Browsing by Author "Hwang, C"

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Analysis of some systematic errors affecting altimeter-derived sea surface gradient with application to geoid determination over Taiwan
(1997-01-01)
Hwang, C
;
交大名義發表
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土木工程學系
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National Chiao Tung University
;
Department of Civil Engineering
This paper analyzes several systematic errors affecting sea surface gradients derived from Seasat, Geosat/ERM, Geosat/GM, ERS-1/35d, ERS-1/GM and TOPEX/POSEIDON altimetry. Considering the data noises, the conclusion is: (1) only Seasat needs to correct for the non-geocentricity induced error, (2) only Seasat and Geosat/GM need to correct for the one cycle per revolution error, (3) only Seasat, ERS-1/GM and Geosat/GM need to correct for the tide model error; over shallow waters it is suggested to use a local tide model not solely from altimetry. The effects of the sea surface topography on gravity and geoid computations from altimetry are significant over areas with major oceanographic phenomena. In conclusion, sea surface gradient is a better data type than sea surface height. Sea surface gradients from altimetry, land gravity anomalies, ship gravity anomalies and elevation data were then used to calculate the geoid over Taiwan by least-squares collocation. The inclusion of sea surface gradients improves the geoid prediction by 27% when comparing the GPS-derived and the predicted geoidal heights, and by 30% when comparing the observed and the geoid-derived deflections of the vertical. The predicted geoid along coastal areas is accurate to 2 cm and can help GPS to do the third-order leveling.
Circulations and eddies over the South China Sea derived from TOPEX/Poseidon altimetry
(2000-10-15)
Hwang, C
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Chen, SA
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土木工程學系
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Department of Civil Engineering
TOPEX/Poseidon (T/P) altimeter data were used to derive time-varying circulation and eddies over the South China Sea (SCS) for 1993-1999. Large variabilities of sea surface heights and dense distribution of eddies were found to occur along a band across the SCS basin. A crossover method is introduced to compute pointwise velocity components. The TiP-derived circulations and eddies are consistent with the drifter results from the World Ocean Circulation Experiment data center. (1) Because of monsoons, the SCS circulation is largely cyclonic in winter, is largely anticyclonic in summer, and is modulated interannually by El Nino - Southern Oscillation. (2) Both cold- and warm-core eddies were found in the waters east of Vietnam, west of Luzon, and west of the Luzon Strait, but their times of occurrence and the way they evolved vary interannually. (3) The reversal of the alongshore currents east of Vietnam was in complete accordance with wind stress. The locations and kinematic properties of eddies over SCS were estimated by the least squares method. The averaged vorticities of the cold- and warm-core eddies are 1.684x10(-6) and -1.738x10(-6) rad s(-1), respectively, and the shearing and stretching deformations are one thousandth of the vorticities. The angular velocity of eddy decreases with increasing radius. Coherence analysis suggests that the interannual and seasonal variations of circulation are largely due to wind stress, and the semi-annual variation is largely due to wind stress curl. The angular velocities of eddies over the central SCS basin are coherent with the magnitudes and signs of wind stress curl. The seasonal steric anomaly was also computed, and its spatial pattern and scale are quite different from those of circulation and eddy.
Fast integration of low orbiter's trajectory perturbed by the earth's non-sphericity
(1998-10-01)
Hwang, C
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Lin, MJ
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土木工程學系
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Department of Civil Engineering
A fast algorithm is proposed to integrate the trajectory of a low obiter perturbed by the earth's nonsphericity. The algorithm uses a separation degree to define the low-degree and the high-degree acceleration components, the former computed rigorously, and the latter interpolated from gridded accelerations. An FFT method is used to grid the accelerations. An optimal grid type for the algorithm depends on the trajectory's permissible error, speed, and memory capacity. Using the non-spherical accelerations computed from EGM96 to harmonic degree 360, orbit integrations were performed for a low orbiter at an altitude of 170 km. For a separation degree of 50, the new algorithm, together with the predict-pseudo correct method, speeds up the integration by 145 times compared to the conventional algorithm while keeping the errors in position and velocity below 10(-4) m and 10(-7) mis for a 3-day arc.
Fast integration of low orbiter's trajectory perturbed by the earth's non-sphericity
(1998-10-01)
Hwang, C
;
Lin, MJ
;
土木工程學系
;
Department of Civil Engineering
A fast algorithm is proposed to integrate the trajectory of a low obiter perturbed by the earth's nonsphericity. The algorithm uses a separation degree to define the low-degree and the high-degree acceleration components, the former computed rigorously, and the latter interpolated from gridded accelerations. An FFT method is used to grid the accelerations. An optimal grid type for the algorithm depends on the trajectory's permissible error, speed, and memory capacity. Using the non-spherical accelerations computed from EGM96 to harmonic degree 360, orbit integrations were performed for a low orbiter at an altitude of 170 km. For a separation degree of 50, the new algorithm, together with the predict-pseudo correct method, speeds up the integration by 145 times compared to the conventional algorithm while keeping the errors in position and velocity below 10(-4) m and 10(-7) mis for a 3-day arc.
Fourier and wavelet analyses of TOPEX/Poseidon-derived sea level anomaly over the South China Sea: A contribution to the South China Sea Monsoon Experiment
(2000-12-15)
Hwang, C
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Chen, SA
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土木工程學系
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Department of Civil Engineering
We processed 5.6 years of TOPEX/Poseidon altimeter data and obtained time series of sea level anomaly (SLA) over the South China Sea (SCS). Fourier analysis shows that sea level variability of the SCS contains major components with periods larger than 180 days and is dominated by the annual and semiannual components. Tidal aliasing creates 30-180 day components that can be misinterpreted as wind-induced variabilities. Continuous and multiresolution wavelet analyses show that the SLA of the SCS has monthly to interannual components of time-varying amplitudes, and the regional slope of SLA is 8.9 mm yr(-1), which may be caused by the decadal climate change. Coherences of SLA with wind stress anomalies (WSA) and sea surface temperature anomalies (STA) are significant at the annual and semi-annual components. At periods of 2-5 years the wavelet coefficients of SLA, WSA, and STA have the same pattern, but WSA leads SLA, and STA follows SLA. The zero crossing of SLA in spring is highly correlated with the onset of the summer monsoon. The interannual variability of SLA is correlated with El Nino-Southern Oscillation, and most important is that when the El Nino-like wavelet coefficients of SLA over the warm pool northeast of Australia or the SCS change curvature from negative to positive, an El Nino is likely to develop. This is a contribution to the South China Sea Monsoon Experiment (SCSMEX).
Global derivation of marine gravity anomalies from Seasat, Geosat, ERS-1 and TOPEX/POSEIDON altimeter data
(1998-08-01)
Hwang, C
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Kao, EC
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Parsons, B
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土木工程學系
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Department of Civil Engineering
Marine gravity anomalies over the area 82 degrees S to 82 degrees N and 0 degrees E to 360 degrees E on a 2' x 2' grid have been derived from Seasat, Geosat, ERS-1 and TOPEX/POSEIDON altimeter data. The inverse Vening Meinesz formula with a 1-D FFT method was used to compute gravity anomalies from gridded north-south and west-east geoid gradients, in a remove-restore procedure with the EGM96 gravity model as the reference field. This paper presents a new gridding technique and an altimeter data management system that help to extract date efficiently. In the 12 areas where ship-measured gravity and satellite-derived gravity were compared, rms agreements of 5-14 mgal were obtained. Special discussions of the gravity anomalies over selected inland seas, the Arctic, Antarctica and coastal waters are presented. The derived gravity anomalies show detailed tectonic structures of the world ocean. The global data set will be continuously updated with more altimeter data included in the derivation.
Gravity recovery using COSMIC GPS data: application of orbital perturbation theory
(2001-05-01)
Hwang, C
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土木工程學系
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Department of Civil Engineering
COSMIC is a joint Taiwan-US mission to study the atmosphere using the Global Positioning System (GPS) occultation technique. Improved formulas are developed for the radial, along-track, and cross-track perturbations, which are more accurate than the commonly used order-zero formulas. The formulas are used to simulate gravity recovery using the geodetic GPS data of COSMIC in the operational phase. Results show that the EGM96 model can be improved up to degree 26 using 1 year of COSMIC data. TOPEX/POSEIDON altimeter data are used to derive a temporal gravity variation. COSMIC cannot reproduce this gravity variation perfectly because of data noise and orbital configuration, but the recovered field clearly shows the gravity signature due to mass movement in an El Nine.
Inverse Vening Meinesz formula and deflection-geoid formula: applications to the predictions of gravity and geoid over the South China Sea
(1998-05-01)
Hwang, C
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土木工程學系
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Department of Civil Engineering
Using the spherical harmonic representations of the earth's disturbing potential and its functionals, we derive the inverse Vening Meinesz formula, which converts deflection of the vertical to gravity anomaly using the gradient of the H function. The deflection-geoid formula is also derived that converts deflection to geoidal undulation using the gradient of the C function. The two formulae are implemented by the 1D FFT and the 2D FFT methods. The innermost zone effect is derived. The inverse Vening Meinesz formula is employed to compute gravity anomalies and geoidal undulations over the South China Sea using deflections from Seasat, Geosat, ERS-1 and TOPEX//POSEIDON satellite altimetry. The 1D FFT yields the best result of 9.9-mgal rms difference with the shipborne gravity anomalies. Using the simulated deflections from EGM96, the deflection-geoid formula yields a 4-cm rms difference with the EGM96-generated geoid. The predicted gravity anomalies and geoidal undulations can be used to study the tectonic structure and the ocean circulations of the South China Sea.
Orthometric corrections from leveling, gravity, density and elevation data: a case study in Taiwan
(2003-08-01)
Hwang, C
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Hsiao, YS
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土木工程學系
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Department of Civil Engineering
A new orthometric correction (OC) formula is presented and tested with various mean gravity reduction methods using leveling, gravity, elevation, and density data. For mean gravity computations, the Helmert method, a modified Helmert method with variable density and gravity anomaly gradient, and a modified Mader method were used. An improved method of terrain correction computation based on Gaussian quadrature is used in the modified Mader method. These methods produce different results and yield OCs that are greater than 10 cm between adjacent benchmarks (separated by similar to2 km) at elevations over 3000 in. Applying OC reduces misclosures at closed leveling circuits and improves the results of leveling network adjustments. Variable density yields variation of OC at millimeter level everywhere, while gravity anomaly gradient introduces variation of OC of greater than 10 cm at higher elevations, suggesting that these quantities must be considered in OC. The modified Mader method is recommended for computing OC.
Spherical harmonic analysis and synthesis using FFT: Application to temporal gravity variation
(2006-05-01)
Hwang, C
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Kao, YC
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土木工程學系
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Department of Civil Engineering
FFT and complex algebra-based methods or spherical harmonic analysis and synthesis are presented. Two computer programs in FORTRAN are developed based on the methods. Both general and special cases are discussed. Special cases involve the analyses of gravity changes of the hydrological origin and the atmospheric origin. Functionals of the Earth's gravity field such as gravity anomaly and geoidal height can also be computed via synthesis. Thermal-corrected sea level anomaly from TOPEX/Poseidon and atmospheric pressure from ECMWF are used to compute changes of geopotential coefficients due to oceanic and atmospheric mass redistributions. Interesting phenomena in the changes of geopotential coefficients have been identified. The two computer programs can facilitate analyses and syntheses of gravity products from satellite missions such as GRACE. (c) 2005 Elsevier Ltd. All rights reserved.
The kinematics of mesoscale eddies from TOPEX/Poseidon altimetry over the Subtropical Counter Current
(2004)
Hwang, C
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Kao, R
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Wu, CR
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土木工程學系
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Department of Civil Engineering
TOPEX/POSEIDON altimetry data were used to track mesoscale eddies over the Subtropical Counter Current (STCC). The radii, centers, vorticities, shearing deformation rates, stretching deformation rates, divergences and center velocities of all identified eddies over STCC were determined using a model that assumes constant velocity gradients. A case study was made for a cyclonic eddy and an anticyclonic eddy, with time series of eddy kinematic parameters computed. Both eddies survive for about 220 days and propagate westwards along over 22degreesN-24degreesN to reach the Kuroshio Current east coast of Taiwan. Sea surface temperature data confirm the existence of these two eddies. The radii of both eddies vary and their shapes are mostly elliptical during propagation. The anticyclonic eddy propagated almost westwards with oscillating north-south components, and the mean speed is 8.3 km/day. The cyclonic eddy moved southwestwards before reaching 130degreesE and then moved northwestwards, with a mean speed of 7.6 km/day. The propagations of these two eddies are basically consistent with the standard theory of eddy propagation but with larger speeds and varying propagating directions.
TOPEX/Poseidon observations of mesoscale eddies over the Subtropical Countercurrent: Kinematic characteristics of an anticyclonic eddy and a cyclonic eddy
(2004-08-17)
Hwang, C
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Wu, CR
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Kao, R
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土木工程學系
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Department of Civil Engineering
Relative dynamic heights and geostrophic fields were derived from TOPEX/Poseidon altimetry data and then used to track mesoscale eddies over the Subtropical Countercurrent (STCC). The radii, centers, vorticities, shearing deformation rates, stretching deformation rates, divergences, and center velocities of all identified eddies over the STCC were determined using a model that assumes constant velocity gradients. Most eddies are concentrated in a zonal band near 22degreesN, and there is an interannual variation in the number of eddies. A case study was made for a cyclonic eddy and an anticyclonic eddy, with time series of eddy kinematic parameters computed. Both eddies survive for similar to220 days and propagate westward along over 22degreesN-24degreesN to reach the Kuroshio Current east coast of Taiwan, where the eddies were dissipated and in turn affected the Kuroshio Current in many ways. Sea surface temperature data and drifter data confirm the existence of these two eddies. The radii of both eddies vary and their shapes are mostly elliptical during propagation. The anticyclonic eddy propagated almost westward with oscillating north-south components, and the mean speed is 8.3 km/day. The cyclonic eddy moved southwestward before reaching 130degreesE and then moved northwestward, with a mean speed of 7.6 km/day. The propagations of these two eddies are basically consistent with the standard theory of eddy propagation but with larger speeds. The propagating direction could be altered while passing steep bottom topography or merging with the other eddies.