Medium Range Prediction by a GFDL Global Spectral Model: Results for Three Winter Cases and Sensitivity to Dissipation

Abstract

Abstract A preliminary evaluation is made of the medium range predictive capability of a GFDL global spectral model of the atmosphere, based upon three winter blocking cases. Analogous forecasts by a GFDL global grid point model provide a background standard of comparison. The spectral model is rhomboidally truncated at wavenumber 30, has 9 sigma levels, incorporates sub-grid scale physical processes commonly associated with general circulation models and employs semi-implicit time differencing. The grid point model has somewhat finer horizontal resolution and fairly similar sub-grid scale physical processes, and employs explicit time differencing. The spectral model is up to 6 times more economical. The level of forecast skill for the 5 to 15 day range is generally less than practically useful and is more case-dependent than spectral versus grid point model-dependent. In the most successful case, i.e., 16 January 1979, an observed Atlantic blocking ridge is simulated quite well, especially by the spectra...