Synoptic- and Meso-.ALPHA.-Scale Variations of the Baiu Front Simulated in an AGCM

Abstract

The present paper studies features of the Baiu front and associated precipitation systems simulated in the climatological SST run by an AGCM T106L52 (primitive equation spectral model, the maximum wave-number of 106, with 52 vertical levels) in comparison with the features found in observational studies. The Baiu front is properly simulated in 1-20 June Y07 (the 7th year after the spin-up integration). In this “Baiu phase”, the large-scale circulation systems, such as the upper cold lows and blocking ridge in the northern latitudes, westward extending Pacific subtropical anticyclone, monsoon westerly, and subtropical jet stream, are simultaneously maintained. Under this condition, the synoptic- and meso-ascale variations of the Baiu front are simulated. The southward shift of the front occurs when a synopticscale cyclone develops in association with a westerly trough. When westerly troughs are inactive, weak subsynoptic-scale depression is formed in the frontal zone. A few meso-α-scale precipitation systems are generated in the trailing portion of the preceding depression, and form a “Baiu precipitation system family” with a length of∼2000 km. This indicates that the simulation of the realistic Baiu front depends on the maintenance of proper large- and synoptic-scale circulation systems. However, the Baiu precipitation zone disappears in the early July, as the west-east elongating barotropic anticyclone, which is usually seen over Japan in August in the real atmosphere, is situated over ∼37°N.