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Zheng Shen and Liming Mei

Abstract

With the help of fractal geometry used to model the intermittency of energy input from wind to wave components, the theoretical spectra of the equilibrium range in wind-generated gravity waves proposed by Phillips are refined.

On account of the intermittency, it is proven that the classical frequency spectral exponent 4 must he replaced by 4 + (2 − D), where D is the informational entropy dimension of the support subset, upon which the energy input from the wind to the gravity waves in the equilibrium range is concentrated. To a first approximation, it is found that D ≈ 1.88 and 4 + (2 − D) ≈ 4.12. The variation of the Toba constant is found to be proportional to (u 2 */gL 0)(2−D)/2, where L 0 is the wavelength of the longest wave component in the equilibrium range, that is, the lower limit wavenumber above which the processes of energy input from wind, spectral flux divergence, and loss by breaking are all significant and proportional. The refined wavenumber spectrum is less sensitive to wind strength than the original.

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Zheng Shen, Wei Wang, and Liming Mei

Abstract

One central problem in the study of wind-generated gravity waves is the energy balancing process in the equilibrium spectral subrange. In considering the predicted equilibrium spectral forms from physical models proposed by Kitaigorodskii, other investigators accepted that the statistical equilibrium state is effectively characterized by the wave action conservation law: δEt+C⃗g·∇E = 0, where E is the wave energy spectrum and C⃗g = ∇kω(k) is the group velocity. Here the continuous wavelet transform is used to analyze typical sets of wind-generated gravity wave data obtained both in the ocean and in a wind-wave channel. This “space scale” analysis is shown to provide the first visual evidence that when the fetch is not very short, the wave action conservation law mentioned above is not convenient to describe the dynamics of the wave components in the equilibrium range estimated from its energy spectrum.

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Norden E. Huang, Hsing H. Shih, Zheng Shen, Steven R. Long, and Kuang L. Fan

Abstract

Using a process denoted here as the empirical mode decomposition and the Hilbert spectral analysis, the ages of the seiches on the Caribbean coast of Puerto Rico are determined from their dispersion characteristics with respect to time. The ages deduced from this method are less than a day; therefore, the seiches could be locally generated.

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Shi Liu, Song Yang, Yi Lian, Dawei Zheng, Min Wen, Gang Tu, Baizhu Shen, Zongting Gao, and Donghai Wang

Abstract

The time–frequency characteristics of the variations of temperature and precipitation over the city of Changchun in northeast China and their associations with large-scale atmospheric and oceanic conditions are analyzed. It is found that the variations of the regional climate are characterized by strong semiannual signals. For precipitation, the amplitude of semiannual signal is about half of that of the annual cycle. The relationships of the Changchun temperature and precipitation with local winds and large-scale patterns of atmospheric circulation and sea surface temperature are also strongest on annual and semiannual time scales. These strong semiannual signals are potentially helpful for improving the prediction of the regional climate.

On the annual time scale, the northeast China climate is affected by both the thermal contrast between the Asian continent and the tropical Indo-Pacific Oceans and that between the continent and the extratropical North Pacific. These effects are manifested by the cyclonic (anticyclonic) pattern over the Asian continent (North Pacific) and the strong southerly flow over East Asia and northwestern Pacific associated with increases in temperature and precipitation. On the semiannual time scale, the northeast China climate is mainly related to the large-scale circulation pattern centered over the North Pacific, with its western portion over northeast China, North and South Korea, and Japan. While temperature signals are related to extratropical atmospheric process more apparently, both extratropical and tropical influences are seen in the semiannual variation of precipitation.

There exist strong relationships between Changchun temperature and precipitation and the North Pacific Oscillation (NPO) in the frequency band up to 7 months. Temperature increases and precipitation decreases when NPO is positive. The relationships were weak before 1980 but became stronger afterward, associated with the strengthening of the East Asian trough.

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