Case Study of Local Heavy Rainfall, Focusing on GPS Precipitable Water Vapor: Rainfall Event Observed in Tokyo on July 4, 2000

Abstract

The present study examined the essential features of local heavy rainfalls observed in Tokyo on July 4, 2000. The following results were obtained: 1) The atmosphere was unstable while the Japanese islands were covered by the Pacific high. 2) The easterly surface winds converged with southerly winds (E-S-type wind system) in the central part of the urban area several hours before the precipitation occurred. 3) The cold air outflow associated with the precipitation system flowed into the convergence zone formed by the E-S wind system. Examining the spatial distribution of and temporal change in the precipitable water vapor (PWV) on the Kanto plain shows that the PWV tends to increase before rainfall occurs both over the mountains and on the plain. However, it does not appear that the PWV increased in Tokyo due to the urban heat island circulation. Therefore, the urban heat island did not produce heavy rainfall by inducing a strong upwind and drawing in water vapor in this case.