The Midlatitude Development of Regional Errors in a Global GCM

Abstract

The growth of geographically confined errors is studied in six experiments with a five-level global spectral (rhomboidal 30) general circulation model. Each experiment consists of 36 identical twin integrations with the initial errors localized in the same region. The centers of the regions are 180°, 120°W, 60°W, 0°, 60°E, and 120°E; all at 45°N. The perturbations are derived from differences in model states taken from a long integration. They reflect the natural distribution of variance in the model atmosphere, and are representative of realistic analysis errors. By day 10 the errors have propagated (predominantly eastward) until they have reached either of the oceanic baroclinic waveguides and have amplified dramatically there. Errors originating in the central Pacific or western North America amplify over the Atlantic. Errors from the European, central Asian, and East Asian regions grow most strongly over the Pacific. Errors originating over the Atlantic show a mixed behavior. The rates of propagation involved are consistent with the downstream development of baroclinic instability. The behavior of the errors normalized by the climatological variance is similar. The cause of preferential error growth in the oceanic waveguides is the markedly baroclinic structure of the errors in these regions. The largest 10-day errors originate in the Pacific and grow and propagate into the Atlantic, while the smallest ten-day forecast errors arise from initial errors over Europe. The largest 10-day relative error (error divided by its initial value) arise from errors originally confined to East Asia and Asia, which develop over the Pacific. The experiments with perturbations centered at 60°E, 120°E, and 180° were repeated using initial errors that were identical in structure and magnitude but were zonally translated. At day 10 the errors that originate over the East Asian coast and developed in the mid-Pacific were the largest. The relative error in these modified experiments behaved very much like that in the original experiments. After the initial period of growth, a stagnation of the errors is seen in the primary of the two oceanic areas, accompanied by error growth in the other (secondary) oceanic region. For certain experiments (those with initial errors in the Pacific, Asia, and East Asia) the latter development can be quite rapid. There is a substantial variation in the growth rates from case to case, with the rms of the most rapidly growing perturbation as large as three times that of the average. The identity of the pair of states leading to the largest error depends on the forecast time.