Early Detection of Baby-Rain-Cell Aloft in a Severe Storm and Risk Projection for Urban Flash Flood

Abstract

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.

Investigation of transition signals from single cell to multicell thunderstorms in the Kinki region, Japan by using vertical vorticity and multi-parameter radar analysis

“…The pseudo-vorticity was estimated by applying the method proposed by Nakakita et al (2017) with a spatial resolution of 50 m × 50 m in each elevation angle at 1 min intervals. Using the radial velocity speed, the vorticity can be calculated by using Eq.…”

“…The localized heavy rainfall from cumulonimbus clouds, which commonly occurs in the summer season, is potentially hazardous. In 2008, a flash flood occurred in the Toga River located in Kobe, Japan due to the downpour of Guerillaheavy rainfall from the development of isolated cumulonimbus clouds (Nakakita et al, 2013(Nakakita et al, , 2017. The rapid rise of water level in the river basin caused the death of five people.…”

“…Referring to the case of the Toga River basin, Nakakita et al (2013Nakakita et al ( , 2017 developed prediction methods on the first radar echo aloft that was described as a "baby-rain-cell" in a single cumulonimbus cloud by using Doppler weather radar observation. They confirmed the presence of a pair of positive and negative vertical vortex tubes as well as an updraft in the early stage of a rain-cell.…”

“…The vertical vorticity analysis is mostly investigated in supercell thunderstorms owing to the exhibition of updraft rotation. According to a study by Nakakita et al (2017), a vorticity pair in the baby-rain-cells was detected, which was related to the existence of an updraft in the single-cell. Therefore, the characteristics and mechanisms for the development from single-cell to multicell thunderstorms, which contribute to localized heavy rainfall, motivated the authors to discover the significant criteria related to the severity of thunderstorms.…”

Feasibility of Liquid Water Path Estimation of Over Land Using Satellite-Based Ka-Band Passive Microwave Data

“…(Corresponding author: Rie Seto.) that water content among hydrometeor types within the cloud system [1] [2] Because the features of rainfall carried by a cloud system can vary depending on the distribution of water particle types within the cloud system, even with the same total water content, the amounts of water in each type of hydrometeor are essential variables that constitute initial conditions for rainfall prediction. Furthermore, such variables are vital for comprehension of cloud and precipitation processes, as well as evaluation of rainfall prediction models.…”