Chung Yen Kuo

Vertical crustal motion determined by satellite altimetry and tide gauge data in Fennoscandia

[1] We present a new method of combining satellite altimetry and tide gauge data to obtain improved estimates of absolute (or geocentric) vertical crustal motion at tide gauges within a semi-enclosed sea. As an illustration, we combine TOPEX/POSEIDON altimetry data (1992-2001) and 25 long-term (>40 years) tide gauge records around the Baltic Sea region of Fennoscandia, an area where crustal deformation is dominated by glacial isostatic adjustment (GIA). A comparison of the estimated vertical motion, at 1-11 mm/yr, with independent solutions from 10 collocated BIFROST GPS sites, shows a difference of 0.2 ± 0.9 mm/yr, thus verifying the accuracy and robustness of the procedure. The solution uncertainty is estimated at 0.4 mm/yr, which is significantly lower than previous analyses of this type. We conclude that our technique can potentially provide accurate vertical motion observations globally where long-term tide gauge records exist.

Louisiana Wetland Water Level Monitoring Using Retracked TOPEX/POSEIDON Altimetry

Previous studies using satellite radar altimetry to observe inland river and wetland water level changes usually spatially average high-rate (10-Hz for TOPEX, 18-Hz for Envisat) measurements. Here we develop a technique to apply retracking of TOPEX waveforms by optimizing the estimated retracked gate positions using the Offset Center of Gravity retracker. This study, for the first time, utilizes stacking of retracked TOPEX data over Louisiana wetland and concludes that the water level observed by each of 10-Hz data with along-track sampling of ∼660 m exhibit variations, indicating detection of wetland dynamics. After further validations using nearby river gauges, we conclude that TOPEX is capable of measuring accurate water level changes beneath heavy-vegetation canopy region (swamp forest), and that it revealed wetland dynamic flow characteristics along track with spatial scale of 660 m or longer.

Calibration of JASON-1 Altimeter over Lake Erie Special Issue: Jason-1 Calibration/Validation

This article describes absolute calibration results for both JASON-1 and TOPEX Side B (TSB) altimeters obtained at the Lake Erie calibration site, Marblehead, Ohio, USA. Using 15 overflights, the estimated JASON altimeter bias at Marblehead is 58 ± 38 mm, with an uncertainty of 19 mm based on detailed error analysis. Assuming that the TSB bias is negligible, relative bias estimates using both data from the TSB-JASON formation flight period and data from 48 water level gauges around the entire Great Lakes confirmed the Marblehead results. Global analyses using both the formation flight data and dual-satellite (TSB and JASON) crossovers yield a similar relative bias estimate of 146 ± 59 mm, which agrees well with open ocean absolute calibration results obtained at Harvest, Corsica, and Bass Strait (e.g., Watson et al. 2003). We find that there is a strong dependence of bias estimates on the choice of sea state bias (SSB) models. Results indicate that the invariant JASON instrument bias estimated oceanwide is 71 mm, with additional biases of 76 mm or 28 mm contributed by the choice of Collecte Localisation Satellites (CLS) SSB or Center for Space Research (CSR) SSB model, respectively. Similar analysis in the Great Lakes yields the invariant JASON instrument bias at 19 mm, with the SSB contributed biases at 58 mm or 13 mm, respectively. The reason for the discrepancy is currently unknown and warrants further investigation. Finally, comparison of the TOPEX/POSEIDON mission (1992–2002) data with the Great Lakes water level gauge measurements yields a negligible TOPEX altimeter drift of 0.1 mm/yr.

Validation of Jason-2 Altimeter Data by Waveform Retracking over California Coastal Ocean

We validated Jason-2 satellite altimeter Sensor Geophysical Data Records (SGDR) by retracking 20-Hz radar waveforms over the California coastal ocean using cycles 7–34, corresponding to September 2008–June 2009. The performance of the ocean, ice, threshold, and modified threshold retrackers are examined using a reference geoid based on Earth Gravitational Model 2008 (EGM08). Over the shallow ocean (depth < 200 m), the modified threshold retracker, which is developed for noisy waveforms with preleading edge bump, outperforms the other retrackers. It is also shown that retracking can improve the precision of sea surface heights (SSHs) for areas beyond 2–5 km from the shore. Although the ocean retracker generally performs well over the deep ocean (depth > 200 m), the ocean-retracked SSHs from some of the cycles are found to be less precise when the waveforms do not conform to the Brown ocean model. We found that the retrackers developed for nonocean surfaces can improve the noisy ocean-retracked SSHs. Among the retrackers tested here, the ice retracker overall provides the most precise SSH estimates over the deep ocean in average using cycles 7–34 in the study region.

Application of retracked satellite altimetry for inland hydrologic studies

We explored the application of satellite radar altimetry for the monitoring of small inland bodies of water and hydrologic studies using a water-detection algorithm, optimally retracked TOPEX/POSEIDON data at 10-Hz sampling, and investigated the use of radar backscatter to improve land cover classification. The procedure was demonstrated over Manitoba and south-western (SW) Ontario, and the Amazon River Basin study regions. Compared with an L-band synthetic aperture radar data generated water-land cover mask, the water-detection algorithm detected more water points over the Amazon basin. High correlation of 0.98 between the retracked 10-Hz altimetry and the gauge measurements in Manitoba confirmed that the retracked TOPEX data are more accurate than the non-retracked data, and with higher along-track spatial resolution by virtue of its higher sampling at 10 Hz.

The Improved Retrieval of Coastal Sea Surface Heights by Retracking Modified Radar Altimetry Waveforms

Measuring sea surface height (SSH) using satellite altimetry in coastal ( from coasts) and shallow water region has long been a challenge since the radar altimeter waveforms are often contaminated by complex coastal topography and do not conform to theoretical Brown waveform shapes. The land contamination or surface variation due to ocean dynamics induce spurious peaks in altimeter waveforms that deviate from Browns theoretical model as the altimeter footprint approaches or leaves the shoreline. These spurious peaks should be mitigated to minimize the error in the determination of the leading edge and associated track offset in the waveform retracking process. Here, we introduce a novel algorithm to modify coastal waveforms (0.5-7 km from coasts, using 20 Hz altimetry data), thus improving coastal data coverage and accuracy. We apply our processing algorithm and use various retrackers to compare retrieved coastal SSHs in four study regions in North America, using both Envisat and Jason-2 altimetry. The retrieved altimetry data in the 1-7 km coastal zone indicate that the 20% Threshold retracker with modified waveform has a RMSE of 21 cm as compared with in situ tide gauge data, which corresponds to a 63% improvement in accuracy compared to the use of the original deep-ocean waveform retracker.

Retracked Jason-2 altimetry over small water bodies: Case study of Bajhang River, Taiwan

River surface heights (RSHs) are retrieved from Jason-2 (J2) 20-Hz radar altimeter waveforms over the narrow Bajhang River in Taiwan using waveform retracking and water detection techniques. Our technique is demonstrated over the narrow river, with changing widths of 100–200 m in the dry season and up to 1 km after heavy rains, which is significantly smaller in size compared to previous studies. Here, we used radar backscatter coefficients and accurately geolocated Google Earth Images validated by GPS campaigns for water detection and employed optimized waveform retrackers to retrieve 20-Hz accurate RSHs. The waveforms of the selected J2 ground tracks are carefully analyzed to remove land-contaminated data. Several waveform retracking algorithms, including offset center of gravity (OCOG), threshold, modified threshold, and ice retrackers are used to find the optimal algorithm by comparing the retracked RSHs with the nearby I-Chu stage gauge records and evaluating the computed improvement percentages (IMPs). The results indicate that the J2-derived RSHs over Bajhang River can be significantly improved by waveform retracking using the original and modified threshold retrackers with a 50% threshold level. The IMP reached 73.53% and the standard deviation of differences between the optimal retracked RSHs and stage gauge records is ∼31 cm. Our successful demonstration of the use of J2 to measure water level changes of the small river with seasonally varying widths implicates a potential new hydrologic application further exploiting satellite altimetry.

Lake Surface Height Calibration of Jason-1 and Jason-2 Over the Great Lakes

This study presents results of Jason-1 (J1) and Jason-2 (J2) radar altimetry absolute calibration (cal/val) over the Marblehead lake water level gauge, with an aid of a GPS buoy, in Lake Erie, the Great Lakes, in North America. The altimeter bias is estimated using the height difference between the altimeter lake surface height and the in situ data in each altimeter 10-day repeat cycle. The altimeter bias estimates for J1 are 81 ± 2 and 70 ± 2 mm for Geophysical Data Record (GDR) Versions ‘B’ and ‘C’, respectively, and 148 ± 5 mm (GDR) and 147 ± 7 mm (IGDR) for J2, respectively. The bias estimates are slightly smaller compared with estimates at other dedicated calibration sites such as the Harvest Platform, the Corsica Site, and the Bass Strait site due in part to the effect of the sea state bias. The J2-to-J1 relative bias determined in the tandem mode over Lake Erie is 84 ± 28 mm, which agrees well with the results from the global analysis and the dedicated sites aforementioned.

Evaluation of Ocean Tide Models Used for Jason-2 Altimetry Corrections

It has been more than a decade since the last comprehensive accuracy assessment of global ocean tide models. Here, we conduct an evaluation of the barotropic ocean tide corrections, which were computed using FES2004 and GOT00.2, and other models on the Jason-2 altimetry Geophysical Data Record (GDR), with a focus on selected coastal regions with energetic ocean dynamics. We compared nine historical and contemporary ocean tide models with pelagic tidal constants and with multiple satellite altimetry mission (T/P, ERS-1/-2, Envisat, GFO, Jason-1/-2) sea level anomalies using variance reduction studies. All accuracy assessment methods show consistent results. We conclude that all the contemporary ocean tide models evaluated have similar performance in the selected coastal regions. However, their accuracies are region-dependent and overall are significantly worse than those in the deep-ocean, which are at the 2–3 cm RMS (root-mean-square) level. The Gulf of Mexico and Northwest Atlantic regions present the least reduction of altimetry sea surface height variability after ocean tides are removed, primarily because of large oceanic variability associated with loop currents in the Gulf of Mexico and the Gulf Stream in the Northwest Atlantic.

Southern Ocean mass variation studies using GRACE and satellite altimetry

The Southern Ocean is a major link between the world oceans via complicated processes associated with the melting and accumulation of the vast Antarctic ice sheets and the surrounding sea ice. The Southern Ocean sea level is poorly observed except from recent near-polar orbiting space geodetic satellites. In this study, the Southern Ocean mass variations at the seasonal scale are compared using three independent data sets: (1) the Gravity Recovery And Climate Recovery Experiment (GRACE) observed ocean bottom pressure (OBP), (2) steric-corrected satellite altimetry (ENVISAT) and, (3) the Estimating the Circulation and Climate of the Ocean (ECCO) model OBP data. The height difference between sea level derived from altimetry and steric sea level contains the vertical displacement of the Earth surface due to elastic loading. Here we provide a formulation of this loading term which has not been considered previously in other studies and demonstrate that it is not negligible, especially for regional studies. In this study, we first conduct a global comparison using steric-corrected JASON-1 altimetry with GRACE to validate our technique and to compare with recent studies. The global ocean mass variation comparison shows excellent agreement with high correlation (∼0.81) and with discrepancies at 3–5 mm RMS. However, the discrepancies in the Southern Ocean are much larger at 12–17 mm RMS. The mis-modeling of geocenter variations and the second degree zonal harmonic (J2) degrade the accuracy of GRACE-derived mass variations, and the choice of ocean temperature data sets and neglecting the loading correction on altimetry affect the OBP comparisons between GRACE and altimetry. This study indicates that the satellite observations (GRACE and ENVISAT) are capable of providing an improved constraint of oceanic mass variations in the Southern Ocean.