3. Case study offshore Hong Kong

In this section, Hong Kong coastal zone is chosen as an example to demonstrate the methodology and potential of the coastal altimetry technology. We focus on Jason-2 data here, and other altimetry missions like HY-2 can also be assimilated to achieve denser coverage and cross validation. To exploit the potential of the current endeavors in coastal altimetry, the three products aforementioned are merged and evaluated.

### 3.1 Study area

Hong Kong (HK) is located just south of the Tropic of Cancer. The climate displays clear seasonal variations. The southwesterly/northeasterly monsoon results in warm wet summers and cool dry winters. HK also frequently suffers from typhoons. On the western side of the HK island flows the Zhujiang River (Pearl River), which brings abundant freshwater, giving rise to a high salinity (hence, sea water density) gradient. All these factors impact significantly on the regional sea level variations.

The HK coast has an extremely complex geomorphology. As shown in Figure 5, many tiny islands lie within the altimeter footprints. Therefore, the corresponding altimeter and radiometer measurements are severely contaminated by land effects. This makes this area particularly relevant for analyzing the performances of coastal altimetry data.

The HK coastal topography is more than irregular. Despite a narrow band between 21.8°N and 22°N, where the depth is steeply falling down to 60 m, the study area has very shallow waters. We can thus expect complex local tides and currents, which can influence sea level variations.

## 3.2 Data sets
