Parametric excitation is a mechanism to influence the energy transport of internal gravity waves. It manifests itself as a decay of a primary wave and growth of two secondary waves through resonant intetaction. Current research status on this issue stays still at the parametric instability in plane monochromatic internal gravity waves. By using a numerical method the temporal and spatial evolution of parametric excitation in propagating gravity wave packets is studied. It is indicated that through resonant interaction two secondary waves can in sevenal hours reach significant intensities above the noise level and possess considerable space area. The energy amounts which two secondary waves extract from the primary wave are different, exhibiting a parameter dependence of the energy transfer in a triad. The primary wave packet provides only a part of its initial energy to excite secondary waves. The primary wave packet is apparently deformed as it decays. This is caused by the localization in the energy transfer characteristics. The energy transfer among the interacting waves is no longer reversible since the propagating wave packets own limited space area. Thus the characteristic bine for the interaction is of particular significance. It represents a stage in which principal energy transfer arises. Beyond the characteristic bine the net energy transfer among the interacting waves becomes rather weak. The decay process for a gravity wave packet propagating from the mesosphere to the lower thermosphere can cop out sufficiently. Resonant interaction is not only able to diffuse wave energy across spectral space but tends to spread wave energy in physical space.
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