Empirical normal mode diagnosis of reanalysis data and dynamical-core experiments

Global data and numerical models are used to investigate the climatology and variability in the upper-troposphere and lower-stratosphere where large-scale low-frequency vorticity modes dominate over high-frequency inertia-gravity waves. An algorithm based on the Empirical Normal Mode decomposition technique is proposed as a diagnostic tool. This technique combines dynamics (a wave-activity of linear dynamics defines the orthogonality between modes) with statistics (the wave-activity amplitude of each mode provides a measure of its statistical significance) and has the unbiased ability to capture neutral normal modes as well as singular modes.; First, the algorithm is applied to winter data provided by the National Centers for Environmental Prediction (NCEP) reanalyses and results indicate that most wave-activity is carried by large-scale, eastward-propagating modes centered at mid- and high-latitudes. The phase-speeds of some leading modes are in good agreement with theoretical predictions of linear dynamics. Some mid-latitude modes exhibit properties that can be explained by the theory of quasi-modes---for example, the leading wavenumber-5 mode with a dipolar pressure field near the tropopause, a propagation speed of 12 m s-1 and a decay rate of 3 days.; In a model study, we use data from two dynamical-core experiments of the Global Environmental Multiscale model: one with the forcing proposed by Held and Suarez, later modified by Williamson et al. (called HSW experiment); the other with the forcing by Boer and Denis (BD). Modes and spectra are similar to those found in the NCEP data study, although details depend on the forcing. For instance, wave-energy amplitudes are higher with the BD forcing and an approximate energy equipartition is observed in the spectrum of wavenumber-1 modes in the studies of NCEP data and BD experiment, but not in the HSW experiment.; The HSW forcing has a relatively strong relaxation acting on the complete temperature field, whereas the BD forcing only acts on the zonal-mean temperature letting the internal dynamics alone drive the wave-activity cascade through the rest of the spectrum. This difference seems to explain why the BD forcing is more successful in reproducing the observed atmospheric wave-activity.