Accuracy of Quantitative Precipitation Estimation Using Operational Weather Radars: A Case Study of Heavy Rainfall on 9–10 September 2015 in the East Kanto Region, Japan

Abstract

On 9–10 September 2015, the East Kanto region of Japan experienced a period of record-breaking heavy rainfall that caused a number of fatalities and serious property damage. The maximum 24-hr rainfall total (0600 UTC 9 September 2015 to 0600 UTC 10 September 2015), about 500 mm, was recorded over Tochigi Prefecture. Spatial and temporal variations in the meteorological and hydrological characteristics of this rainfall event were analyzed using data from the Japan Meteorological Agency’s (JMA) C-band radar network and data from the X-band polarimetric radar network (XRAIN). The rain gauge data available from the Kanto region has a temporal resolution of 10 min. The spatial and temporal resolutions of the JMA C-band radar data are 1000 m and 5 min, respectively, whereas the XRAIN radar has spatial and temporal resolutions of 250 m and 1 min, respectively. Data from the two radar networks were compared, both with each other and with data from various rain gauge networks to validate their accuracy. The 24-hr total rainfall data from both radar networks showed frequency distributions similar to those showed by the rain gauge data. However, the JMA and XRAIN data showed different distributions for the higher rainfall intensity thresholds. There was no relationship evident between rainfall and elevation in either of the radar datasets recorded during this event. The spatial distribution of rainfall over the study area derived from XRAIN showed clear variations, whereas the JMA radar did not. This is most probably related to the coarser spatial and temporal resolutions of the JMA observations. Based on a comparison of data from the rain gauge and radar networks, the XRAIN data more accurately reflected the rain gauge stations than did the JMA data. From a hydrological perspective, the Kinugawa watershed is unique in terms of its topography. The upper part of the watershed is wide and mountainous, whereas the rest is narrow and elongate north–south. The rain echo moved from south to north over the catchment, and the highest 24-hr accumulated rainfall totals were recorded mostly in the upper (northern) part of the Kinugawa watershed, whereas there was less rainfall in the lower (southern) part. This pattern suggests a high probability of serious flooding along the Kinugawa River in the days following such a rainfall event if the heaviest rainfall moves northwards over the watershed.