# Evaluation of Coastal Sea Level Offshore Hong Kong from Jason-2 Altimetry
## 5 Results  
### A First Analysis Based on Jason-2 Waveforms Observed along the Track  
The HK coast considered in this study has a very complex topography, so the waveforms are diverse. In order to go further in our analysis we used the waveform classification provided in the PISTACH product. It classifies the altimeter waveforms into 16 classes, including a \"doubt\" class. Based on considerations on all waveform shapes, we grouped these classes into five categories (see Table 2). In Table 2, for each category, we provide a general description of the surface observed and propose a specific retracker.  
We use these five categories to classify all Jason-2 waveforms used in our study. The resulting percentages of waveforms as a function of category and case (i.e., distance to the coast) are shown in Figure 7. In all three cases the most frequent category is \"Brown\", and the second is \"Distorted Brown\". Not surprisingly, the percentage of Brown-like waveforms decreases when approaching land. In case 2, more than 52% of the waveforms are Brown-like and in case 3 this number decreases to 37%. Concerning the distorted-Brown waveforms, logically, their percentage increases when the distance to  
\begin{table}
\begin{tabular}{p{85.4 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Category** & **Waveform Characteristics** & **Possible Surface Observed** & **Retracking Strategy** \\ \hline Brown & The waveform is close to the Brown model. & The land contribution in the altimeter footprint is null or small. & MLE4 \\ \hline Distorted Brown & The waveform is similar to the Brown model, but with distortions (either with an increasing plateau, or with a sharply decreasing plateau, or with a too broad noise floor). & There is land signature at the fringe of the altimeter footprint, with different reflection than over open ocean surface. & \\ \hline Peak dominated & One (or a few) large peak(s) dominate(s) the waveform; there may be a Brown shape in the waveform, but its maximum power is much lower than the peak(s). & There are one or a few strong bright targets (e.g., extremely calm water surface or effective corner reflector, see [34]) within the altimeter footprint. & MLE based on Gaussian model or improved threshold footprint. \\ \hline Brown + Peak & The waveform is a mixture of Brown shape and one (or a few) peak(s) with comparable power levels. The location of the peak can be in the leading edge or the plateau. & The portions of ocean and land surfaces within the altimeter footprint are equivalent. & MLE based on Bagr (Brown with Asymmetric Gaussian Peak [35]) \\ \hline Others & Unexplained waveform patterns (e.g., very noisy echoes or linear echoes). & Unexplianable surface features (e.g., very composite geomorphology or some extreme events). & Should be rejected. \\ \hline \hline \end{tabular}
\end{table}
Table 2: Overview of the five categories defined from the 16 PISTACH classes.  
the coast decreases. It varies from 17% in case 1 to 27% in case 3. These results suggest that within a shoreline distance of 5 km, a significant number of waveforms are Brown-like and that using a classical open-ocean retracker can still provide good quality sea level data in this very nearshore area. However, the use of ALES is expected to increase the number of retrieved accurate data, which is what we found in the previous section.