Localized torrential rainfall caused heavy rainfall disasters in Summer and Baiu season of Japan called as "Guerrilla-heavy-rainfall" in Japanese media.The rainfall was produced by an isolated cumulonimbus that grew rapidly.To prevent these disasters, Japanese government recently installed networks of X-band polarimeric Doppler radars over major urban areas in Japan.X-band radar is suitable to realize earlier and more detailed detection of the baby cell, because X-band radar has higher sensitivity and spatial resolution.This research investigates the growth of the baby rain cell to become large cumulative clouds by focusing on its vorticity using Doppler velocity.In this research, we define two vortexes with different size, and evaluate a relationship qualitatively between rainfall and the existence of vortex.

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In July 2008, five people were killed by a tragic flash flood caused by a local torrential heavy rainfall in a short time in Toga River. From this tragic accident, we realized that a system which can detect hazardous rain-cells in the earlier stage is strongly needed and would provide an additional 5 to 10 min for evacuation. By analyzing this event, we verified that a first radar echo aloft, by volume scan observation, is a practical and important sign for early warning of flash flood, and we named a first echo as a “baby-rain-cell” of Guerrilla-heavy rainfall. Also, we found a vertical vorticity criterion for identifying hazardous rain-cells and developed a heavy rainfall prediction system that has the important feature of not missing any hazardous rain-cell. Being able to detect heavy rainfall by 23.6 min on average before it reaches the ground, this system is implemented in XRAIN in the Kinki area. Additionally, to resolve the relationship between baby-rain-cell growth and vorticity behavior, we carried out an analysis of vorticity inside baby-rain-cells and verified that a pair of positive and negative vertical vortex tubes as well as an updraft between them existed in a rain-cell in the early stage.