The Development of the Medium-scale Disturbance in the Baiu Front

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Abstract
On July 04, 1969, severe precipitation took place over the Japan Islands with the passage of medium-scale disturbances along the Baiu front. The structure and the process of the development of the disturbance are analysed in detail by using the data of the Severe Rainstorms Research Project and satellite cloud pictures. The satellite and radar pictures show that the disturbance is accompanied by a well organized medium-scale cluster of convective clouds. Detailed analyses of rawinsonde data inside the disturbance revealed that the deepening of the surface pressure is deeply related to the formation of a mid-tropospheric warm core accompanied by a significant field of convergence in the lower troposphere. The high humidity in the warm core and the intense precipitation suggest that the formation of the walm core is due to the process of so-called convective warming, i.e. the release of latent heat and the upward transport of heat energy caused by cumulus convections. The formation of the convergence field is due to the increase of low-level wind velocity over the intense rainfall area. The active cumulus convections in the sheared westerly transport the momentum downward and therefore work to increase the low-level wind velocity.

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