Browsing by Author "Shum, C. K."

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Introduction to the special issue on Tibet: Contemporary geodetic-geophysical observations and interpretations
(2019-02-01)
Hwang, Cheinway
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Shen, Wen-Bin
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Shum, C. K.
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Song, Xiaodong
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土木工程學系
Preface to the Special Issue on Geodynamic and Climate-Change Processes over Tibet, Xinjiang and Siberia
(2011-04-01)
Hwang, Cheinway
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Chao, Benjamin Fong
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Freymueller, Jeffrey T.
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Shen, Wenbin
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Shum, C. K.
Preface to the special issue on satellite altimetry over land and coastal zones: Applications and challenges
(2008-04-01)
Hwang, Cheinway
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Benveniste, Jerome
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Dang, Yamin
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Shum, C. K.
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土木工程學系
Regional surface mass anomalies from GRACE KBR measurements: Application of L-curve regularization and a priori hydrological knowledge
(2012-11-08)
Tangdamrongsub, Natthachet
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Hwang, Cheinway
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Shum, C. K.
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Wang, Lei
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土木工程學系
We present a method of directly estimating surface mass anomalies at regional scales using satellite-to-satellite K-band Ranging (KBR) data from the Gravity Recovery and Climate Experiment (GRACE) twin-satellite mission. Geopotential differences based primarily on KBR measurement are derived using a modified energy integral method with an improved method to calibrate accelerometer measurements. Surface mass anomalies are computed based on a downward continuation process, with optimal regularization parameters estimated using the L-curve criterion method. We derive the covariance functions in both space-and space-time domains and use them as light constraints in the regional gravity estimation process in the Amazon basin study region. The space-time covariance function has a time-correlation distance of 1.27 months, which is evident that observations between neighboring months are correlated and the correlation should be taken into account. However, most of the current GRACE solutions did not consider such temporal correlations. In our study, the artifact in the regional gravity solution is mitigated by using the covariance functions. The averaged commission errors are estimated to be only 6.86% and 5.85% for the solutions based on the space-covariance function (SCF) and the space-time covariance function (STCF), respectively. Our regional gravity solution in the Amazon basin study region, which requires no further post-processing, shows enhanced regional gravity signatures, reduced gravity artifacts, and the gravity solution agrees with NASA/GSFC's GRACE MASCON solution to about 1 cm RMS in terms of water thickness change over the Amazon basin study region. The regional gravity solution also retains the maximum signal energy while suppressing the short wavelength errors.
Remote sensing of glacier distribution and change over the Qinghai-Tibet Plateau
(2016)
Ke, Linghong
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Ding, Xiaoli
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Zhang, Lei
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Shum, C. K.
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Hwang, Cheinway
Glacier change over the Qinghai-Tibet Plateau (QTP) provides an important clue to climate change in the remote high-altitude areas, and impacts various aspects of regional environment. Observations of mountain glaciers over the QTP are yet far from sufficient. Remote sensing techniques enable mapping and monitoring glaciers over an extensive region, from different dimensions and with regular revisit time. At present, mapping debris-covered glaciers and estimating glacier mass balances are one of the most challenging issues. This study illustrates the use of remote sensing techniques for glacier mapping and change detection, with multi-temporal and multi-source data. We show contrasting behavior of different type glaciers (temperate, semi-continental and continental) in three typical densely glacierized sub-regions of the plateau, mainly based on elevation data from ICESat and Landsat imagery. Glacier-climate interaction is analyzed further. Here we stress the challenges of applying remote sensing techniques for mapping and monitoring glaciers over the QTP, and prospects in future studies.
SOLID EARTH DEFORMATION MONITORING USING SATELLITE ALTIMETRY IN SOUTHWESTERN COASTS IN TAIWAN
(2010)
Cheng, Kai-chien
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Hwang, Cheinway
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Chung, Yu-Hsiang
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Huang, Ching-Yao
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Wei, Shiang-Hung
A novel method of using waveform retracked altimeter observations on land for the monitoring of the solid Earth deformation in Taiwan is described in the article. The southwestern coasts of Taiwan have been experiencing a significant land subsidence in the rate of similar to 8 cm/year, due to, in part, the excess extraction of the underground water. The first-order leveling networks and the GPS control points available in the area were used to verified the results of the waveform retracked altimeter observation. In this study, we proposed to use TOPEX/Poseidon (T/P) altimeter land observations from 1992-2002 on one T/P groundtrack that passes the southwestern coasts of Taiwan where is part of the Chia-Nan Plain with the main vegetation being rice paddy. This special vegetation and flat terrain provide an opportunity for successful waveform retracking. The comparison between T/P retracked result and that from the leveling network and GPS control points are presented.
Time-varying land subsidence detected by radar altimetry: California, Taiwan and north China
(2016-06-21)
Hwang, Cheinway
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Yang, Yuande
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Kao, Ricky
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Han, Jiancheng
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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.