Gravity waves are wide spread in the summer mesopause region and polar mesospheric clouds (PMCs) serve as a natural manifestation of these wave activities. A majority of the PMC population appears to be “fibered” by at least semi-organized wave-like structures, among which the highest occurrence frequency is at wavelengths < 100km. In this study a wave tracking approach has been designed to identify the gravity waves at wavelengths of ~25-50km in the PMCs measured by Cloud Imaging and Particle Size experiment (CIPS) aboard the AIM satellite. In the wave tracking approach, a circular region (~500km radius) is placed at a given location in the CIPS cloud domain and then a wavelet transform is carried out along all 360-degree radial directions. Regions with a large mean wavelet power are determined as being wave active. The PMC mean brightness over such a circular region is within ~50×10-6 sr-1 and the median value is approximately ~15-20×10-6sr-1. The whole season analysis in 2007 shows that distinctly better organized wave features possess only a moderate level of wavelet power (i.e., 8-10×10-12 sr-2) and occur mostly in dimmer orbits. The maximum daily frequency of such occurrences reaches ~5% which was obtained in early to middle summer time. While regions with moderate to high level of wavelet power show a strong correlation with the seasonal cloud brightness variation and therefore the maximum daily frequency (~30%) occurs in the middle to late summer when the PMCs are the brightest. In these later cases no distinct wave features are present. Rather, the components with the scales of ~25-50km exhibit as spatially alternating and wide spread bright spots and dim voids overlapped on a background filled with semi-organized wave-like features. We infer from this study that such features resulted from nonlinear or complex interaction of multi-scale gravity waves.

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*email: ping-ping.rong@hamptonu.edu
*Preference: Poster