Gravity wave characteristics in the middle atmosphere derived from the Empirical Mode Decomposition method

Abstract. The recently developed Empirical Mode Decomposition (EMD) method is applied to analyzing gravity wave characteristics in the middle atmosphere. By establishing a close connection between the fundamental Intrinsic Mode Functions (IMFs) derived from the EMD method and WKB solutions of a dispersive-dissipative wave equation, we show that the EMD method can provide useful insights into physical processes in the ß I 11 _. _ _ I I 1 ' ß 1- ß.• ' i i ß., oom•nam. A local u•spers•ve-cnss•pauve pnenomena are mmcne atmospnere wave power spectrum function P is introduced which provides a quantitative description of the spectrum at any particular location within a data series. The sharp localization of P in space and wavenumber leads to an identification of unphysical small scale oscillations by falling spheres embedded in the wind profiles above 60 km. Further analyses of the horizontal wind profiles derived from the Dynamics Adapted Network for the Atmosphere (DYANA) campaign suggest that for horizontal wind fluctuations with vertical wavenumber m _< 3 km- • (or vertical wavelength L z-> 2 km) the previously observed m-3 Fourier spectra could be produced by a linear wave packet whose characteristic vertical wavenumber decreases with altitude. For small vertical scale disturbances with m> 3 km- • (L z < 2 km) a near-3 slope in the marginal distribution exists locally in the middle atmosphere with a great degree of universality, suggesting that nonlinear energy cascade processes may dominate the spectral formation in this wavenumber ange. 1