# Hayashi spectra of the northern hemisphere mid-latitude atmospheric variability in the NCEP–NCAR and ECMWF reanalyses

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## Abstract

We compare 45 years of the reanalyses of National Center for Environmental Prediction–National Center for Atmospheric Research and European Center for Mid-Range Weather Forecast in terms of their representation of the mid-latitude winter atmospheric variability for the overlapping time frame 1957–2002. We adopt the classical approach of computing the Hayashi spectra of the 500 hPa geopotential height fields and we introduce an ad hoc integral measure of the variability observed in the Northern Hemisphere on different spectral subdomains. Discrepancies are found especially in the pre-satellite years of the records in the high frequency-high wavenumber propagating waves. This implies that in the pre-satellite period the two datasets have a different representation of the baroclinic available energy conversion processes. Minor differences are also found in the description of low frequency–low wavenumber standing waves. We observe a positive impact of the satellite data on the representation of wave activity over the oceanic sectors in the period starting from 1979, in particular on the description of high frequency variability. Since in the pre-satellite period the assimilated data are more scarce, predominately over the oceans, and of lower quality than found later on, they provide a weaker constraint to the model dynamics. Therefore, the resulting discrepancies in the reanalysis products may be mainly attributed to differences in the models’ behaviour.

## Notes

### Acknowledgments

The authors wish to thank A. Sutera, A. Speranza and anonymous reviewers for useful suggestions and comments. NCEP data have been provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado, from their web site at http://www.cdc.noaa.gov/. The ECMWF ERA-40 data have been obtained from the ECMWF data server at http://data.ecmwf.int/data/.

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