \\ **Measurements** \\ \end{tabular} & \
\begin{tabular}{c} \(\mathit{SLA}_{\mathit{TG}}\) **vs.** \(\mathit{SLA}_{\mathit{L3}}\) \\ \end{tabular} &
\begin{tabular}{c} \(\mathit{SLA}_{\mathit{TG}}\) **vs.** \(\mathit{SLA}_{\mathit{NEMO}}\) \\ \end{tabular} \\ \hline
1 & Dirahami & 116 & 0.83 & 0.17 & 0.93 & 0.26 \\
2 & Kunda & 22 & 0.88 & 0.22 & 0.92 & 0.29 \\
3 & Narva-Joesuu & 32 & 0.71 & 0.35 & 0.97 & 0.29 \\
4 & Loksa & 7 & 0.82 & 0.14 & 0.83 & 0.21 \\
5 & Pirita & 5 & 0.90 & 0.09 & 0.94 & 0.21 \\
6 & Paldiski & 5 & 0.79 & 0.17 & 0.87 & 0.26 \\
7 & Ristna & 73 & 0.86 & 0.17 & 0.92 & 0.30 \\
8 & Heltermaa & 20 & 0.88 & 0.09 & 0.83 & 0.27 \\
9 & Virtsu & 19 & 0.81 & 0.15 & 0.90 & 0.26 \\
10 & Roomassaare & 18 & 0.73 & 0.18 & 0.89 & 0.27 \\
11 & Ruhnu & 82 & 0.86 & 0.20 & 0.92 & 0.31 \\
12 & Haädemesete & 41 & 0.76 & 0.19 & 0.88 & 0.30 \\
13 & Párnu & 8 & 0.86 & 0.17 & 0.97 & 0.27 \\ \hline
14 & Muuga & 15 & 0.80 & 0.17 & 0.83 & 0.27 \\
15 & Tallinna Sadam (MSI) & 4 & 0.97 & 0.08 & 0.95 & 0.24 \\
16 & Paldiski (MSI) & 15 & 0.83 & 0.23 & 0.88 & 0.31 \\
17 & Lehtma & 82 & 0.85 & 0.14 & 0.93 & 0.21 \\
18 & Heltermaa (MSI) & 21 & 0.85 & 0.10 & 0.87 & 0.24 \\
19 & Triigi & 5 & 0.77 & 0.09 & 0.91 & 0.22 \\
20 & Virtsu & 17 & 0.82 & 0.23 & 0.91 & 0.33 \\
21 & Rohukila (MSI) & 18 & 0.94 & 0.17 & 0.96 & 0.25 \\ \hline  & Total/weighted & & & & & \\  & average & 625 & 0.83 & 0.18 & 0.91 & 0.27 \\ \hline  & & & & & & \\ \end{tabular}
\end{table} 
Table 6: Overview of tide gauges (Figure 2) used for comparison with CMES products in the Baltic Sea.