Spectral Properties of Analyzed and Forecast Global 500 mb fields

Abstract

Global ECMWF data for winter and summer seasons are used to study the spectral structure of 500 mb analyzed height variance and forecast error variance, as well as the effect of spectral truncation in the height and temperature variance calculations. The high wavenumber region of the variance of the analyses is nearly homogeneous and isotropic and follows the ?5 power law both in the zonal, two-dimensional and meridional wavenumber indexes. The high wavenumber region is dominated by transient waves. The low wavenumber region is dominated by the stationary part, being anisotropic and seasonally variable. The transition between the two regions is at wavenumbers 7?8. The temperature variance follows a ?4 power law in the high wavenumber region. The ECMWF forecast error variance is dominated by the nonsystematic error, the maximum of which shifts from medium to long zonal waves during the forecast. The nonsystematic error variance spectrum is quite similar to the transient variance in the analyses.