# Evaluation of Coastal Sea Level Offshore Hong Kong from Jason-2 Altimetry
## 5 Results
### Relative Performances of MLE4, MLE3, and ALES Near Hong Kong  
Here, we examine the relative performance of the MLE3, MLE4, and ALES retrackers in the H-Kcoastal zone. Because ALES and PISTACH are L2 products, we used the L3 X-TRACK product to project ALES and PISTACH MLE3 and MLE4 SLA onto regular 1-Hz reference points along the track, allowing us to obtain SLA time series over the study period. Figure 11a shows the percentage of valid measurements obtained for the different retrackers. MLE3 seems to be the most robust near the coast in the sense that it provides more valid data. Figure 11b shows the time-averaged SLA values for the three retrackers. Over the open ocean, ALES and MLE4 agree well (within 1 cm); MLE3 has a negative bias of 1-2 cm. Near the coast, ALES shows less variations, while MLE3 and MLE4 display large peaks likely due to retracking errors. Figure 11c shows the trend estimates for the different retrackers. ALES displays the smoothest pattern, which seems to indicate a better performance than MLE3 and MLE4.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Data Source** & **Case 1** & **Case 2** & **Case 3** \\ \hline ALES & 13.05/344 & 12.49/348 & 11.19/2 \\ MLE3 & 13.29/338 & 12.39/339 & 13.17/343 \\ MLE4 & 12.96/339 & 11.44/340 & 11.12/348 \\ XTRACK & & 13.23/338 & \\ Tide Gauge & & 11.46/311 & \\ \hline \hline \end{tabular}
\end{table}
Table 4: Estimated annual amplitude (cm) and phase (degree) as a function of sea level data source and case.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Data Source** & **Case 1** & **Case 2** & **Case 3** \\ \hline ALES & 6.03/235 & 5.67/239 & 7.38/260 \\ MLE3 & 6.17/241 & 6.76/252 & 9.24/270 \\ MLE4 & 6.02/236 & 6.96/245 & 11.80/262 \\ XTRACK & & 6.81/223 & \\ Tide Gauge & & 7.62/236 & \\ \hline \hline \end{tabular}
\end{table}
Table 5: Estimated semiannual amplitude (cm) and phase (degree) as a function of sea level data source and case.  
The sea level residuals obtained after removing the trend and seasonal signal are shown in Figure 12 for MLE3, MLE4, ALES and the tide gauge data. A 3-month low pass filter was applied to the different SLA time series. For most cycles, the altimetry-based SLA have variations similar to the tide gauge, but the latter occasionally shows larger anomalies. In the first few cycles, the tide gauge residuals have surprising low values, and around cycle 135-160 have a few large negative peaks. Very local small-scale tides, waves, and currents may cause these sea level signals. The standard deviations of the altimetry SLA residuals with respect to the tide gauge residuals, before and after the 3-month smoothing, are given in Table 6. The improvement due to the smoothing is significant, the standard deviations decreasing by more than 50%. The consistency between the altimetry and tide gauge residuals is about 5 cm, which is encouraging given that the study area which is quite complex. ALES SLA has slightly larger standard deviation with respect to tide gauge sea level.  
Figure 11: (a) Percentage of valid measurements; (b,c) mean values and 6.5-yr trend estimates for ALES, MLE3, and MLE4 retrackers. Results are presented as a function of latitude.