Two-dimensional turbulence properties of the ECMWF reanalyses

1999 Tellus: Series A, Dynamic Meteorology and Oceanography  
A spherical harmonic analysis is made of the stationary and transient rotational motions during 14 winter and 15 summer seasons from the reanalyses of the European Centre for Medium-Range Weather Forecasts (ECMWF) for the northern hemisphere. Vertically-integrated kinetic energy, enstrophy, rotational non-linear interactions and the baroclinic source term are diagnosed as a function of total wavenumber n. The contributions to the non-linear transfers from triads involving wavenumber and
more » ... enumber and frequency bands of meterological relevance are mapped. The inter-seasonal and intra-seasonal variability are computed. The non-linear energy and enstrophy tendencies and fluxes are examined and compared to existing geophysical turbulence theories. The transient divergent kinetic energy is less than the rotational energy. The spectral energy slope seen in the range n~10-40 is roughly −2.5~−2.6. Based on the variability of the slope on seasonal and 10-day time scales, this slope is significantly different than −3. There is no indication of the −5/3 mesoscale energy regime seen in observations. A broad enstrophy dissipation regimes is seen for n>40. Non-linear terms transfer transient energy from a band centered at n~15 to one at n~7, with the latter predominantly associated with non-zonal (zonal wavenumber m≠0) flow. Non-linear terms transfer mean energy from n=7 to the mean zonal flow m=0, n=3 and n=5. The non-linear transfer of transient energy is quite variable, with about 16% of 10-day periods yielding a tendency twice the mean, and 16% showing no upscale tendency whatever. This variability is greatly reduced when interactions involving only synoptic scales (n~10-40) are retained. The latter set of interactions are associated mostly with triads involving both high and low frequencies, with associated periods in the 1-9 day and 11-90 range, respectively. Non-local planetary wave advective interactions play an important rô le in the downscale transfer of enstrophy. More local interactions involving synoptic scales dominate the non-linear energy transfers. The main seasonal effects are a weakening in summer of the total energy and shifting to higher wave number of the peak, and a distinct shift to smaller scales in the transition between the large-scale and synoptic-scale regimes in the energy budget. The predominant time scale for non-linear maintenance of the planetary waves (about 9 days) is roughly the same as that of the baroclinic support of larger synoptic scale waves. The time scale of baroclinic conversion which maintains the smaller synoptic waves (n~20-40) is shorter (about 4 days).
doi:10.1034/j.1600-0870.1996.00015.x fatcat:dtwi3xoh4renhatlksoct2nr3i