Dawei Zheng

Least Squares-Based Filter for Remote SensingImage Noise Reduction

The Vondrak filter is a unique technique for smoothing data. The filter aims to achieve a balance between the fidelity and the smoothness of the filtered results. It can therefore preserve the original attributes of the observational data while, at the same time, smooth out the noise. We reformulate the 1-D Vondrak filter that has been widely used in data processing in fields such as astronomy and geophysics and then extend it into two dimensions. The method of conjugate gradients is used to solve the least squares optimization problem. The proposed 2-D filter is a powerful tool for enhancing the quality of various geoscience and remote sensing data such as satellite images. Various tests with simulated and real synthetic aperture radar interferograms show that the new filter is very effective in removing the noise.

Depiction of the Variations of Great Plains Precipitation and Its Relationship with Tropical Central-Eastern Pacific SST

Abstract Several advanced analysis tools are applied to depict the time–frequency characteristics of the variations of Great Plains (GP) precipitation and its relationship with tropical central-eastern Pacific Ocean sea surface temperature (SST). These tools are advantageous because they reveal the detailed features of the dominant time scales of precipitation variations, the combined effects of multiscale oscillating signals on the intensity of precipitation, and the variations of SST–precipitation relationships in time and frequency domains. The variability of GP precipitation is characterized by strong annual and semiannual signals, which have the most stable oscillating frequencies and the largest amplitudes. However, nonseasonal signals, which are less oscillatory and have smaller amplitudes and more variable frequencies with time, also contribute significantly to precipitation variability and may modify the seasonal cycle of GP precipitation. The phase of these nonseasonal signals is in phase (out o...

Pacific warm pool excitation, earth rotation and El Niño southern oscillations

The interannual changes in the Earths rotation rate, and hence in the length of day (LOD), are thought to be caused by the variation of the atmospheric angular momentum (AAM). However, there is still a considerable portion of the LOD variations that remain unexplained. Through analyzing the non-atmospheric LOD excitation contributed by the Western Pacific Warm Pool (WPWP) during the period of 1970-2000, the positive effects of the WPWP on the interannual LOD variation are found, although the scale of the warm pool is much smaller than that of the solid Earth. These effects are specifically intensified by the El Nino events, since more components of the LOD-AAM were accounted for by the warm pool excitation in the strong El Nino years. Changes in the Earths rotation rate has attracted significant attention, not only because it is an important geodetic issue but also because it has significant value as a global measure of variations within the hydrosphere, atmosphere, cryosphere and solid Earth, and hence the global changes.

Filtering method for SAR interferograms with strong noise

We develop a filtering method for SAR interferograms with strong noise based on the concept of the Vondrák filter and the commonly used Goldstein filter. The one‐dimensional Vondrák filter is first extended to two dimensions, and then utilized to smooth the Fourier spectra of overlapped interferogram patches. The smoothed spectra are then back‐transformed into the interferogram space. Experimental results with simulated and real datasets show that the proposed filter can eliminate up to 70% of noise in a SAR interferogram. The results also show a 15% to 35% improvement over some of the existing filters when the noise in an interferogram is strong, and the stronger the noise in an interferogram, the greater is the improvement.

Earth rotation and ENSO events:combined excitation of interannual LOD variations by multiscale atmospheric oscillations

Time series of the length of day characterizing the rate of Earth rotation, the atmospheric angular momentum and the Southern Oscillation Index from 1962 to 2000 are used to reexamine the relationships between the ENSO events and the changes in the length of day, as well as the global atmospheric angular momentum. Particular attention is given to the different effects of the 1982-1983 and 1997-1998 ENSO events on the variations of Earth rotation. The combined effects of multiscale atmospheric oscillations (seasonal, quasi-biennial and ENSO time scales) on the anomalous variations of the interannual rates of Earth rotation are revealed in this paper by studying the wavelet spectra of the data series

Movement of earth rotation and activities of atmosphere and ocean

The rotation of the earth, including the variation of the rotational rate and polar motion, represents the statement of the earth’s overall movement and interactions among the solid earth, atmosphere and ocean on a variety of space-time scales. They make the earth’s complex dynamical system under the conservation of angular momentum. The application and development of recent space geodetic techniques greatly promote the researches on the interactions between the earth rotation and the activities of atmosphere and ocean. This review will mainly report the progress in researches on the earth rotation and the activities of atmosphere and ocean as well as the air-sea interaction in the tropics, and prospect the direction for future theoretical investigations.

Interannual polar motion with relation to the North Atlantic Oscillation

The North Atlantic Oscillation (NAO) is an interannual, north-south seesaw in the atmosphere and ocean. The associated motion and mass redistribution will inevitably cause variations in the Earths polar motion. The present paper studies the relation of NAO with interannual polar motion. Two monthly series for the period of 1964-1994 are employed: the NAO index (NAOI), which is used as a proxy to measure the intensity of NAG, and the polar motion series. The latter is further processed to obtain the interannual polar motion excitation function psi by deconvolution and subsequent removal of the drift, the seasonal terms and short period variations. The time-domain cross correlation and frequency-domain coherence studies show significant correlations between NAOI and psi especially on the x-component of psi as anticipated from the general pattern of NAG. This suggests possible significant contribution of NAO in exciting interannual polar motion

SAR interferogram filtering with 2D Vondrak filter

2005 IEEE International Geoscience and Remote Sensing Symposium, IGARSS 2005, Seoul, 25-29 July 2005

Premonitory phenomenon of El Nino event reflected in the observations of LOD and sea level

Data sets of the changes of the length of day, the sea surface temperature in the eastern Pacific and of the sea level in Hong Long from tide gauge observations are used to analyze and reveal the reflections in the observations of the length of day and the sea level changes concerned with the premonitory phenomenon of next EI Niño event. The results from this study indicate that a new EI Niño event has been brewing with the ending of the strong La Niño event that started in early summer of 1998. The estimated formation period of the new EI Niño event will begin before the end of 2000, and the peak period may be reached at around the end of 2001.

Monitoring of 1997 El Niño from TOPEX/Poseidon

In early 1997, one El Niño event occurred. The change of the sea surface height during the event was monitored with TOPEX altimetric data, and this is helpful to the study on the formation and development mechanism of an El Niño event and its impact on the global climatic change.