Regional validation of retracked sea levels from SARAL/Altika over the South China sea and adjacent seas

This paper focuses on assessing the quality of sea level anomaly (SLA) data from the new generation of Ka-band SARAL/AltiKa satellite altimetry over the continental shelf of the South China Sea. The region consists of peninsulas, shallow seas, and small islands that produce complicated altimetric waveform patterns. The improved-accuracy of SLAs data from the MLE4, Ice1 and Ice2 retrackers which are provided in the AVISO-Sensor Geophysical Data Records (SGDR) were optimized in this study. The quality of retracked SLAs is assessed by making comparison with tide gauge data from six stations. In general, the percentage of data availability of Ice-1 retracker is superior ( > 68%) to those of MLE- 4 and Ice-2 retrackers. The improvement of percentage (IMP) also shows that Ice-1 retracker improves the standard deviation > 12% better than those of Ice-2 retracker. Over complex areas of Lubang and Ko Taphao Noi, the temporal correlation of Ice-1 retracker is superior (r > 0.80) to those of MLE4 and Ice-2 retrackers (r 5.8) and lower RMS error ( < 34 cm) than those of Ice-1 retracker. It can be concluded that the Ice-1 and Ice-2 retrackers were superior for the coastal region of Maritime Continent

ALES+: Adapting a homogenous ocean retracker for satellite altimetry to sea ice leads, coastal and inland waters

Water level from sea ice-covered oceans is particularly challenging to retrieve with satellite radar altimeters due to the different shapes assumed by the returned signal compared with the standard open ocean waveforms. Valid measurements are scarce in large areas of the Arctic and Antarctic Oceans, because sea level can only be estimated in the openings in the sea ice (leads and polynyas). Similar signal-related problems affect also measurements in coastal and inland waters. This study presents a fitting (also called retracking) strategy (ALES+) based on a subwaveform retracker that is able to adapt the fitting of the signal depending on the sea state and on the slope of its trailing edge. The algorithm modifies the existing Adaptive Leading Edge Subwaveform retracker originally designed for coastal waters, and is applied to Envisat and ERS-2 missions. The validation in a test area of the Arctic Ocean demonstrates that the presented strategy is more precise than the dedicated ocean and sea ice retrackers available in the mission products. It decreases the retracking open ocean noise by over 1 cm with respect to the standard ocean retracker and is more precise by over 1 cm with respect to the standard sea ice retracker used for fitting specular echoes. Compared to an existing open ocean altimetry dataset, the presented strategy increases the number of sea level retrievals in the sea ice-covered area and the correlation with a local tide gauge. Further tests against in-situ data show that also the quality of coastal retrievals increases compared to the standard ocean product in the last 6 km within the coast. ALES+ improves the sea level determination at high latitudes and is adapted to fit reflections from any water surface. If used in the open ocean and in the coastal zone, it improves the current official products based on ocean retrackers. First results in the inland waters show that the correlation between water heights from ALES+ and from in-situ measurement is always over 0.95

Coastal altimetry products in the Strait of Gibraltar

This paper analyzes the availability and accuracy of coastal altimetry sea level products in the Strait of Gibraltar. All possible repeats of two sections of the Envisat and AltiKa ground-tracks were used in the eastern and western portions of the strait. For Envisat, along-track sea level anomalies (SLAs) at 18-Hz posting rate were computed using ranges from two sources, namely, the official Sensor Geophysical Data Records (SGDRs) and the outputs of a coastal waveform retracker, the Adaptive Leading Edge Subwaveform (ALES) retracker; in addition, SLAs at 1 Hz were obtained from the Centre for Topographic studies of the Ocean and Hydrosphere (CTOH). For AltiKa, along-track SLA at 40 Hz was also computed both from SGDR and ALES ranges. The sea state bias correction was recomputed for the ALES-retracked Envisat SLA. The quality of these altimeter products was validated using two tide gauges located on the southern coast of Spain. For Envisat, the availability of data close to the coast depends crucially on the strategy followed for data screening. Most of the rejected data were due to the radar instrument operating in a low-precision nonocean mode. We observed an improvement of about 20% in the accuracy of the Envisat SLAs from ALES compared to the standard (SGDR) and the reprocessed CTOH data sets. AltiKa shows higher accuracy, with no significant differences between SGDR and ALES. The use of products from both missions allows longer times series, leading to a better understanding of the hydrodynamic processes in the study area

Comparison of sea-ice freeboard distributions from aircraft data and cryosat-2

The only remote sensing technique capable of obtain- ing sea-ice thickness on basin-scale are satellite altime- ter missions, such as the 2010 launched CryoSat-2. It is equipped with a Ku-Band radar altimeter, which mea- sures the height of the ice surface above the sea level. This method requires highly accurate range measure- ments. During the CryoSat Validation Experiment (Cry- oVEx) 2011 in the Lincoln Sea, Cryosat-2 underpasses were accomplished with two aircraft, which carried an airborne laser-scanner, a radar altimeter and an electro- magnetic induction device for direct sea-ice thickness re- trieval. Both aircraft flew in close formation at the same time of a CryoSat-2 overpass. This is a study about the comparison of the sea-ice freeboard and thickness dis- tribution of airborne validation and CryoSat-2 measure- ments within the multi-year sea-ice region of the Lincoln Sea in spring, with respect to the penetration of the Ku- Band signal into the snow

The optimal coastal retracked sea levels from saral/altika satellite altimetry over the southeast asia

The current demand for accurate coastal altimetry data, particularly for the sea level has increased since human activities have become increasingly concentrated along coastal areas. Over coastal region, particularly within 10 km from the coastline, the altimeter footprint is severely contaminated by land and rough coastal sea states. The contamination leads to the low quality observations, thus creating a significant gap in data availability over the coast. The aim of this study is to evaluate the quality of coastal retracked sea level data from AltiKa satellite altimetry over the Southeast Asia region. In this study, high resolution (40 Hz) sea levels derived from the advanced AltiKa satellite altimetry are validated over the Southeast Asia coastal regions. The parameter of sea level is derived based on three standard retracking algorithms which are MLE-4, Ice-1 and Ice-2. The assessments of quantity and quality of the retracked sea levels data are conducted to identify the optimum retracker over the study regions, which are Andaman Sea, Strait of Malacca, South China Sea, Gulf of Thailand and Sulu Sea. The quantitative analysis involves the comparison between AltiKa and Jason-2 waveforms, the computation of percentage of data availability, and the minimum distance of Sea Level Anomaly (SLA) to the coastline. The qualitative analysis involves the relative validation with geoid height and absolute validation with tide gauge. In general, AltiKa measurement can obtain as close as 1 km to the coastline with =85% data availability. The Ice-1 retracker has shown an excellent performance with percentage of data availability at =90% and minimum distance as close as 0.9 km to the coastline. In term of quality, Ice-1 retracker shows the highest improvement of percentage (IMP) values over Andaman Sea, Sulu Sea and Strait of Malacca with IMPs of 19%, 16% and 43%, respectively. The Ice-1 retracker also shows the highest temporal correlation (up to 0.95) and the lowest root mean square (RMS) error up to 8 cm over distance less than 10 km for those three regions. Contrary, over the South China Sea, Ice-2 retracker has better performance when compared to other retrackers with IMP values of 43%. Over distance less than 10 km to the shore, the temporal correlation and RMS error reach up to 0.88 and 7 cm respectively. Over the Gulf of Thailand, the optimum retracker cannot be concluded due to unavailable tide gauge data. The Ice-1 is the optimum retracker over three out of four regions. Therefore, it is used to study the seasonal variability of sea levels over the Southeast Asia. The seasonal variability shows that the mean amplitude is up to 25 cm during the Northeast Monsoon and decreased by 9 cm during the Southwest Monsoon and between 2 to 9 cm during inter-monsoon seasons. In conclusion, the research has significantly contributed in defining the quantity and quality of the AltiKa SLAs in the coastal region of Southeast Asia. The results from comprehensive validation obtained in this research present a significant improvement in identifying the reliability and applicability of the AltiKa datasets and retracking algorithms over the coastal area of the study region

CryoSat-2 Significant Wave Height in Polar Oceans Derived Using a Semi-Analytical Model of Synthetic Aperture Radar 2011–2019

This paper documents the retrieval of significant ocean surface wave heights in the Arctic Ocean from CryoSat-2 data. We use a semi-analytical model for an idealised synthetic aperture satellite radar or pulse-limited radar altimeter echo power. We develop a processing methodology that specifically considers both the Synthetic Aperture and Pulse Limited modes of the radar that change close to the sea ice edge within the Arctic Ocean. All CryoSat-2 echoes to date were matched by our idealised echo revealing wave heights over the period 2011–2019. Our retrieved data were contrasted to existing processing of CryoSat-2 data and wave model data, showing the improved fidelity and accuracy of the semi-analytical echo power model and the newly developed processing methods. We contrasted our data to in situ wave buoy measurements, showing improved data retrievals in seasonal sea ice covered seas. We have shown the importance of directly considering the correct satellite mode of operation in the Arctic Ocean where SAR is the dominant operating mode. Our new data are of specific use for wave model validation close to the sea ice edge and is available at the link in the data availability statement