My scientific interests lie primarily in global geophysics using space-geodetic and seismological observational data; they pertain to a broad range of geophysical areas listed below in the order of effort over the years (in a majority of cases collaborating with colleagues and students):
(1) Earth rotation and low-degree gravitational variations: We studied the geophysical excitations of the length-of-day variations, the polar motion, as well as the Earth’s oblateness (J2) changes (e.g. the 1998 anomaly) due to surface geophysical fluids (atmosphere, ocean, land hydrology, cryosphere, in association with ENSO and other climatic fluctuations), tidal influences, core fluid motion, and co-seismic deformations. For the latter we developed the calculation method using normal-mode summation. We also extended such studies to understand the rotation and gravity of Mars and its two satellites.
(2) Earth’s rotational normal modes: I derived mathematical (convolution) expressions for the excitation of a rotational normal mode in a spinning reference frame (such as the Earth), and applied them to the Chandler wobble and the free-core nutation (e.g. to determine their natural frequencies and Q-values). I derived (multipole) expressions for the mantle-inner core gravitational torques, and delineated their connection to the inner-core libration and inner-core wobble. We found a 6-year in variation the Length-of-day relating to the inner-core libration, which in turn creates a 6-year westward propagating wave-2 rotary motion manifested in the GPS, geomagnetic, and global gravity observations.
(3) Harmonic analysis method: I developed the autoregressive (AR) estimation method in the frequency domain for determining the complex frequency (frequency and Q) of a free oscillation, and further extended it to form a highly sensitive and high-resolution AR-z spectrum. We successfully applied them to the detection and analyses of Earth’s free oscillations, tidal and other harmonic signals in various rotational and gravitational phenomena.
(4) We explored with the utility of empirical orthogonal function (EOF) and wavelet spectrum to study time-variable gravity from the GRACE satellite and ocean radar altimetry observations, particularly for oceanographic phenomena (e.g. Antarctica Circum-polar Current and the Argentine gyre).
(5) I studied the anthropogenic effects of water impoundment of reservoirs on global geophysics, particularly for sea level variation.
(6) I have written numerous general Earth-scientific articles featured in AGU’s EOS, as well as for popular-science magazines (in Chinese) in Taiwan and China.