Revealing short-period normal modes of the atmosphere

Abstract

Barometer and seismometer measurements at Collm, Germany (51.3° N, 13.0° E) for all of 2002 are used to search for atmospheric normal modes (ANMs) in the frequency range 50–310 µHz. The measurements are spectrally analyzed using a 5-day window sliding along the 1-year series with a 1-day step. The subsequent analysis follows two procedures: (a) revealing features in the frequency distribution of the number of statistically significant spectral peaks in histograms built on the basis of these spectra and (b) calculating the multiplication spectra for the raw spectra. The two procedures yield the same result for the two instruments, i.e., reveal a periodicity in the clustering of atmospheric modes on the frequency axis with a period of ∼6 µHz. The fact that this period is close to 7 μHz, which is predicted by the crude theory of gravity—inertia ANMs [3] for their frequency distribution, suggests that ANMs are generated down to as small a period as ∼1 h.