Evolution of Ensemble Perturbations in an Atmospheric General Circulation Model

The properties of the ensemble perturbations evolving in an atmospheric general circulation model (AGCM), known as AFES (AGCM for the Earth Simulator, Ohfuchi et al. 2004) at a relatively high-resolution (T159/L48), have been investigated by analyzing two kinds of ensemble experiments performed on the Earth Simulator. One is a 100-member breeding experiment, where the control run is nudged to the analysis states provided by the Japan Meteorological Agency, and the ninety-nine perturbations are bred around the control run with globally orthogonalizing and rescaling every 12 hours. The other is 40-member local ensemble transform Kalman filter (LETKF) experiments; several kinds of 6-hour cycle perfect model experiments are performed by changing the observing networks and localization scales. The amplitude of the LETKF perturbations is greatly dependent on the observing density as expected. We found that both kinds of ensemble perturbations have large amplitudes in the extratropics and that their dominant spatial scales are slightly smaller than the synoptic scales of O(1000 km). These results are more apparent when their amplitudes are suppressed considerably in the breeding experiment or are controlled appropriately over the densely observed regions in the LETKF experiments. The perturbations seem to grow rapidly in the locally unstable areas induced by the development of mid-latitude depressions.