High Resolution Weather Forecast of CWB-GFS Nested with NCEP-RSM

The complicity of terrain in Taiwan makes it difficult for CWB-GFS T511 (about 25 km) to capture the detail features of the regional circulation, which play an important role in agriculture, fishery, and the disaster prevention, especially the track of an approaching typhoon. In 2017, NCEP-RSM model, which has friendly design of implementing, calculating efficiency, and a built-in option for non-hydrostatic version, was nested into CWB-GFS T511 model to run simultaneously and efficiently via MPMD (multi-program multi-data) structure. The performance and efficiency of the nested model are preliminarily evaluated on the case of 11 September 2017 Typhoon Talim, whose track was not well forecasted on 11 September 00 UTC by most of the operational centers. With 12 km horizontal grid spacing, the nested model shows capability of improving both the typhoon intensity and track of the CWB-GFS prediction. Besides, in aid of the MPMD programming, over 60% of I/O and about 17% of the time to finish a 5-day forecast are reduced in the run of nested model compared with the separate runs of CWB-GFS and NCEP-RSM with the same computer resources of 768 CPUs. This preliminary result indicates the potential ability of the nested model to do the better forecast economically. More description of the nested model and further sensitivity tests of different resolution, wave truncation, and diffusion will be presented.