**5. Conclusion**

This chapter presented the investigations of the forecasts in several torrential rainfall events using Ensemble Kalman Filter. These descriptions are based on my recent studies [17]. In Section 2, I aimed to investigate the impact of dense surface observation data on a hazardous rainfall event in eastern Japan area in September 2015. In this case, a convergence zone extending in meridional direction was maintained by the surrounding wind of two typhoons. It associated to activate the rainband and cause a record-breaking precipitation. So, an impact on the rainband was the primary focus of the study. In this study, two data assimilation experiments were performed. Although the conventional data set (NCEP PREPBUFR) has many kinds of observations, it is difficult to perform precise forecast for the torrential rain. In addition to it, dense surface observation data assimilation contributed to improve forecast accuracy in both rainfall area and amount. Surface observation network is a classical system compared with modern remote sensing instruments. Even though total data size from 4000 stations is the order of 100 kB at one observation time, its data assimilation has certain impact on a torrential rainfall forecast having horizontal scale of the order of 100 km. Sections 3 and 4 showed that the rapid-update data assimilation can be a powerful tool in forecasting a local torrential rainfall through a single case study. In Section 3, a rapid-update ensemble Kalman filtering study for a torrential rainfall event on September 11, 2014, in Kobe city, which locates in the western part of Japan and provides a discussion of an impact on a torrential rainfall forecast. Since the local convective system having 10-km or less special scale grows within 10 minute. The conventional forecast system, which has 1-hour or longer data assimilation cycles and 1-km order horizontal resolution, has hardly resolved an isolated convective system both especially and temporary. To address this issue, 30-second-update, 100-m-mesh experiment was performed. Owing to the innovative NWP system, the torrential rain by an isolated convective system was well represented including its three-dimensional structure, and forecast accuracy was clearly improved. In Section 4, in general, these results were investigated by case studies. As mentioned in Section 3, several issues remained especially in the 100-m-mesh forecast. In order to contribute the results of the research to improve forecasts universally, additional experiments should be conducted and profiled the issues. Moreover, to review the entire parameter settings in Ensemble Kalman Filter is also important.
