A Climatological Spectral Study of the 500 mb Geopotential Height of the Northern Hemisphere

Abstract

A 10-year record of the 500 mb geopotential height for the Northern Hemisphere has been expanded into spherical harmonics and filtered in the time domain. Maps of the root-mean-square (rms) height have been constructed corresponding to different spatial scales and frequency bands. The spatial scales and frequency bands were chosen to emphasize blocking and cyclogenesis and to help isolate spurious, high-frequency parts of the field from the physically meaningful parts. We find that low-frequency fields are dominated by planetary-scale waves at high latitudes and by synoptic-scale waves at mid-latitudes. The medium-frequency fields get substantial contributions from the waves of synoptic scale and shorter. Power spectra of the spherical harmonic expansion coefficients are presented, as well as quadrature spectra for each pair of cosine and sine expansion coefficients. We find large-scale waves have a large amount of low-frequency power and a spectrum rapidly decreasing with frequency. Shorter waves have less low-frequency power but have more slowly decreasing power with frequency. We also find westward propagation dominating the longer waves while the shorter waves propagate eastward. All calculations are performed for both the winter and summer seasons.