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Chih-Pei Chang

Abstract

A new method of using satellite photographs in the study of large-scale cloud motions in the tropics is presented. Cloud clusters are seen to be propagating westward with a phase speed ∼9 m sec−1 in the summer of 1967, and can sometimes be followed all the way across the Pacific. It is suggested that this method can be used in the study of tropical wave disturbances.

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Chih-Pei Chang

Abstract

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Chih-Pei Chang

Abstract

The solutions of the linear equatorial boundary layer are evaluated and found to be dependent on both the depth of the modelled boundary layer and the mode of symmetry about the equator. The inclusion of the frictional effect at levels higher than the usually assumed 1-km boundary layer depth is crucial to the concentration of convergence around the critical latitude. It is therefore suggested that the top of the boundary layer in tropical wave models should be properly selected in order to represent the deep convergence around the critical latitude.

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Chih-Pei Chang

Abstract

In this paper we deal with the interpretation of observed oscillations in the tropical troposphere and stratosphere within the framework of the equatorial wave theory. A difficulty with this problem arises when one compares the short vertical wavelength (or equivalent depth) predicted by the classical theory and the observed large vertical scales associated with the low Doppler-shifted frequencies of the tropospheric oscillations. In this analysis it is shown that the inclusion of simple linear damping, justified by budget studies which revealed the important role of cumulus momentum transport, has a strong influence at low frequencies on the forced equatorial waves and results in two types of dispersive relationships. The first type is characteristic of the regular internal gravity waves which have fast phase speeds and weak vertical attenuation. The second type is dominated by the viscous damping time scale and has slow phase speeds and strong vertical trapping. The theory predicts that the stratosphere oscillations may be identified with the first type and the tropospheric oscillations with the second. In the case of Kelvin waves the results can be used to explain consistently both the observed stratospheric Kelvin waves and the planetary-scale Kelvin-like oscillations in the troposphere including the 40–50 day oscillation and the monsoon and Walker circulations. Possible implications with respect to other waves in the tropics are also discussed.

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Chih-Pei Chang

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Solutions to the wave-CISK (conditional instability of the second kind with cumulus heating being induced by low-level internal wave convergence) system are obtained to study the vertical structure of orginally unstable waves. A diabatic heating profile is specified that resembles those observed and those theoretically derived from simple pararmeterization schemes. Upper and lower bounds for the vertical wavelength of the unstable waves under normal beating conditions are established through analysis of the frequency (stability) equation. The lower bound excludes the possibility of excitation or maintenance of short vertical wavelengths (relative to the vertical scale of heating) by wave-CISK. The calculated growth rates indicate that this result is basically insensitive to the vertical heating profile. The vertical structure of the most unstable waves is also computed and the possible roles played by CISK in large-scale tropical waves are discussed in light of these results.

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Chih-Pei Chang

Abstract

The problem of scale-selection of Kelvin waves in the stratosphere by forcing from tropospheric heating is analyzed using a simple linear model. The effect of vertical wind shear is excluded because the phase speed of the waves is fast relative to the range of the mean zonal wind in the vicinity of the tropopause at which level the upward energy flux due to forcing is evaluated. Results of this analysis modify Holton's (1973) theory in that 1) the forcing is most efficient for the longest zonal wavelength even if the heat sources are distributed randomly, and 2) the most favored vertical wavelength of the excited waves is about twice the vertical scale of beating. The calculated vertical wavelengths exceed slightly those observed and the discrepancies are discussed.

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