Browsing by Author "Yang, Yuande"

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A fixed full-matrix method for determining ice sheet height change from satellite altimeter: an ENVISAT case study in East Antarctica with backscatter analysis
(2014-09-01)
Yang, Yuande
;
Hwang, Cheinway
;
E, Dongchen
;
土木工程學系
;
Department of Civil Engineering
A new method, called the fixed full-matrix method (FFM), is used to compute height changes at crossovers of satellite altimeter ground tracks. Using the ENVISAT data in East Antarctica, FFM results in crossovers of altimeter heights that are 1.9 and 79 times more than those from the fixed half method (FHM) and the one-row method (ORM). The mean standard error of height changes is about 14 cm from ORM, which is reduced to 7 cm by FHM and to 3 cm by FFM. Unlike FHM, FFM leads to uniform errors in the first-half and second-half height-change time series. FFM has the advantage in improving the accuracy of the change of height and backscattered power over ORM and FHM. Assisted by the ICESat-derived height changes, we determine the optimal threshold correlation coefficient (TCC) for a best correction for the backscatter effect on ENVISAT height changes. The TCC value of 0.92 yields an optimal result for FFM. With this value, FFM yields ENVISAT-derived height change rates in East Antarctica mostly falling between and 3 cm/year, and matching the ICESat result to 0.94 cm/year. The ENVISAT result will provide a constraint on the current mass balance result along the Chinese expedition route CHINARE.
A subwaveform threshold retracker for ERS-1 altimetry: A case study in the Antarctic Ocean
(2012-04-01)
Yang, Yuande
;
Hwang, Cheinway
;
Hsu, Hung-Jui
;
E, Dongchen
;
Wang, Haihong
Based on a correlation analysis method, a subwaveform threshold retracker is developed and coded in FORTRAN for satellite altimetry to determine the leading edge and retracking gate, and to improve the precision of sea surface heights (SSHs) and gravity anomalies (GAs). Using ERS-1/ERM waveforms, the subwaveform threshold retracker outperforms full-waveform threshold retrackers at the tide gage Port Station. A direct comparison between retracked SSHs and in situ SSHs is made at tide gage Port Station. Here the subwaveform retracking improves SSH precision from 0.241 to 0.193 m, yielding an improvement percentage (IMP) of 20%. Using ERS-1/GM waveforms, the subwaveform threshold retracker outperforms the Beta-5 and full-waveform threshold retrackers over the Bellingshausen and Amundsen Seas (BAS) in the Antarctic Ocean. The standard deviations of raw and retracked SSHs are 0.157 and 0.070 and 1.836 and 0.220 m over the ice-free and ice-covered oceans, corresponding to IMPs of 54.4% and 88%, respectively. Use of retracking improves the precision of GAs by up to 46.6% when comparing altimeter-derived and shipborne GAs. (C) 2011 Elsevier Ltd. All rights reserved.
GPS-derived velocity and strain fields around Dome Argus, Antarctica
(2014-01-01)
Yang, Yuande
;
Sun, Bo
;
Wang, Zemin
;
Ding, Minghu
;
Hwang, Cheinway
Knowledge of the surface topography, velocity field and strain field at an ice-core site is critical to the accurate interpretation of ice-core records. At Dome Argus (Dome A), where a Chinese deep ice-core drilling project is being carried out, we have produced an accurate surface digital elevation model from GPS measurements in January 2013 at 47 sites. We identify two peaks at Dome A, with the northern peak similar to 7 cm higher than the southern peak. Repeat GPS measurements at 12 sites in 2008 and 2013 provide a surface velocity field around the dome. The surface velocity ranges from 3.1 +/- 2.6 to 29.4 +/- 1.2 cm a(-1), with a mean of 11.1 +/- 2.4 cm a(-1). The surface flow directions are near perpendicular to the surface elevation contours. Velocities from GPS are lower than derived from satellite radar interferometry (InSAR). From GPS velocities, the accuracy of velocity from the existing InSAR velocity field is determined, resulting in a standard deviation of 0.570 m a(-1) in speed and 117.5 degrees in direction. This result is consistent with the reported accuracy of InSAR, showing the value of in situ GPS measurements for assessing and correcting remote-sensing results. A surface strain field for the drilling site over Dome A is calculated from 24 strain triangles, showing north-south extension, east-west compression and vertical layer thinning.
Space-Time Evolution of Greenland Ice Sheet Elevation and Mass From Envisat and GRACE Data
(2019-08-01)
Yang, Yuande
;
Li, Fei
;
Hwang, Cheinway
;
Ding, Minghu
;
Ran, Jiangjun
Precise measurements of height changes (HCs) are important for improved estimates of mass balance of the Greenland Ice Sheet (GrIS). Here we determine 10 years of precise, high-resolution HCs of the GrIS from Envisat radar altimeter using a subwaveform retracker and a modified repeat-track method. The HCs show clear seasonal changes and monotonic declines over glaciers on the coasts such as Zachariae IsstrOm. We enhance mass change estimates from Gravity Recovery and Climate Experiment (GRACE) data using HCs and densities from interannual correlations between GRACE-derived mass changes and HCs. We estimate the mass changes of eight drainage basins with our combined mass change. The largest mass change between 2002 and 2012 occurred in the northwest basin and the smallest in the northeast basin. We separate the ice and snow HC rates to derive a ratio (f) between them to characterize the relative importance of ice or snow to mass change. The snow HC rates are mostly positive over the GrIS, except on the margins of the west coast and Zachariae IsstrOm. The mean ice HC rate is -6.6 3.9 cm/year over 2002-2006, which accelerated to -13.9 2.4 cm/year over 2007-2012. The f factors show a clear post-2006 ice dominance in the GrIS mass loss, particularly on the west coast, with a mean 91.4% ice contribution over 2002-2006, increasing to 94.5% over 2007-2012. This indicates increasing mass loss after 2006. A coincident radar altimeter and gravimetry mission is important for studying mass balance and separating snow and ice contributions over space and time.
Time-varying land subsidence detected by radar altimetry: California, Taiwan and north China
(2016-06-21)
Hwang, Cheinway
;
Yang, Yuande
;
Kao, Ricky
;
Han, Jiancheng
;
Shum, C. K.
Contemporary applications of radar altimetry include sea-level rise, ocean circulation, marine gravity, and icesheet elevation change. Unlike InSAR and GNSS, which are widely used to map surface deformation, altimetry is neither reliant on highly temporally-correlated ground features nor as limited by the available spatial coverage, and can provide long-term temporal subsidence monitoring capability. Here we use multi-mission radar altimetry with an approximately 23 year data-span to quantify land subsidence in cropland areas. Subsidence rates from TOPEX/POSEIDON, JASON-1, ENVISAT, and JASON-2 during 1992-2015 show time-varying trends with respect to displacement over time in California's San Joaquin Valley and central Taiwan, possibly related to changes in land use, climatic conditions (drought) and regulatory measures affecting groundwater use. Near Hanford, California, subsidence rates reach 18 cm yr(-1) with a cumulative subsidence of 206 cm, which potentially could adversely affect operations of the planned California High-Speed Rail. The maximum subsidence rate in central Taiwan is 8 cm yr(-1). Radar altimetry also reveals time-varying subsidence in the North China Plain consistent with the declines of groundwater storage and existing water infrastructure detected by the Gravity Recovery And Climate Experiment (GRACE) satellites, with rates reaching 20 cm yr(-1) and cumulative subsidence as much as 155 cm.