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. The author finds 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. All calculations are performed for both the winter and summer seasons.