Radar Echo Population of Air-Mass Thunderstorms and Nowcasting of Thunderstorm-Induced Local Heavy Rainfalls Part 1: Statistical Characteristics

Abstract

A radar echo population of 179 thunderstorms generated in the Tokyo metropolitan area on August 5, 2008, when the Zoshigaya rainstorm occurred in the center of Tokyo, is presented. Analysis was made using three-dimensional radar data from the Japan Meteorological Agency. One third of total convective cells had diameters of less than 3.5 km and the average diameter was 5.5 km. The mode of lifetimes of cells was from 20 to 40 minutes, and 88% of cells disappeared within 60 minutes after their initiation. The echo-top height of half of the cells reached 15 km, which was the limit of radar observation. Although the rainfall amount estimated from the radar echo was less than 40 mm for half of the cells, whereas one third of total cells counted were estimated atmore than 60 mm. Vertically integrated liquid water (VIL) ranged from 1.4 to 42.4 kgm-2. Maximum VIL was equivalent to 70% of precipitable water estimated from upper sounding on that day. The speed of cell travel was less than 2 ms-1in accordance with the weak wind velocities in the lower to middle troposphere. The time from echo initiation to rainfall peak was as short as 10 to 30 minutes for almost all cells. Thunderstorms composing the Zoshigaya rainstorm ranked at the highest class in horizontal size, lifetime and total rainfall amount among 179 thunderstorms. The horizontal size of cells in the thunderstorms was nearly equal to those reported for other areas in the world, whereas the echo top height was higher than in the other cases.