# Evaluation of Coastal Sea Level Offshore Hong Kong from Jason-2 Altimetry
## 5 Results
### Sea Level Trend
#### 5.2.3 Solutions Derived from the Different Retrackers  
For each of the three cases defined in Section 2 and each retracker, we computed a spatially averaged 20-Hz SLA time series (see Section 4.1.1) as well as the associated 20-Hz noise level (estimated by the standard deviation of the 20-Hz SLA series). The results are shown in Figure 10. In all cases, ALES solution provides the lowest noise level after editing, and MLE4 is slightly less noisy than MLE3. Concerning the three experimental retrackers used in PISTACH, ICE3 has the lowest noise level, and RED3 is slightly less noisy than ICE1. Note that a problem was detected in some of the PISTACH SLA solutions: around cycle #150, both ICE3 and RED3 have much larger SLA than ICE1, the relative bias being about 0.2 m. Comparison with the tide gauge sea level showed that ICE1 SLA roughly followed the tide gauge data, while ICE3 and RED3 data display large jumps. The latter severely influences the corresponding sea level trend estimates. Sea level trends obtained from Equation (5) are summarized in Table 3 (except for OCE3 in PISTACH, which is the same as MLE3). Figure 9: Regional 6.5 yr-long trends in sea level derived from filtered X-TRACK data along the Jason-2 pass 153. Vertical blue bars represent the uncertainty on the trend estimation. From Table 3 we note that, using the tide gauge as reference, case 1 provides the closest estimate to the tide gauge trend. Besides, we note a difference of less than 1 mm/yr for ALES and M LE3. This can easily be explained. As shown in Figure 2, case 2 includes a number of small islands while case 1 contains more land-free areas, thus more accurate SLA data. Finally, case 3 has very few valid altimetry measurements (usually less than five valid points), which is not enough to provide robust results. MLE3 and ALES trends are both close to the tide gauge trend (within 0.5 mm/yr). The trends estimated from MLE4 are slightly lower than for ALES and MLE3 but the difference is within the error bar. The trends deduced from the PISTACH retrackers highly disagree with the tide gauge trend: both ICE3 and RED3 show unrealistic large values (\(>\)+5 cm/yr), while ICE1 shows a negative trend of \(-\)2 cm/yr. The ICE1 retracker may be inherently not accurate enough to derive trends, but concerning ICE3 and RED3 retrackers, since the processing procedure used in this work is homogeneous, the large errors may not come from the retracker algorithm itself, but more likely from errors in the basic products. For example, in a new PISTACH version, the 18 cm calibration bias used in Equation (2) may no longer be applied in the computation of R. Since the retracked offset E parameter is no more accessible in the current PISTACH version, this hypothesis could not be verified. Anyway, in the remaining part of the study we discard ICE1, ICE3, and RED3 solutions and concentrate on MLE3, MLE4, and ALES which, in the context of our study, appear as the best available retrackers to capture coherent coastal sea level signals from altimetry. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Data Source** & **Case 1** & **Case 2** & **Case 3** \\ \hline ALES & +5.9 \(\pm\) 1.5 & +9.7 \(\pm\) 1.6 & +17.3 \(\pm\) 2.3 \\ MLE3 & +5.0 \(\pm\) 1.6 & +8.1 \(\pm\) 2.2 & +2.6 \(\pm\) 3.1 \\ MLE4 & +4.2 \(\pm\) 1.6 & +4.3 \(\pm\) 2.2 & +5.5 \(\pm\) 3.5 \\ ICE1 & \(-\)29.1 \(\pm\) 2.4 & \(-\)27.5 \(\pm\) 2.8 & \(-\)22.9 \(\pm\) 4.0 \\ ICE3 & +57.5 \(\pm\) 2.3 & +60.1 \(\pm\) 2.5 & +52.9 \(\pm\) 3.0 \\ RED3 & +55.3 \(\pm\) 2.1 & +58.0 \(\pm\) 2.3 & +58.5 \(\pm\) 3.0 \\ \hline XTRACK & & +8.6 \(\pm\) 2.2 & \\ Tide Gauge (in-situ reference) & & +5.2 \(\pm\) 2.0 & \\ Tide Gauge (After VLM correction) & & +5.5 \(\pm\) 2.0 & \\ Regional trend from the CCI data & & 2.7 \(\pm\) 2.0 & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Estimated linear trend and associated uncertainty (mm/yr) as a function of sea level data source and case. Figure 10: _Cont._  
Figure 10: SLA means and noise levels.