Periodic Evolution of Multiscale Precipitation Systems Developed within a Baiu Frontal Cloud Cluster

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  • WAKAZUKI Yasutaka
    Hydrospheric Atmospheric Research Center, Nagoya University Advanced Earth Science and Technology Organization, Meteorological Research Institute (MRI), Tsukuba
  • TSUBOKI Kazuhisa
    Hydrospheric Atmospheric Research Center, Nagoya University
  • TAKEDA Takao
    Hydrospheric Atmospheric Research Center, Nagoya University

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Abstract

A meso-α-scale cloud cluster (CC) was observed in the Baiu frontal zone, located southwest of Kyushu, Japan, on July 7, 1996. It was characterized by a lifespan of approximately 20 hours, stationary motion, and heavy precipitation of over 200 mm in a period of 12 hours. This paper contains a report of the characteristic features of internal multiscale precipitation systems observed within the CC, and their periodic evolutions as newly found facts.<br>The CC consisted of a convective rainfall region, characterized by a meso-βL-scale (100-200 km) line-shaped convective system (convective line, MβLCL), and a weak stratiform rainfall region on the lee side of the MβLCL. The MβLCL consisted of several band-shaped meso-βS-scale (20-100 km) convective systems (MβSCSs), and each MβSCS consisted of meso-γ-scale cumulonimbus clouds. The environmental atmosphere was characterized by a warm and moist inflow in the shallow layer (below 500 m in height) associated with a cyclonic circulation of slow moving and shallow depression. Additionally, the CC was located within a large temperature gradient zone in the lower atmosphere (below 4000 m in height) along the Baiu front. These were favorable environments for the generation and maintenance of theMβLCL. The MβLCLs were periodically generated with an interval of 5-6 hours in almost the same region, and they showed a common evolution of structure and processes.<br>The behavior of cold pools formed by the MβLCLs on the northern side of the MβLCL played a key role in the periodic evolution. The developed cold pool intensified the MβSCSs in the MβLCL and modified one of the MβSCSs into an arc-shaped MβSCS, that had similar characteristics to those ofsquall lines. Simultaneously, the arc-shaped MβSCS became the sole MβSCS that constituted MβLCL. Finally, the MβLCL decayed with the expansion of the cold pool. Another MβLCL was generated when the favorable environment was restored after the cold pool dissipated, and the evolution process of the MβLCL was repeated.

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