A turbulence analysis is made of transient motions during 15 winter and 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 wave number n, and in terms of contributions of low and high frequencies. The inter-seasonal and intra-seasonal variability is computed. The non-linear energy and enstrophy tendencies and fluxes are examined and compared to existing geophysical turbulence theories. The main results are: (a) The spectral 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. (b) There is no indication of the -5/3 energy regime for small scales seen in aircraft observations. (c) Non-linear terms transfer energy from a band at n ~15 to one at n~7. This transfer is quite variable, with very roughly 16% of ten-day periods yielding a tendency twice the mean, and 16% showing no upscale tendency whatever. This variability is greatly reduced when only interactions involving synoptic and sub-synoptic scales are retained. (d) Non-local planetary/sub-synoptic and planetary/small scale interactions play an important role in the downscale transfer of enstrophy, and are responsible for about 1/3 of the total tendency at very small scales in winter. More local interactions involving synoptic and sub-synoptic scales dominate the non-linear energy transfers, and are important (along with interactions between sub-synoptic and small scales) for the enstrophy transfer. (e) The high frequencies dominate the local interactions in the enstrophy transfer described above in (d). Interactions between low and high frequencies dominate the synoptic and sub-synoptic upscale energy transfers described above in (c). (f) The main seasonal effects are a dramatic weakening in summer of the total energy and shifting to higher wave number of the peak, and a distinct shift to higher scales in the transition between the large-scale and synoptic-scale regimes in the energy budget. The summer spectra are slightly steeper over the range for which a single slope is well-defined.
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Center for Ocean-Land-Atmosphere Studies
last update: 5 January 1998
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