Evolution and Air flow Structure of a Kanto Thunderstorm on 21 July 1999 (the Nerima Heavy Rainfall Event)

Abstract

Data from the Automated Meteorological Data Acquisition System (AMeDAS), global positioning system-derived precipitable water vapor (GPS-PWV), conventional and Doppler radar observations, and results from a numerical simulation by the Japan Meteorological Agency Non-Hydrostatic Model (JMANHM) were used to investigate the evolution and structure of the convective systems that caused the Nerima heavy rainfall, which was a disastrous rainfall event in the Tokyo metropolitan area, with hourly precipitation amount reaching 110 mm.Two types of convective systems comprised the thunderstorms that caused the rainfall event: a thunderstorm developed in a warm and moist region where southerly inflow from Tokyo and Sagami bays and northeasterly flow over the northern Kanto Plain converged, and a convective band that organized to the west of the first system. The first convective system quickly decayed after the mature stage, because divergent flow from intense rainfall prevented the southerly inflow from reaching the updraft region. The middle-level airflow characterized by cold temperature invaded this system from the north after its mature stage. Because it entered the updraft region of the system, it did not enhance the convection through the intensification of the cold outflow that produces the convergence with the low-level inflow from the south. However, abundant water vapor in the region of convergence, resulted in heavy rainfall in spite of the short duration of the system. For the second convective system, water vapor of low-level southerly inflow directly fed into the band, and thus the band maintained its intensity after the first system decayed. Low-level northerly airflows that lifted up the southerly inflow were far more intense than that of the first system. These northerly airflows acted to organize the convective band and forced it southeastward. The middle-level cold airflow that also invaded the system after the mature stage entered the downdraft region, resulting in an enhancement of the convection. However, due to rapid propagation speed of the second band, the rainfall duration at a fixed point was relatively short, so that the band did not produce floods.For prediction of thunderstorms, monitoring of low-level convergence zones of moist air was found to be possible, using indexes of accumulation and convergence of water vapor, as well as the Doppler radar radial wind in the non-precipitation weak echoes.