Estimates of Tropical Analysis Differences in Daily Values Produced by Two Operational Centers

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Abstract
In order to asses the uncertainty of daily synoptic analyses for the atmospheric state, the intercomparison of three First GARP Global Experiment (FGGE) level IIIb datasets is conducted. The original analyses and reanalyses produced by the European Centre for Medium Range Weather Forecasts (ECMWF) are compared with the reanalyses produced by the National Meteorological Center (NMC) system, operational in early 1987. Daily values of vorticity ζ, divergence δ, temperature T, static stability Γ, mixing ratio q, vertical motion ω, and diagnosed diabatic heating rate are compared for the period of 26 January–11 February 1979. The spatial mean and variance, temporal mean and variance, two-dimensional wavenumber power spectrum, anomaly correlation, and normalized square difference are used for comparison. Also, equivalent blackbody temperatures from the TIROS-N are used as a proxy to the vertical motion and diagnosed diabatic heating rates in the tropics. Data are interpolated onto the σ coordinates of the NCAR Community Climate Model (CCM 1 ) with 12 vertical levels. Global data are expanded in spherical harmonics with two resolutions [triangular 13 (T 13) and T42] in order to investigate how data agreement changes depending on the horizontal length scale. Other questions to be investigated are: What aspects of the analyses have improved in the FGGE reanalyses produced at the ECMWF and NMC? What aspects of the analyses are still unsatisfactory? What can be done to further improve the analyses? Since data agreement tends to be weak in the tropics, attention is focused in the tropical belt of 30°N–30°S. One enlightening finding is that both ζ and δ of the NMC reanalyses agree more closely with the ECMWF reanalyses than their earlier analyses. This result may indicate that both the data quality and the analysis techniques have improved. More good news is found in that the agreement of ζ at T13 is excellent, while there is only slight disagreement at T42, indicating that FGGE has succeeded in describing the quasi-rotational state of the atmosphere, even in the tropics. The bad news is that interanalysis agreement of δ and q>/ is poor. Similarly, the analyses of ω show the least agreement, indicating the need for further improvement in describing the diabatically driven irrotational circulations of the tropics.

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