Abstract

We studied disastrous heavy rainfall episodes in 2017-2019 summer in SW Japan, especially in the Kyushu region using tropospheric delay data from the dense Global Satellite Navigation System network GEONET (GNSS Earth Observation Network). This region often suffers from extremely heavy rains associated with stationary fronts during summer. In this study, we first analyze behaviors of water vapor on July 6, 2018, using tropospheric parameters obtained from the database at University of Nevada Reno. The data set includes tropospheric delay gradient vectors (G), as well as zenith tropospheric delays, estimated every 5 minutes. At first, we interpolated G to obtain those at grid points and calculated their convergence, similar to the quantity proposed by Shoji (2013) as water vapor concentration index. We obtained zenith wet delay from ZTD by removing zenith hydrostatic delay. The raw ZWD values do not really reflect the wetness of the atmosphere above the GNSS station because they largely depend on the station altitudes. To study the dynamics of water vapor before heavy rains, we estimated ZWD converted to the values at sea-level. In the inversion scheme, we used G at all GEONET stations and ZWD data at low-altitude ( 50 mm/hour) events occurred, i.e., July 5, 2017, July 6, 2018, and August 27, 2019. Next, we performed high time resolution analysis (every 5 minutes) on the days of heavy rain. The results suggest that both WVC and sea-level ZWD go up prior to the onset of the rain, and ZWD decreases rapidly once the heavy rain started.

Highlights

  • Disastrous heavy rains in summer 2017 to 2019 in SW Japan caused a lot of damage to property and human lives

  • We propose a new method to use tropospheric delay gradients to study heavy rain phenomena in Japan using the data from the GEONET data

  • The purpose of this study is to show the implication of utilizing tropospheric delay gradients, in addition to zenith wet delay (ZWD), to improve our understanding of water vapor dynamics during heavy rains

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Summary

Introduction

Disastrous heavy rains in summer 2017 to 2019 in SW Japan caused a lot of damage to property and human lives. Precipitation records at meteorological stations show extreme rainfalls from June 28 to July 8, 2018, especially in the northern part of the Kyushu District. This was obtained by a weather radar with a 250-m resolution every 5 min. Such meteorological radars have been operated by JMA at 20 stations throughout Japan. Water vapor transported along the front from SW is thought to have caused the heavy rain

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