Browsing by Author "Deng, Xiaoli"

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Coastal gravity anomalies from retracked Geosat/GM altimetry: Improvement, limitation and the role of airborne gravity data
(2006-07-01)
Hwang, Cheinway
;
Guo, Jinyun
;
Deng, Xiaoli
;
Hsu, Hsin-Ying
;
Liu, Yuting
We process geophysical and waveform data records of the Geosat/GM (geodetic mission) satellite altimeter mission for waveform retracking and applications. An improved threshold retracker is developed. The performances of the Beta-5, threshold and improved threshold retrackers are assessed over waters around Taiwan. The improved threshold retracker outperforms the other two. The improvement in the accuracy of sea surface height (SSH) is investigated according to marine zone and the distance of waters to the shore. The improvement rate increases closer to the land, with the largest improvement rate of about 20% in waters within 10 km of the shore. Over waters around islands and coasts, there are still retracked SSHs with large errors. Least-squares collocation is used to compute gravity anomalies from the Geosat/GM altimeter data. Use of retracked SSHs improves the accuracy of gravity anomalies by about 11%. Adding airborne gravity data further improves the accuracy, especially in the immediate vicinity of the coasts. Tide model errors over coastal waters remain a problem in altimetry applications, even if the waveforms are properly retracked.
Seasonal and interannual variations of the Leeuwin Current off Western Australia from TOPEX/Poseidon satellite altimetry
(2008-04-01)
Deng, Xiaoli
;
Hwang, Cheinway
;
Coleman, Richard
;
Featherstone, Will E.
;
土木工程學系
The Leeuwin Current (LC) is an eastern boundary current flowing strongly southwards along the Western Australian coastline. The current varies seasonally, with weaker southwards flow occurring during the austral summer (November to March), when southerly winds are strongest. Nearly 9.5 years of TOPEX/Poseidon-derived sea-surface heights, in conjunction with the AUSGeoid98 gravimetric geoid model, have been used to investigate the temporal characteristics of the LC over the area bounded by 20 to 45 degrees S and 108 to 130 degrees E. The ocean dynamic height associated with the LC is used to compute the geostrophic parameters of volume transport, axis velocity, height jump, width, and axis locations of the current: These parameter estimates are then used to analyse the seasonal and interannual variability of the LC, as well as effects related to the El Nino Southern Oscillation (ENSO). Our results show that the LC is highly variable in terms of seasonal volume transport and is closely linked to ENSO events at interannual time-scales. Seasonally, when assuming an averaged isobath of 80 m, the LC has maximum volume transport of up to -7.2 Sv (10(6) m(3) s(-1), a negative sign means southwards transport) in April - June and -8.4 Sv in May - August in regions west and south of Western Australia, respectively. Inter-annually, the LC has higher volume transports than the average transport during 1993 - 2003 in most La Nina years, and the opposite in most El Nino years, which is linked to inter-basin exchange between the Pacific and Indian Oceans.