Developing 2-way feedback nested-domain LETKF data assimilation system and application to high-resolution typhoon analyses and forecasts

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This project ended in the middle of the second fiscal year because the principal investigator moved outside of Japan. During the one-year and four-month period of this project, the online nested-domain LETKF has been newly implemented in the SCALE-LETKF system. Combined with the existing online nested-domain function in the SCALE-RM model, the SCALE-LETKF has become capable of running both data assimilation cycles and extended ensemble forecasts with multiple nested domains in an online framework. The design is efficient in both the computational and human-effort aspects.The data assimilation experiments with the September 2015 Kanto-Tohoku Heavy Rainfall case were conducted using this new online nesting function. Compared with the old offline downscaling experiment in Lien et al. (2017, SOLA), the online nested-domain experiment led to improved forecast skill, due to further performing data assimilation in the inner high-resolution domain and frequent coupling of the two domains in the model forecast steps.The principal investigator also coauthored two papers by Honda et al. (2018a,b, MWR, JGR) for typhoon data assimilation studies using this SCALE-LETKF system.All source code developed in this project is publicly available at https://github.com/gylien/scale-letkf, with an MIT license.Due to early termination of this project, the 2-way feedback LETKF analysis with multiple nested domains is still under development. The principal investigator may be able to continue working on it and concluding the work in his next position.